2020 Volume 26 Issue 5 Pages 561-578
Flaxseed can provide synergistic health benefits which are attributed to its enriched bioactive substances such as alpha-linolenic acid (ALA), flaxseed gums, flaxseed protein and lignan. The research on the advantages of flaxseed including anti-inflammation, anti-cancer, antioxidant, antidiabetic etc., have been confirmed. Generally, flaxseed can be consumed directly as a whole or milled form. At present, in order to improve the nutritive values of food products, whole or milled flaxseed as a functional additive, has been incorporated into many staple products. However, the addition of flaxeed into food products can significantly affect physical and chemical properties such as texture, sensory etc. We are facing a challenge that how to keep the stability of the bioactives in flaxseed during the production of enriched flaxseed products. Therefore, in this paper, the latest studies regarding the health advantages of whole or milled flaxseeds and their applications in food products were mainly presented.
Flaxseed is regarded as one of the oldest arable crop oilseeds, which means “very useful” in Latin (Goyal et al., 2018; Liu et al., 2018). Human using flaxseed as a part of diet can date back to the early 1990's (Marambe and Wanadsundara, 2017). Flaxseed is grown in around 50 countries (Edel et al., 2015). Among of these countries, Canada and China accounts for more than 30% flaxseed trades (i). Flaxseed has been widely applied in food, cosmetic, medicinal or feed industries (Bakowsk-Barczak et al., 2020; Bekhit et al., 2018).
The advantages of flaxseed and its derived products indicated by animals or human subjects, are generally attributed to its bioactive substances including ALA, dietary fiber, protein and lignan etc (Bechlin et al., 2019; Giarola et al., 2019; Lan et al., 2020). The contents of main compositions in whole or milled flaxseed are given in Table 1. The content of oil in flaxseed depends on many factors such as cultivars, environmental conditions and analysis methods etc (Deng et al., 2017; Dunford, 2015; Zhang et al., 2017; Zou et al., 2017). Flaxseed oil is available commercially, and has received GRAS status (Goyal et al., 2018). The content of ALA in flaxseed oil is usually ranged from 36 to 62% (Deng et al., 2017; Moghadam et al., 2019). ALA can improve immunity, and reduce occurred rates of cardiovascular disease, cancer, diabetes, arthritis and the diseases of gastrointestinal tract due to its excellent anti-inflammatory activity (Goyal et al., 2018; Kheira et al., 2019; Mosavat et al., 2018; Yadav et al., 2018; Zhu et al., 2020).
| Flaxseed forms | Total fat (g) | C18:3 undifferentiated (g) | Protein (g) | Carbohydrate (g) | Ash (g) | Dietary fiber (g) | Moisture (g) | Energy (kcal) |
|---|---|---|---|---|---|---|---|---|
| Whole flaxseed | 42.16 | 22.81 | 18.29 | 28.88 | 3.72 | 27.3 | 6.96 | 534 |
| Milled flaxseed | 42.16 | 22.81 | 18.29 | 28.78 | 3.72 | 27.3 | 6.96 | 534 |
Data was obtained from the United States Department of Agriculture, Agricultural Research Service (USDA, Agricultural Research Service, 2019). https://fdc.nal.usda.gov/fdc-app.html#/food-details/169414/nutrients (ii)
The main form of flaxseed lignan is secoisolariciresinol diglucoside (SDG) which can reach up to 1 099 mg/100 g (Gerstenmeyer et al., 2013). The levels of other lignans in flaxseed (isolariciresinol, lariciresinol, pinoresinol and matairesinol), are low (Bekhit et al., 2018; Gerstenmeyer et al., 2013). Flaxseed lignan mainly participates in the metabolism of hormones because of its weak estrogenic and antiestrogenic activity. Many health benefits from flaxseed lignan including anticancer, antidiabetic, anti-inflammatory, antihyperlipidemic, antiartherosclerotic, and antioxidative ability, have been reported (Aqeel et al., 2019; Figueiredo et al., 2017; Goyal et al., 2018).
The level of flaxseed dietary fiber varies from 20 to 28% (Mercier et al., 2014). As many another dietary fibers, flaxseed dietary fibers consist of soluble and insoluble fibers (Goyal et al., 2018; Liu et al., 2018), which can decrease the incidence risk of many chronic diseases, and keep the balance of intestinal flora (Luo et al., 2018).
The protein content in flaxseed is around 20% (Kaur et al., 2017; Marambe and Wanadsundara, 2017). In terms of nutrition, although flaxseed protein is not a complete protein, it shows comparable nutritive value to soy protein (Bekhit et al., 2018; Marambe and Wanadsundara, 2017). Many evidence show flaxseed protein is advantageous to our health for example anti-diabetic effect, anti-bacterial activity, angiotensin-converting enzyme inhibition and antioxidant capacity (Franck et al., 2019; Marie et al., 2019; Nwachuku and Aluko, 2018; Wei et al., 2018). Regarding the minerals in flaxseed, it is enriched in calcium, magnesium and phosphorus (Oliveira et al., 2017; Tuncel et al., 2017). The research have indicated that the consumption of 100 g flaxseed/day can meet most requisite demands for these minerals (Kaur et al., 2017). Vitamin E with many kinds of tocopherols in flaxseed is in existence (Tavarini et al., 2019). Grzegorz et al. (2019) reported that total content of β/γ- tocopherol in flaxseed varied from 216∼457 mg/kg. It has a good antioxidant ability, and protects low-density lipoprotein from oxidizing (Ren et al., 2017; Tavarini et al., 2019).
In brief, because of the existence of bioactives, for example ALA, dietary fiber, protein, lignan, flaxseed offers many potential health benefits, e.g., anti-inflammation, anti-cancer, antioxidant, antidiabetic, reducing blood fat, and weight loss (Bechlin et al., 2019; Giarola et al., 2019; Lan et al., 2020). Researchers and food manufacturers are trying to develop flaxseed-based products to meet consumers' health food demands. Currently, many kinds of flaxseed-derived products are available on the market, which include whole seeds, milled or ground flaxseed (flour), flaxseed oil, flaxseed blending oils, dehulled flaxseed, flaxseed hull (flaxseed gum), cyclic peptides, flaxseed protein concentrates, and lignan extracts (Kajla et al., 2015; Shim et al., 2015). Consumption flaxseed solely in our dietary is a rather boring thing, and maybe some people can not accept the poor sensory of flaxseed. Therefore, flaxseed as whole or milled form has been added into many normal foods, for example breads, muffins, crackers, energy bars, cookies, snacks, soups, and cake etc (Kajla et al., 2015; Mercier et al., 2014; Rodriguez-Leyva et al., 2013). All these applications have showed that flaxseed can be utilized as a nutrient supplement in food products. Due to poor stability of flaxseed oil, food industry prefers to incorporate whole or milled flaxseed into the products (Barriuso et al., 2013; Mercier et al., 2014). However, we are still facing many challenges during the production of flaxseed-based products. Even if whole or milled flaxseed have higher stability than flaxseed oil, flaxseed needs to meet adverse productive conditions that will reduce the stability of flaxseed. One of primary objectives for the development of flaxseed-based products is to keep the stability of bioactive compounds in flaxseed. Adding flaxseed into foods may also significantly influence on the physicochemical properties (Mercier et al., 2014, 2016). Many flaxseed-based products are made from the mixture of wheat flour and flaxseed flour. Incorporation flaxseed flour into food products may destroy the gluten network of pasta, and thus leads to obtained foods with poor structure (Mercier et al., 2014). Another objective for producing flaxseed-based products needs to keep the structural integrity of the products, and preserves the advantages of bioactives during the production and storage periods (Mercier et al., 2014). Therefore, in this paper, the latest research on health benefits of whole or milled flaxseed and its applications in foods were reviewed. What's more, the stability of ALA and SDG in foods was also discussed.
The treatment of cardiovascular diseases (CVDs) by flaxseed CVDs, one of most serious diseases threatening our health, which are highly related to many risk factors, such as hypertension, high serum lipid profile, inflammation etc (Edel et al., 2015; Ursoniu et al., 2016). Many studies have indicated that flaxseed shows cardioprotective activity due to bioactive compounds in flaxseed (Cassani et al., 2015; Edel et al., 2015). The hypocholesterolemic activity of flaxseed has been evaluated in rats, mice, rabbits and human models. Research on cardio-protective effect of flaxseed are showed in Table 2.
| Flaxseed forms and addition amounts | Model system | Results | References |
|---|---|---|---|
| 30 g milled flaxseed/day | Human patientswith peripheral artery disease | The dietary of milled flaxseed could significantly reduce the levels of total cholesterol and LDL-cholesterol. | Edel et al. (2015) |
| 60 g flaxseed powder/day | Men withcardiovascular risk factors | The dietary of flaxseed powder not only reduce triglyceride level, but also decrease inflammatory makers levels (C-reactive protein, tumor necrosis factor α). | Cassani et al. (2015) |
| 10% milled flaxseed/diet | Rats with arrhythmias and heart dysfunction | Supplement of flaxseed could significantly reduce the incidence of arrhythmias. The levels of myocardial fibrosis and tumor necrosis factor-. were significantly reduced. | Parikh et al. (2019) |
| 30 g milled flaxseed/day | Patients with cardiovascular disease | Bagels contained with flaxseed were easiest to be accepted by the patients with cardiovascular disease. | Austria et al. (2016) |
| 40g ground flaxseed/day | Hemodialysis patients with dyslipidemia | The dietary of ground flaxseed could significantly decrease triglycerides, total cholesterol, LDL-cholesterol levels, while the level of HDL-cholesterol increased significantly. | Khalatbari Soltani et al. (2013) |
Askarpour et al. (2020) investigated impact of flaxseed supplementation on inflammatory markers. The authors found that flaxseed supplementation could significantly decrease the levels of C-reactive protein, interleukin-6 and vascular cell adhesion protein. Parikh et al. (2019) showed that supplement of flaxseed could significantly reduce the incidence of arrhythmias due to the reduction of inflammatory markers. It is indicated that dietary flaxseed showed a great potential method for the treatment of arrhythmias. Through above studies, we could know flaxseed can reduce the occurrence of cardiovascular disease through the prevention or control of dyslipidemia, hypertension or inflammation. Austria et al. (2016) explored which foods containing milled flaxseed were best accepted for the patients with cardiovascular disease. Milled flaxseed (30 g) was incorporated into many kinds of foods such as muffins, snack bars, biscuits, bagels, and pasta etc. After one year study, the results showed that bagels contained with flaxseed were best accepted by the patients with cardiovascular disease (Austria et al., 2016).
Flaxseed in cancer treatment Many researchers have indicated that ingestion flaxseed can reduce incidence risk of cancer (Table 3). Anti-cancer ability of flaxseed is mainly attributed to the presence of ALA and lignan. Although many research studies indicate flaxseed has anti-cancer ability for many types of cancer, it is very difficult to cure it due to its complicated formation mechanisms. Lowcock et al. (2013) found consumption of flaxseed or flaxseed breads could significantly low the occurrence of breast cancer. Hu et al. (2019) investigated impact of flaxseed extract on the development of human breast cancer MCF-7 cells. The authors found flaxseed oil could induce the apoptosis of MCF-7 cells, which was indicated by the loss of mitochondrial membrane potential and caspase cascade reaction. Pal et al. (2019) investigated impact of dietary flaxseed on the apoptosis and angiogenesis of ovarian cancer in chicken. The results indicated that the supplement of flaxseed could improve the apoptosis. Therefore, flaxseed can be used as a dietary intervention to reduce cancer risk (Pal et al., 2019). However, more research should be carried out to explain the anti-caner mechanisms when whole or milled flaxseed is used as an anti-cancer dietary supplement.
| Flaxseed forms and amounts | Model System | Results | References |
|---|---|---|---|
| 15% flaxseed meal/diet | Chicken | The supplement of flaxseed could improve apoptosis, and inhibit the formation of angiogenesis in ovarian cancer cells. | Pal et al. (2019) |
| 0∼320 µg /mL flaxseed extracts | Human breast cancer MCF-7 cells | Flaxseed extracts could induce the apoptosis of MCF-7 cells. | Hu et al. (2019) |
| 16 g flaxseed/kg of body weight | Rats | Dietary flaxseed could induce the expression of the genes involved in proliferation, cell cycle arrest and mitochondrial apoptosis. | Hernandez-Salazar et al. (2013) |
| 28 g flaxseed/day | Adolescents | Flaxseeds could significantly reduce diastolic blood pressure. However, flaxseeds did not differentially influenc on plasma glucose and inflammatory profile, plasma lipid responses | Machado et al. (2015) |
| 40 g flaxseed/day | Perimenopausal overweight women | Waist circumference reduced in flaxseed group. Only dietary golden flaxseed could reduce body weight and fat mass. | Aguilar et al. (2017) |
| 25% flaxseed flour/diet | Adult male rats | The body adiposity was significantly improved in flaxseed flour group. | de Abreu et al. (2017) |
Flaxseed in obesity treatment Obesity, a public health problem in the world, can significantly increase the incidence of many other chronic diseases (de Abreu et al., 2017). Some studies have demonstrated that feeding flaxseed can reduce fat absorption, and improve the lipids excretion by fecal due to dietary fiber and ALA in flaxseed (Table 3). Machado et al. (2015) compared effect of dietary flaxseed on health parameters in overweight adolescents. The authors found dietary flaxseed could significantly reduce diastolic blood pressure, but did not differentially affect inflammatory profile and plasma lipid responses (Machado et al., 2015). Aguilar et al. (2017) also studied effect of dietary flaxseed on body compositions of overweight perimenopausal women. The results indicated flaxseed could induce the reduction of waist circumference. However, only dietary golden flaxseed could reduce body weight and fat mass. Therefore, golden flaxseed presented higher ability to modify body compositions of perimenopausal women compared to brown flaxseed (Aguilar et al., 2017). However, in the study of de Abreu et al. (2017), the authors found body adiposity of adult male rats was significantly improved after flaxseed flour was feed during the investigated periods.
Flaxseed in diabetes treatment Although anti-diabetes mechanisms of dietary flaxseed are not fully understood, many evidence have showed that ingestion of flaxseed can control the development of diabetes through improving immunity, as well as reducing the incidence risk of obesity, blood glucose and dyslipidemia (Table 4). Anti-diabetes activity of flaxseed is attributed to dietary fiber, lignan, and ALA presented in flaxseed oil. Soltanian and Janghorbani (2018) investigated effect of dietary flaxseed on the health of constipated patients with type 2 diabetes. The results showed that dietary flaxseed could significantly decrease weight, cholesterol, body mass index, triglycerides and LDLC. The authors suggested that flaxseed cookies could be used as a dietary strategy to constipation patients with type 2 diabetes. What's more, compared to psyllium, flaxseed showed better ability to decrease constipation symptoms, glycemia, weight and lipid levels (Soltanian and Janghorbani, 2018). Also, in the study of Ricklefs-Johnson et al. (2017), the authors reported that flaxseed had higher ability to reduce waist circumference and oxidative stress in patients with type 2 diabetes compared to psyllium. In addition, the study also showed nitric oxide bioavailability in the patients with type 2 diabetes increased after dieting flaxseed. Afzal et al. (2020) evaluated effect of dietary flaxseed powder and extract on the status of Sprague Dawley rats with hyperglycemia and hypercholesterolemia. The results showed that flaxseed extract (5.0%) incorporated into based diet presented better ability to control cholesterol, glucose, triaclglycerols and LDL-C, compared to flaxseed powder (10%). Thus, flaxseed extract showed higher bio-efficacy to treat hyperglycemia and hypercholesterol compared to flaxseed powder (Afzal et al., 2020). However, Taylor et al. (2010) found that blood glucose of the patients with type 2 diabetes was not reduced for 12 weeks after being ingested milled flaxseed (32 g/day). At present, effect of ingestion of whole or milled flaxseed by diabetic patients or animals are limited. More studies in this field should be carried out.
| Flaxseed forms and amounts | Model System | Results | References |
|---|---|---|---|
| 20 g flaxseed pre-mixed in cookies/day | Constipated patients with type 2 diabetes | Dietary flaxseed could significantly decrease weight, cholesterol, body mass index, fasting plasma glucose, triglycerides, HDLC and LDLC. | Soltanian and Janghorbani (2018) |
| 20 g flaxseed pre-mixed in cookies/day | Patients with type 2 diabetes | Flaxseed exhibited higher ability to reduce waist circumference and oxidative stress in patients with type 2 diabetes compared to psyllium. | Ricklefs-Johnson et al. (2017) |
| 10% flaxseed powder or 5.0% flaxseed extract/diet | Sprague Dawley rats with hyperglycemia and hypercholesterolemia | The ingestion of flaxseed could significantly decrease inflammatory markers levels, such as neutrophil count, white blood cell count, serum amyloid A, C-reactive protein. | Afzal et al. (2020) |
| 32 g milled flaxseed/day | Adults with well-controlled type 2 diabetes | Blood glucose in the patients was not reduced after being ingested milled flaxseed. | Taylor et al. (2010) |
| 100, 300, 500 mg/kg flaxseed extract | Mice | Flaxseed extract exhibited antidiarrheal and antispasmodic activities by its antimotility and antisecretory effects. | Palla et al. (2016) |
| 30 g ground flaxseed/day | Patients with uclerative colitis | Dietary flaxseed and flaxseed oil could significantly decrease in Mayo score, fecal calprotectin, ESR, IL-6, INF-γ, waist circumference, diastolic blood pressure and systolic blood pressure. | Morshedzadeh et al. (2019) |
Flaxseed in inflammatory bowl disease (IBD) treatment IBD is one kind of gut-associated chronic disease (Morshedzadeh et al., 2019; Power et al., 2016). Dietary flaxseed or ALA in flaxseed oil has been utilized to treat IBD (Table 4). Almousa et al. (2018) explored the relationship between the levels of serum pyruvate kinase M2 in patients with IBD. The authors found the levels of serum pyruvate kinase M2 in IBD patients were 6-fold higher than those in healthy groups. Flaxseed could significantly suppress the expression of serum pyruvate kinase M2 induced by LPS (500 ng/mL) (Almousa et al., 2018). Morshedzadeh et al. (2019) investigated that impact of ground flaxseed and flaxseed oil on serum levels of inflammatory markers in patients with IBD. It was indicated dietary flaxseed and flaxseed oil could significantly decrease in Mayo score, fecal calprotectin, ESR, IL-6, INF-γ and waist circumference (Morshedzadeh et al., 2019). Palla et al. (2016) investigated mechanisms of flaxseed extracts for the treatment of IBD. The results showed that the protection for the gut mucosal could be generated through modulating cytokines, and anti-inflammatory effect of flaxseed (Palla et al., 2016).
Other health activities of flaxseed Flaxseed has showed beneficial effects on many forms of renal injury. Tou et al. (2015) investigated effect of ω-3 fatty acids supplementation on polycystic kidney disease. The results indicated that ALA in flaxseed oil played an important role in renal protection. Flaxseed also has lung protective activity. Christofidou-Solomidou et al. (2014) found that flaxseed could provide a potential radio-protection for murine lungs through modulating the miRNA profiles. Mucci et al. (2015) studied impact of maternal dietary flaxseed on the young male offspring rats with neonatal hypoxic-ischemic encephalopathy. The results showed that dietary flaxseed increased the content of the hippocampal DHA, reduced depressive behavior, and thus showed neuroprotective effect. Mourvaki et al. (2010) found that ingestion of flaxseed could improve sperm quality through increasing the integrity sperm membrane and the viability of sperm due to the presence of ALA in flaxseed.
Maira et al. (2018) investigated impact of dietary flaxseed flour on bone health in adult Wistar male rats at 180 d. The results showed that dietary flaxseed flour was favourable to bone health through improving total and spine bone mineral density, total bone area, and osteocalcin. Ribeiro et al. (2017) investigated impact of feeding flaxseed flour on bone structure of dam rats during lactation and post-weaning. The results showed that flaxseed group showed higher breaking strength, maximum force, and rigidity due to the presence of flaxseed oil. In summary, all these positive effects indicate that whole or milled flaxseed can be used as a dietary supplement. Generally, dietary flaxseed 30 g per day can satisfy our health demand.
Many products enriched with flaxseeds, for example milk, yogurts, soups, snack bars, spaghetti, rice paper, chutney powder, chips, cookies, puffs, buns, meat patties and biscuits, have been investigated (Cukelj et al., 2017; Kajla et al., 2015; Mercier et al., 2014). Although incorporation of flaxseed into foods can improve its nutritional value, it is very hard to develop it due to many challenges we are facing (Bekhit et al., 2018). Many research have indicated the addition of flaxseed significantly influences on the characteristics of products, such as colour, texture, storage stability and sensory etc (Bekhit et al., 2018). Furthermore, impact of flaxseed enrichment on the quality of foods are highly depended on many factors, such as food systems, processing methods, and levels of the addition etc. Therefore, in order to minimize impact of flaxseed enrichment on the characteristics of products, the addition of flaxseed is generally controlled at a certain level. In this section, effect of the addition of flaxseed into various foods on the quality of the obtained foods was discussed (Table 5, Figure 1). As indicated in Table 5 and Figure 1, many whole flaxseed-based products have been developed. Not only the incorporation amounts of whole or milled flaxseed but also the incorporation forms of flaxseed can significantly affect the product quality including storage stability, textural properties, sensory characteristics etc.
| Food types | The addition amounts of flaxseed | References |
|---|---|---|
| White bread | 10 g roasted flaxseed flour/bread | Marpalle et al. (2015) |
| Cookies | Wheat flour and roasted flaxseed flour blends (100:00, 90:10, 80:20 and 70:30) | Kaur et al. (2019) |
| Iranian toast | 10∼30% flaxseed flour | Pourabedin et al. (2017) |
| Muffins | 5.0∼30% flaxseed meal (germinated or ungerminated) | Kaur et al. (2020) |
| Muffins | Substitution of wheat flour with 10∼40% ground flaxseed | Kaur and Kaur (2018) |
| Cookies | Substitution of wheat flour with 10∼30% flaxseed flour | Kaur et al. (2017) |
| Biscuits | 10% milled flaxseed on flour basis | Cukelj et al. (2017) |
| Inegol meatball | 10% flaxseed flour | Turp (2016) |
| Sausage | 3.0, 6.0% flaxseed powder | Ghafouri-Oskuei et al. (2020) |
| Yogurts | 1.0, 3.0, 5.0% ground flaxseed powder | Marand et al. (2020) |
| Stirred probiotic yogurt | 2.0, 4.0% flaxseed | Mousavi et al. (2019) |
| Hamburgers | 7.5% flaxseed meal | Hautrive et al. (2019) |
| Pasta | 15, 30% flaxseed flour | Villeneuve et al. (2013) |
| Chips | 10∼20% flaxseed flour | Yuksel et al. (2014) |
| Rice paper | 10% ground flaxseed | Cameron and Hosseinian. (2013) |
| Bars | 0∼20% flaxseed flour | Mridula et al. (2013) |
Various flaxseed-based products
A: Flaxseed; B: Milled flaxseed; C: Roasted milled flaxseed; D: Pasta; E: Bread; F: Chinese bread; G: Bar; H: Cookies; I: Dehulled flaxseed; J: Muffins; K; Sprouted flaxseed powder; L: Flaxseed yogurt
Whole or milled flaxseed In general, compared to another flaxseed forms, oxidative stability of whole flaxseed may be the highest. Many internal and external factors including processing methods, moisture content, storage environment, packaging etc., can significantly affect flaxseed quality. Among many processing methods for whole flaxseed, roasted treatment of flaxseed is one of most popular methods. It can improve sensory properties, reduce microbial loads, and low the levels of poisonous compounds in flaxseed (Wei et al., 2018; Yu et al., 2019 a, b). The quality of flaxseed such as texture, overall acceptability etc., can be significantly improved after being roasted treatment. In general, the moisture of flaxseed decreases as roasting temperature increased and roasting time extended. The moisture content in roasted flaxseed can reach to 0.10∼0.30% after flaxseed is roasted at 160 or 180 °C for 24 minutes (Hall et al., 2006). If the roasting temperature is higher than 180 °C, the color and ALA content in flaxseed would significantly decrease. It is suggested that roasting temperature is below 140 °C (Hall et al., 2006). Yu et al. (2019b) found that the flavour of pressed flaxseed oil changed from nutty aroma to strong burnt taste, and the color became darker with the increase of roasting temperature. Roasting temperature could significantly affect ALA contents in flaxseed oil. Wei et al. (2018) found that sufficient amount of favorable volatile aroma compounds were formed when flaxseed was roasted at 200 °C for 30∼40 min. Compared to that of unroasted flaxseed, storage stability of roasted flaxseed decreased. The increase of PV values in roasted flaxseed during investigated storage period, was observed due to low water activity of roasted flaxseed (Hall et al., 2006).
Milled treatment for whole flaxseed is another one of most popular methods. Unheated or heated milled flaxseed can be available on the market. Milled flaxseed without heating treatment is usually stored in an inert gas. Compared to milled flaxseed with heat treatment, unheated milled flaxseed may have poorer quality due to anti-nutritional factors and bacteria inside of flaxseed (Shim et al., 2015). After flaxseed is milled, the thermal, oxidative and storage stability will be altered. Many researches regarding the storage stability of milled flaxseed have been reported. It would be highlighted in the following section in this paper. Ghosh et al. (2004) investigated impact of replacement wheat flour tortilla with milled flaxseed flour on dough quality. The results showed that milled flaxseed addition significantly affected the texture and color of dough. Due to the mucilage in flaxseed, water absorption of flour mixtures increased, and the strength of the formed dough decreased. Also, substitution wheat flour tortilla with milled flaxseed flour could make the dough darker (Ghosh et al., 2004). Shah et al. (2018) evaluated storage stability of whole/milled flaxseeeds treated by vacuum steam pasteurization method. Microbial counts in whole/milled flaxseeds after vacuum steam pasteurization were significantly reduced. The results indicated that vacuum steam pasteurization could be utilized to the sterilization of flaxseed based products.
Sprouted flaxseed Germination is an early growth stage of seed, which peels the seed hull (Shim et al., 2015). Germination can increase the contents of nutrients, reduce the contents of anti-nutritional compounds, and thus improves the bioavailability of the nutrients (Shim et al., 2015). The most popular sprouted product on the market is sprouted soybean. Also, flaxseed can be de-hulled by germination (Lv and Huang, 2015). Sprouted flaxseed may be powdered, and used as a dietary supplement (Lv and Huang, 2015; Shim et al., 2015). During the preparation of sprouted flaxseed, it should pay close attention to the content changes of poisonous compound cyanogenic acid monoglucosides (Shim et al., 2015). Kaur et al. (2013) investigated impact of the incorporation of germinated flaxseed meal on the quality of breads. The results showed that breads enriched 10% flaxseed, regardless of ungerminated or germinated flaxseed, showed better sensory properties, as well as higher acceptability than control (Kaur et al., 2013). Wang et al. (2016) investigated impact of the development of flaxseed germination on antioxidant and antiproliferative capacities of flaxseed. The results showed that the highest antioxidant and antiproliferative capacities of germinated flaxseed presented after 10-day germination. The author indicated flaxseed germinated for 8∼10 days could be used as a functional additive incorporated into our diet (Wang et al., 2016). Lv and Huang (2015) also patented a preparation method for sprouted flaxseed powder. Prior to soaked flaxseed being germinated for 175 h at 25 °C, it should be treated at 7 °C for 48 min. Afterwards, germinated flaxseed was dried, and crushed. Under the optimized conditions, ALA content in sprouted flaxseed powder could reach 22.5%. It is an effective and reliable way for the consumers to ingest flaxseed conveniently. Kaur et al. (2020) investigated baking and sensory quality of germinated and ungerminated flaxseed muffins, and found muffins with 10% germinated flaxseed meal exhibited high organoleptic score and good textural property.
Baking applications
Breads In general, effect of flaxseed on breads volume depends on the usage amounts of flaxseed. The specific volume of breads is reduced because the addition of flaxseed destroys the structure of the gluten. However, dietary fibers in flaxseed can improve water retention ability of the dough, and result in dough expansion due to the high internal pressure during the baking. Addition of flaxseed also can increase the darkness of breads, that results in lower acceptance by the consumers (Pourabedin et al., 2017; Maidana et al., 2020; Wandersleben et al., 2018). One reason is that dark and brownish color of flaxseed itself can decrease the brightness of breads, the other one is that Maillard reaction compounds are formed.
Pourabedin et al. (2017) prepared Iranian toast incorporated with flaxseed flour, and investigated its characteristics. The results showed that the addition of flaxseed flour could increase water absorption and development time of dough, and reduce dough stability, extensibility and brightness in contrast with control toast. Wandersleben et al. (2018) obtained breads composed of flaxseed and lupine flours, and found it contained higher levels of dietary fiber and proteins compared to control samples. The consumer survey showed that the bread presented more than 90% acceptability (Wandersleben et al., 2018). Roozegar et al. (2015) prepared Taftoon flaxseed bread, and investigated its characteristics. The authors found the addition of (un)coated ground flaxseed could significantly decrease water absorption of dough, but, significantly improved the stability and the development time of dough. The sensory evaluation tests indicated that Taftoon bread with 20% ground flaxseeds exhibited higher acceptance. Maidana et al. (2020) reported the fermentation of chia and flaxseed sourdoughs could significantly improve the specific volume of the obtained breads.
Cakes Lee et al. (2004) investigated the potential of flaxseed powder and nutrim oat bran as a shortening replacer in cake making. The authors indicated that with the increase of the replacement of flaxseed powder, the cohesiveness and springiness gradually increased. In addition, the addition of flaxseed powder improved the darkness of cake (Lee et al., 2004). Moraes et al. (2010) investigated the stability of cake incorporated milled flaxseed. The results showed that ALA in cake exhibited a good stability due to the presence of the antioxidants in flaxseed. Addition of milled flaxseed could also increase water retention. The maximum addition of milled flaxseed in breads could reach up to 30% (Moraes et al., 2010).
Muffins In the study of Ramcharitar et al. (2005), the authors showed that texture and flavor of muffins were positively correlated to overall acceptability. Most of the panel thought flaxseed-free muffins appeared better acceptability than those enriched with 11.6% milled flaxseed (Ramcharitar et al., 2005). Sudha et al. (2010) investigated effect of the addition of milled flaxseed on the characteristics of muffins. The authors found the substitution of semolina by flaxseed flour could improve water adsorption and preservation ability, reduce dough development time, and finally led to poorer dough stability due to the dilution of the gluten. Muffins contained roasted milled flaxseeds exhibited higher overall acceptability than those incorporated with raw flaxseeds (Sudha et al., 2010). Kaur and Kaur (2018) investigated impact of the partial substitution of wheat flour by ground flaxseed flour on the properties of muffins. The results showed that increasing substitution of flaxseed could decrease the volume of muffins. In addition, the colour of muffin crumb was darker as the substitution increased. The authors suggested that the highest replacement of wheat flour by ground flaxseed flour was 10% (Kaur and Kaur, 2018).
Cookies Kaur et al. (2017) investigated impact of the substitution of wheat flour by flaxseed flour on the properties of cookies. The results showed that the color of cookies became darker, and spread factors significantly increased as replacement degree increased. The authors suggested the highest addition of flaxssed flour into cookies was up to 10%. Kaur et al. (2019) also investigated impact of the addition of flaxseed flour on the characteristics of cookies. The results showed that resilience, springiness, hardness and chewiness of cookies significantly reduced, and gumminess, cohesiveness and fracturability significantly improved as the addition of flaxseed flour increased.
Extruded products
Pasta In general, fresh pasta presented a short storage stability due to its high water activity (Mercier et al., 2014). Many research have indicated the incorporation of flaxseed can significantly influence on the quality of pasta due to many reasons such as the dilution of the gluten, the disruption of the gluten network, the enrichment of dietary fibers, protein or phenolics. Yalla and Manthey (2006) investigated effect of the addition of flaxseed on physical characteristics of the dough. It was showed that the presence of flaxseed could reduce the viscosity of the dough because of the disruption of the gluten structure. Sinha and Manthey (2008) studied impact of the incorporation of flaxseed on dough strength, cooking and color properties of pasta. The authors observed the incorporation of flaxseed could decrease the firmness of the dough. What's more, the firmness of cooked pasta contained flaxseed was reduced around 30% compared to that of flaxseed-free pasta. Flaxseed pasta was with bower brightness and weaker yellow color in contrast with flaxseed-free pasta (Sinha and Manthey, 2008). Villeneuve et al. (2013) investigated impact of flaxseed enrichment on the properties of pasta. The authors indicated that the increasing addition of flaxseed could significantly affect color and texture properties of pasta. Compared to pasta contained 15% flaxseed, pasta contained 30% flaxseed appeared darker in color, and thus led to lower consumer acceptability. The lower swelling ability for pasta enriched with 30% flaxseed was observed because the disruption of the gluten could reduce water adsorption ability of pasta (Villeneuve et al., 2013).
Macaroni Lee et al. (2003) investigated effect of various treatments on the quality of macaroni enriched with milled flaxseed. The results showed that after being treated, all samples showed similar appearance property. However, treatment forms could significantly affect the firmness of the cooked macaroni. Macaroni treated by boiling heating showed the highest firmness, while samples treated by microwave-heating appeared the lowest firmness. ALA in macaroni presented the high stability during the investigated three kinds of treatments. Hall et al. (2005) also investigated ALA stability in macaroni enriched with treated or un-treated milled flaxseed. The results showed that pretreatment, the formation or drying process of macaroni did not significantly affect ALA stability. It is suggested the addition of milled flaxseed into macaroni may be a good way to enhance the dietary consumption of ALA.
Snack bars Mridula et al. (2013) investigated impact of the incorporation of flaxseed (0∼20%) on the properties of oat cereal bars. The results showed that incorporation of flaxseed did not significantly influence on the firmness of cereal bars. However, the brightness of oat cereal bars enriched flaxseed was significantly reduced. The average brightness of oat cereal bars decreased 23% when cereal bars ware incorporated with 20% flaxseed. In addition, cereal bars enriched with 5.0% flaxseed exhibited the highest acceptability. Storage conditions could significantly affect overall acceptance of the consumers for the bars. Therefore, the authors suggested that oat bars contained flaxseed had better be kept under refrigerated conditions in a short period (Mridula et al., 2013). Khouryieh and Aramouni (2012) also investigated that impact of the incorporation of on the acceptability of oat bars. The authors found the texture and acceptability of oat bars were not affected when the incorporation of flaxseed was below 12%. What's more, the authors also found flavor used in the preparation of oat bars significantly impacted the consumer overall acceptance.
Flaxseed yogurt Mousavi et al. (2019) investigated physicochemical, textural and sensory properties of flaxseed yogurt. The authors found that the supplementary quantity of flaxseed could markedly influence on the viability of L. acidophilus, and the textural property of formed yogurt. The incorporation of flaxseed was more beneficial to the growth of L. acidophilus compared to the control. After flaxseed was incorporated into the yogurt, the textual attributes (for example the viscosity, cohesiveness, hardness and gumminess etc.) increased, but the syneresis and adhesiveness of yogurt reduced. In addition, high levels of flaxseed could decrease in the yogurt's desirable sensory scores (Mousavi et al., 2019). Marand et al. (2020) prepared yogurt incorporated with flaxseed powder and evaluated its physicochemical and sensory properties. The authors found the incorporation of flaxseed markedly improved water holding ability and the viscosity of formed yogurts. Color attributes of yogurt was markedly affected by the incorporation of flaxseed. The sensory evaluation results indicated that yogurts enriched flaxseed powder were acceptable (Marand et al., 2020).
The products based on milled flaxseed or dehulled flaxseed have been developed in our group. One kind of products based on dehulled flaxseed, is flaxseed yogurt (Tang, 2019a). Dehulled milled flaxseed and whey protein were co-fermented by Streptococcus thermophilus and Streptococcus acidophilus. The formed flaxseed yogurt showed fat-reducing ability. We also developed flaxseed yogurt using milled flaxseed as raw material (Tang, 2019b). Milled flaxseed and milk were co-fermented by Streptococcus thermophilus and Lactobacillus bulgaricus. This type of flaxseed yogurt is beneficial for menopause women. Another kind of product based on dehulled flaxseed, is liquid flaxseed drink (Tang and Bian, 2018; Tang et al., 2017). As many another plant protein drinks, dehulled flaxeed need to be treated by milling and homogenizing in order to obtain stable flaxseed protein beverage (Figure 2). It has been commercialized since 2018. The shelf life of the obtained flaxseed protein beverage can reach upto 10 months.
Production process for flaxseed protein beverage
Meat products The consumption of meat products is very popular in our daily diet. The biggest drawback for meat products may be the poor levels of unsaturated fatty acids, even if meat products are good sources of multiple nutrients (Ghafouri-Oskuei et al., 2020; Lorenzo et al., 2018; Pateiro et al., 2018). Flaxseed as a functional additive has been incorporated into many kinds of meat products for example sausages, patties, meat batter, meat ball etc (Reyes-Padilla et al., 2018; Zajac et al., 2017). Ghafouri-Oskuei et al. (2020) incorporated flaxseed and tomato powder into beef sausage, and studied its quality properties. The authors found the incorporation of tomato and flaxseed obviously decreased pH and moisture content, and increased the contents of protein, ash, carbohydrate and fiber. The maximum addition of tomato and flaxseed up to 3.0% in cooked or fried sausages was suggested (Ghafouri-Oskuei et al., 2020). Flaxseed and tomato paste also were added into beef patties (Valenzuela-Melendres et al., 2018). ALA contents in beef patties could significantly increase with the increase of flaxseed addition. In addition, adding flaxseed and tomato paste could significantly reduce texture quality parameters in the investigated beef patties. Whereas, the incorporation of the mixture of flaxseed and tomato paste negatively affected the sensory characteristics of the cooked beef patties. Beef patties with 10% flaxseed plus 10% tomato paste showed the highest acceptable sensory scores (Valenzuela-Melendres et al., 2018).
Turp (2016) investigated impact of cooking methods on the properties of Inegol meatball incorporated with flaxseed. The results showed that cooking methods could significantly affect meatball proximate composition. The author indicated that ohmic cooking might be the best cooking method based on ALA level in meatballs. The obtained meatballs by ohmic cooking presented brighter color and harder texture compared to those made by traditional methods (Turp 2016). Yogesh et al. (2015) investigated effect of the additive quantity of flaxseed on the texture and sensory characteristics of meat batter. The authors found the incorporation of flaxseed could significantly reduce the hardness and chewiness values compared to the control samples. Based on sensory evaluation, incorporating flaxseed decreased the sensory scores for meat batters (Yogesh et al., 2015). Zajac et al. (2017) improved the nutritional values of liver sausages through the incorporation of flaxseed. The authors found the spreadability and hardness of the obtained sausages significantly increased after flaxseed was incorporated. In addition, the incorporation of flaxseed significantly improved nutritional values of the liver sausages due to high ALA level in the liver sausages. The evaluation results showed the liver sausages with 10% flaxseed presented the highest acceptable scores (Zajac et al., 2017).
Other food products Rao et al. (2013) produced flaxseed chutney powder, and found the developed flaxseed chutney powder had a good storage stability. The sensory property did not change at room temperature during the investigated storage periods. Kaur and Das (2015) developed barley flaxseed based functional dry soup mix enriched with β-glucans and ALA, which could be called as “health food” (Kaur and Das, 2015). Cameron and Hosseinian (2013) developed flaxseed rice paper contained ALA and dietary fibre. The obtained flaxseed rice paper exhibited good antioxidant ability, which could reach up to 231.7 µmol Trolox equivalents/g. Yuksel et al. (2014) developed flaxseed chips, and found incorporating flaxseed flour could increase the contents of dry matter, ALA and protein in chips. As frying temperature increased, the firmness of chips reduced, but overall acceptability of chips increased. The suggested conditions for the production of flaxseed chips were frying temperature 180 °C, frying time 50 s, and flaxseed level 10% (Yuksel et al., 2014).
ALA in flaxseed is very easy to be oxidized, particularly ALA in extracted flaxseed oil. Generally, the rancidity of flaxseed-based products is mainly due to ALA oxidation during the production or storage period. We can obtain the information regarding the oxidative stability of flaxseed-based products through measuring normal physical and chemical parameters such as acid values (AVs), panisidine values, peroxide values (PVs), free fatty acids (FFAs), conjugated diene (CD), oxygen consumption etc (Edel et al., 2015).
Stability of ALA in flaxsseeds ALA in whole flaxseed appears very stable, even if flaxseed goes through many kinds of thermal processing (Table 6). ALA content did not change after being treated at 100 °C for 1 h, or 178 °C for 1.5 h (Edel et al., 2015; Hall et al., 2006). However, if roasting temperature for flaxseed was higher than 180 °C, temperature would significantly affect ALA stability, and the oxidative stability of obtained flaxseed oil would significantly reduce (Yu et al., 2019b). Yu et al. (2019a) pretreated flaxseeds by normal-pressure roasting, microwave and high-pressure roasting, respectively, and then pressed to produce flaxseed oil. The results showed that these pretreatments for flaxseed did not significantly affect fatty acid compositions. The oxidative stability of obtained flaxseed oil by normal-pressure pretreatment (170 °C, 45 min) was best, followed by high-pressure roasting (1.0 MPa) (Yu et al., 2019a). Shah et al. (2018) treated whole or milled flaxseeds through vacuum steam-pasteurization, and characterized the quality of obtained flaxseed oil. Through the obtained results of many parameters such as AVs, SDG content, oil content, fatty acid profiles, PVs etc., vacuum steam-pasteurization showed little impact on chemical stability of both pasteurized whole/milled flaxseeds. The author suggested the pasteurization of vacuum steam had a potential application in reducing microbial contamination of flaxseed without significantly affecting chemical stability (Shah et al., 2018).
| Flaxseed sources | Results | References |
|---|---|---|
| Whole and milled flaxseed | Vacuum steam-pasteurization showed little impact on chemical stability of both pasteurized whole/milled flaxseeds. No significant changes of SDG concentration between pasteurized and unpasteurized milled flaxseed were observed. | Shah et al. (2018) |
| Whole flaxseed | Packaging could significantly affect lipid content in flaxseed. Flaxseed stored by polyethylene-coated package exhibited better storage stability. | Bechlin et al. (2019) |
| Milled flaxseed | The size of milled flaxseed could significantly affect the ALA stability. Coarse milled flaxseed showed better oxidation stability compared to fine milled flaxseed. | Schorno et al. (2010) |
| Whole flaxseed | Lignans were tolerant to high temperature. The treatments including the grinding, alkaline extraction, heating, and enzymatic hydrolysis, could improve the recovery rate of lignans. | Gerstenmeyer et al. (2013) |
In addition, ALA in flaxseed appears a relatively high storage stability. ALA content in flaxseed did not change when being stored in glass jars or sealed glass tubes under frozen conditions for around 9 months (Hall et al., 2006). Bechlin et al. (2019) found packaging could significantly affect lipid content for both varieties samples. Flaxseed stored by polyethylene-coated package showed better stability after 90 days of storage. Color values of brown flaxseed stored in polyethylene-coated paper package did not alter after being exposed to ozone gas for 120 min (Bechlin et al., 2019).
The hull of flaxseed can inhibit the contact between oxygen and ALA, and thus can protect ALA from being oxidized. However, after flaxseed hull is destroyed by crushing, grinding or milling, stability of ALA in flaxseed decreases due to higher absorption of oxygen. Many factors, such as size of milled flaxseed, storage temperature, storage time, packaging forms etc., can significantly affect ALA stability (Edel et al., 2015). Generally, ALA is more easily oxidized with the decrease of particle size of milled flaxseed. ALA in coarsely milled flaxseed can be unaffected (Edel et al., 2015; Hall et al., 2006). The studies regarding the stability of ALA and SDG in milled flaxseed is very limited. The scientists are more interested in the research of the stability of ALA and SDG in real food products. Schorno et al. (2010) studied storage stability of milled flaxseed stored in open aluminum pans. The results showed that milled flaxseed on top layer appeared more serious oxidation compared to that on the bottom layer after being stored for 84 days at 50 °C. ALA stability increased with the increase of flaxseed sizes. The authors suggested that milled flaxseed samples should be taken from the center section of samples when milled flaxseed samples were stored in open containers (Schorno et al., 2010). Based on above results, milled flaxseed should be stored in sealed plastic bags as massive quantities per unit.
Gerstenmeyer et al. (2013) investigated impact of baking conditions on lignan stability in milled flaxseed. The results showed that lignan content in milled flaxseed after being baked treatment at 250 °C for 3.5 min, was higher than that in untreated flaxseed. After milled flaxseed was heated, the porosity of flaxseed increased, and resulted in the increase of lignan recovery. Heating did not result in the formation of SDG. SDG recovery was highly depended on treatment methods such as alkaline and enzymatic extraction of lignan, grinding flaxseed etc. Shah et al. (2018) found that no significant changes of SDG concentration were between pasteurized and unpasteurized milled flaxseed.
Stability of ALA and SDG contained in flaxseed-based foods The stability of ALA or SDG in flaxseed based foods is highly related to the producing process and storage conditions. Many studies have indicated some processes such as screw-pressing, mechanical fractionation, and the addition of flaxseed forms can significantly influence the stability of ALA and SDG in the final products (Table 7).
| Food products | Results | References |
|---|---|---|
| Milled flaxseed in milk, yogurt, cheese | SDG exhibited high stability during the process of cheese making. | Hyvarinen et al. (2006b) |
| Milled flaxseed in biscuit | Baking process did not influence on the levels of SDG in biscuit samples. | Cukelj et al. (2017) |
| Flaxseed in oat bars | ALA in oat bars appeared oxidized during the investigated storage. Oat bars had better be stored at refrigerated conditions. | Mridula et al. (2013) |
| Roasted and ground flaxseed in cookies | No significant difference for storage properties between control and flaxseed cookie samples after being stored for 90 days, was found. | Ravjiv et al. (2012) |
| Whole flaxseed in muffins | PV for germinated flaxseed muffins showed a higher elevated rate compared to ungerminated flaxseed muffins at investigated conditions. | Kaur et al. (2020) |
| Roasted flaxseed in bread | PVs, FFAs and p-anisidine values in breads could only be kept stably up to 3∼5 days at investigated storage conditions. | Marpalle et al. (2015) |
| Flaxseed in breads | Higher substitution for wheat flour by flaxseed flour led to lower PV in flaxseed breads. | Pourabedin et al. (2017) |
| Milled flaxseed in spaghetti | Hydration/extrusion process could destroy ALA in milled flaxseed. ALA showed a high stability during the process of drying and cooking for spaghetti. | Manthey et al. (2002) |
| Flaxseed in liver sausages | TBARs in liver sausages with flaxseed increased during the inviestigated storage peirod. | Zajac et al. (2017) |
| Flaxseed in meat batter | TBARs continually increased during the storage period in obtained meat batter samples. | Yogesh et al. (2015) |
| Milled flaxseed in macaroni | ALA in dried pasta or macaroni exhibited highly stable during the storage period. Higher drying temperatures were more favour to remain ALA oxidation stability. | Hall et al. (2005) |
| Flaxseed chutney powder | A slight increase of fatty acids levels was observed in investigated samples during 6 months of storage. | Rao et al. (2013) |
ALA, alpha-linolenic acid; TBARs, Thiobarbituric acid reactive substances; PVs, Peroxide values; SDG, Secoisolariciresinol diglucoside
Stability of ALA in food products
Baked products
Breads ALA in flaxseed-based foods should be kept stably during processing and storage periods. Many favourable factors, such as product matrix, lignan content, Maillard reactions, carbon dioxide produced during fermentation etc., can keep ALA stability in flaxseed-based foods (Kasote, 2013; Mercier et al., 2014). Pourabedin et al. (2017) investigated effect of baking process on ALA stability in breads. The authors revealed higher replacement of wheat flour by flaxseed flour led to lower PVs in flaxseed breads. It was indicated that the phenolics and antioxidants in flaxseed flour might decrease ALA oxidation during baking process (Pourabedin et al., 2017). Marpalle et al. (2015) found that PV in breads contained 10% ground roasted flaxseed significantly increased from 6.6 to 13 meq O2/kg samples when being stored at 30 or 4 °C during five days storage period. Osuna et al. (2018) reported PV in flaxseed based bread could be significantly reduced through the addition of antioxidant compounds such as ascorbic acid or α-tocopherol.
Muffins Kaur et al. (2020) analyzed the changes of PVs in flaxseed muffins stored at room temperature improved faster compared to those of the controls under refrigerated conditions. PVs for germinated flaxseed muffins showed a higher growth rate in contrast with ungerminated flaxseed muffins during the investigated storage period (Kaur et al., 2020).
Cookies Ravjiv et al. (2012) investigated storage stability of flaxseed cookies, and found flaxseed cookies exhibited similar storage characteristics with controls after being stored after 90 days of storage. During the storage period, ALA in the control samples remained negligible levels, and ALA levels in flaxseed cookies samples were kept from 4.75∼5.31% (Ravjiv et al., 2012).
Biscuits Cukelj et al. (2017) reported that PV significantly increased in all investigated flaxseed biscuits during 180 days of storage. FFAs in flaxseed biscuits were two times higher that those in control samples after 180 days storage (Cukelj et al., 2017).
Extruded Products
Pasta The preparation process for pasta enriched with milled flaxseed includes hydration, extrusion, drying and cooking. Each step can affect ALA stability in pasta at different levels (Hall et al., 2006; Mercier et al., 2014). Manthey et al. (2008) reported that the presence of lipid oxidation was observed during the process of hydration and extrusion. Packaging conditions and storage time could significantly affect PVs. Lipid oxidation in fresh pasta could be minimized by vacuum packaging. Villeneuve et al. (2013) investigated impact of production process and simulated digestive process on the stability of ALA in flaxseed pasta. The results showed that ALA showed very highly stable during the process of flour mixing, dough extrusion, pasta drying and cooking. Flaxseed forms could significantly affect the release of ALA in pasta due to the different bound ability of ALA and pasta matrix in pasta samples.
Spaghetti Manthey et al. (2002) investigated effect of process conditions on ALA stability in flaxseed-enriched dried spaghetti. The results showed that that CDs of flaxseed-enriched dried spaghetti samples were higher than those of original semolina-flaxseed mixture. It meant that ALA was oxidized during the extrusion process. The authors considered that ALA oxidation was mainly occurred at hydration step.
Macaroni Hall et al. (2005) studied storage stability of ALA in flaxseed-fortified macaroni. The results showed that ALA in the investigated flaxseed macaroni exhibited a high stability during the employed storage periods Drying temperature could significantly affect ALA stability in flaxseed-fortified macaroni samples. CDs in macaroni dried at 90 or 40 °C significantly increased after 24, 36 weeks of storage, respectively. It is indicated that macaroni had better dry at low temperature in terms of keeping ALA stability (Hall et al., 2005).
Bars Mridula et al. (2013) investigated the storage stability of ALA in oat bars enriched with 15% flaxseed at 25 or 4 °C for 90 days. The results showed that storage temperatures did not significantly affect ALA stability in oat bars after the storage of one month. Afterwards, ALA in oat bars stored 25 °C showed a poorer stability than that stored 4 °C for 90 days. ALA levels in oat bars decreased by 10.3% at 25 °C and 4.5% at 4 °C after 90 d of storage, respectively. And, the degradation of ALA in oat bars was linearly positive correction with fatty acid contents. It is suggested that ALA in oat bars appeared oxidized during the investigated storage. Oat bars had better be preserved at refrigerated conditions (Mridula et al., 2013).
Meat products Yogesh et al. (2015) studied storage stability of ALA in raw and cooked meat batter for 8 days of storage at refrigerated condition. The results showed that higher TBARs value was found in meat batter samples with 5.0% flaxseed, and TBARs value continually increased due to the degradation of ALA. Zajac et al. (2017) also found TBARs value in liver sausages with 5.0% flaxseed was higher compared to those in flaxseed -free liver sausages. In addition, TBARs value in liver sausages with flaxseed increased during storage period (Zajac et al., 2017). Hautrive et al. (2019) investigated impact of fat substitution by flaxseed flour on oxidative properties of hamburgers. The substitution of pork fat with flaxseed flour could improve the development of lipid oxidation. The hamburger incorporated with flaxseed flour showed higher TBARs values due to high concentration of ALA in flaxseed compared to that incorporated with defatted flaxseed flour (Hautrive et al., 2019).
Flaxseed adjunct Chetana and Sunkireddy (2011) prepared chikki by 20% substitution of peanut with flaxseed flour, and found PVs of all chikki samples increased during investigated storage time. Addition of the antioxidants into chikki samples could improve storage stability of the products due to the inhibition degradation of ALA. Rao et al. (2013) investigated storage stability of flaxseed chutney powder under room temperature for 6 months. A slight increase of FFAs was found in investigated powder samples after 6 months of storage due to the oxidation of ALA presented in powder (Rao et al., 2013).
Stability of SDG in foods Many physical and chemical factors, such as processing conditions, storage time, storage temperature, package type etc., can affect the stability of SDG in foods (Mercier et al., 2014). Hyvarinen et al. (2006a) investigated impact of processing process and storage conditions on the stability of SDG in various bakery products enriched with flaxseed. The results showed SDG presented a high stability at high baking temperature (160∼225 °C), was relatively stable during the storage period. SDG content did not change after being stored in plastic bags at investigated temperatures for 2 months. Hyvarinen et al. (2006b) also investigated the stability of SDG in dairy-based products (milk, yogurt and cheese) enriched with milled flaxseed. The results showed that SDG can be kept stably during the fermention process. It is suggested that SDG could be preserved during cheese making process. Furthermore, the content of SDG in yogurt could be remained under frozen conditions for the storage of 21 days. However, the levels of SDG in whey-based drinks reduced at 8 °C after the storage of 6 months (Hyvarinen et al., 2006b). Cukelj et al. (2017) obtained functional biscuits with SDG concentration ranged from 101 to 117 mg/kg. SDG levels in biscuits samples were not affected by baking process. According to the above-mentioned studies, it is indicated that SDG showed the relatively high stability towards investigated baking temperatures and a variety of storage conditions (Edel et al., 2015).
Flaxseed as a whole or milled form has been consumed for many years. Increasing people are aware of the importance of flaxseed due to the presented bioactive compounds such as ALA, lignan, protein dietary fibre etc. Another aspect, growing evidence have indicated that whole or milled flaxseed is not only safe, but also can provide health benefits to our body. Therefore, whole/milled flaxseeds as one kind of dietary supplements are highly recommended. Compared to ALA in flaxseed oil, ALA in whole/milled flaxseeds shows higher stability. The stability of ALA or SDG in whole flaxseed-based foods is associated with many factors such as processing conditions, storage conditions, food forms etc. Researchers and food companies have developed many flaxseed-enriched products to satisfy consumers' demand. However, many challenges are facing during the development of flaxseed-based products. We should better understand impact of the incorporation of flaxseed on the macroscopic structure and mechanical characteristics of the products, and find the balance between nutrition and sensory value of products. What's more, more work regarding the stability and tissue bioavailability of ALA and SDG in flaxseed-based products need to be carried out in the future.
Ethics statements
This is a review paper, which does not include animal or human experiments.
Declarations of Competing interest
None.
Acknowledgments This work was supported by the High Level Major Research Achievements Training program of Tourism College of Zhejiang (No. 2019GCC004), Research Foundation of Education Department of Zhejiang (No. Y201941046), 2019 Employee Faculty Special Research Project of Tourism College of Zhejiang (No. 2019XJZD04) and the Project from Sichuan State Key Laboratory of Culinary Science (No. PRKX201917).
