Postharvest Technologies for Quality Maintenance of Sprouts
2022 Volume 10 Pages 239-256
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2022 Volume 10 Pages 239-256
Sprouts are of great interest to consumers owing to their easy growth, beneficial traits, and health-promoting compounds. However, maintaining freshness after harvest and improving the shelf life of sprouts is challenging because of their rapid deterioration rate. This is the result of several factors, including high respiration rate, rapid dehydration, discoloration, texture changes, and high susceptibility to several foodborne pathogens. Therefore, various decontamination and storage techniques have been used to maintain quality and eliminate pathogens during postharvest handling of sprouts. This review summarizes sprout quality attributes and their dynamic changes during storage. Additionally, approved postharvest technologies to maintain sprout quality, minimize microbial growth, and prolong shelf life are discussed. Finally, the need for further research to develop or modify postharvest technologies, which can ensure both the safety and quality of this popular vegetable, is considered.
Sprouts from various plant seeds have attracted overwhelming consumer interest recently owing to an increasing focus on healthy living and longevity. Sprouts are the product obtained from the germination of seeds and their development in water or other mediums. They are harvested before the development of true leaves and are intended to be eaten whole, including the seeds [1]. During the transformation process of sprouts, the bioactive phytochemical content, antioxidant capacity, and digestibility are increased beyond that of their seeds or mature counterparts [2]. Additionally, because of low costs and requirements of simple equipment and supplies, interest from sprout-producing industries and health-conscious consumers has increased rapidly.
Cultivated crops and species used for sprouting purposes belong mainly to the Poaceae (cereal crops), Brassicaceae (vegetables and herbs), and Fabaceae (legumes) families, which are also used in scientific research because they provide a plethora of information on their nutritional benefits. Sprouts are attractive owing to traits such as unique colors, intense smells, delicate textures, and variable tastes [2]. Among the Fabaceae family, bean sprouts, such as mung bean and soybean sprouts, are the most consumed. They are also used in Asian cuisine and vegetarian diets. Owing to their appealing nutty or bean-like flavor and crispy texture, bean sprouts have become a popular nourishment food in many Asian countries, including Japan, Korea, and China, over the years. Currently, alfalfa, buckwheat, red cabbage, and broccoli sprouts have also gained popularity and are consumed worldwide because of their beneficial health impacts [3].
Sprouts are mainly used for homemade preparations and in fresh food markets. They can be eaten raw as appetizers and salads as well as added to stir fries, sautéed vegetables, pastas, and even desserts. Moreover, the nutritional value and sensory properties of certain food products can be enhanced by the addition of sprout powder. For example, soybean sprouts are used as additives in antiaging and cosmetic whitening products [4], and the use of wheat sprouts in tortillas enhance shelf life and sensory attributes [5]. On the other hand, this type of processing might deteriorate the nutritional value of sprouts. However, a potential opportunity has been proposed for sprouts to be used as supplements in animal feed, which would then be transferred to humans [6].
Despite the increasing popularity of sprouts as a healthy food, their relatively high water content (up to 95%) and delicate texture make them susceptible to damage. Additionally, difficulties in storage management lead to rapid postharvest quality deterioration. For instance, sprouts easily dehydrate, have a high respiration rate, and rapidly lose certain nutrients after harvest [7, 8]. Diminished sprout quantity and quality also cause economic losses that subsequently limit market value.
There is always a demand among consumers for safe and high-quality food products that support health and longevity. Considering this fact, fundamental research on suitable handling practices and safety measures is required to extend shelf life and preserve sprout quality. Thus, this review provides a concise overview of postharvest quality aspects, emerging postharvest technologies with an emphasis on quality maintenance, and techniques for the shelf life extension of sprouts.
Postharvest quality attributes of fruits and vegetables are generally the result of chemical components or physical characteristics or a combination of these factors, including flavor (taste and aroma), appearance (color), texture, and nutritional compounds [9]. In the case of sprouts, its unique sensory attributes vary between species. For example, sprouts from Fabaceae family seeds, such as lentils, peas, beans, and some legumes, taste bitter because of their high polyphenol content [10]. In contrast, high amounts of glucosinolates give a spicy taste to Cruciferous family sprouts such as radish and broccoli sprouts [11, 12]. Color is another important external quality parameter that affects overall acceptability. A color change from green to yellow during postharvest storage is a major problem of green vegetables. The content of chlorophyll, a green-color-producing pigment, significantly decreases with storage time in broccoli and alfalfa sprouts and results in a yellow color [13, 14]. Several studies have been conducted on the nutritional properties of sprouts as a quality parameter. Contents of titratable acid, ascorbic acid, carotenoids, soluble sugar, and protein were usually tested to evaluate the storage quality of mung beans and Brussels sprouts [13, 15, 16]. These nutritional characteristics decrease over the storage period.
Edible sprouts are a good source of various phytonutrients and bioactive compounds. For instance, sulforaphane has been found in broccoli sprouts, isoflavone in soybean sprouts, resveratrol in pea sprouts, and glucoraphanin in rocket sprouts. High concentrations of these compounds contribute to antioxidant, antibacterial, and anticancer properties [17]. Biogenic amines such as histamine, tyramine, tryptamine, putrescine, cadaverine, and phenylethylamine are usually formed by decarboxylation of amino acids. They have psychoactive and vasoactive properties, which can cause toxicological effects on human health [18]. The presence of biogenic amines in food is considered as an indicator of bacterial spoilage or the safety of food because high concentrations of amines such as histamine, putrescine, and cadaverine are associated with microbial activity [18, 19, 20]. The relationship between intense microbial activity of Enterobacteriaceae and high accumulation level of biogenic amines has also been proposed in sprouts [19]. Biogenic amines are also endogenously produced during the germination process of sprouts [21]. The predominant increase in histamine concentration was found in lupin and fenugreek sprouts at 5 days of the germination process [22]. In lentils and radish sprouts, a high putrescine level was detected during germination [20]. Biogenic amines of sprouts were also monitored during storage conditions. For example, cadaverine and putrescine were found in broccoli sprouts during refrigerated storage [23]. The content of total phenols, flavonoids, isoflavone, glucosinolate and biogenic amine and the accumulation of these compounds are also important as quality indices for sprouts. Therefore, it is necessary to evaluate changes in extrinsic and intrinsic properties during the postharvest storage of sprouts to optimize their quality and palatability.
Sprouts are characterized as highly perishable produce owing to their high respiration rate, high moisture content, high sensitivity to physical injury, and availability of nutrients for pathogenic organisms. Generally, the degree of degradation or perishability of harvested produce is proportional to its respiration rate. Sprouts that are stored at high temperatures exhibit higher respiration rates, which in turn result in decreased weight and nutritional quality and color changes [7, 16, 24]. Additionally, sprouts have a fine texture and delicate structure that can easily deteriorate during harvesting and subsequent storage and transportation. Rotting caused by bacteria or fungal attacks may also occur due to improper handling practices, which could result in food loss and pose biohazard risks to human health [25].
3.1 BrowningBrowning of sprouts is a major problem during postharvest handling that adversely affects consumer acceptance. This undesirable event is especially relevant for sprouts as they are rich in polyphenols and are therefore highly susceptible to enzymatic browning. Polyphenol oxidase (PPO) is the key enzyme related to browning. Phenylalanine ammonia-lyase (PAL) is another enzyme that participates in the browning reaction by synthesizing phenolic compounds during storage. Generally, browning appears when subcellular de-compartmentation is occurred by stress or injury which leads to the contact between PPO enzymes and phenolic compounds. Fresh sprouts can be easily turned brown due to microbial growth, cold stress, hot air drying, and physical injury such as cutting [26, 27, 28, 29]. Figure 1 shows a generalized scheme for enzymatic browning in sprouts since they have a similar browning mechanism to that of fresh-cut produce [27]. According to the figure, postharvest stress causes damage to the cell membrane system and induces PAL activity by signaling molecules [30]. This induction of PAL activity results in the biosynthesis of phenolic compounds, the substrates of browning reaction. It is reported that phenolic compounds (synthesized by PAL) are stored in vacuoles. When cell compartmentation is gradually disrupted, phenolic compounds consequently leak out from vacuole to cytosol. The eluted phenolics then interact with active PPO that present in cytosol, and ultimately leading to the form of quinone (browning compound) [28]. Quinone can undergo further non-enzymatic polymerization to form brown pigments that eventually appeared as tissue browning [27]. In mung bean sprouts, flavonoids were identified as the main substrate for PPO activity, leading to browning [24]. Biochemical characteristics of PPOs from stored lentil sprouts were analyzed by Sikora et al. [31] who found that the optimum pH for PPO was 4.5–5.5 and optimal temperature was 35 °C. They also reported that catechol was the suitable substrate for lentil sprout PPO. Therefore, characterization of PPO in different sprouts during postharvest storage is important to provide information for developing novel methods to effectively control browning.
Figure 1: Schematic diagram of enzymatic browning mechanism in sprouts during postharvest stress
Softening, or texture degradation, is a quality disorder that limits the shelf life of sprouts and occurs during the process of postharvest senescence. Several factors, such as weight (moisture) loss, cell wall modification, and heat treatment, result in softer sprouts. It has been reported that the decrease in Brussels sprout firmness is inversely correlated with the increase in weight loss [32]. The reduced firmness of Brussels sprouts was also detected after immersion in 100 °C water for 4 min [33]. Loss of cell-cell connection in structural molecules is easily induced by heating, and heat treatment increases membrane destruction and subsequent leakage, leading to reduced firmness [34]. Polygalacturonase (PG) and pectin methylesterase (PME) are the major cell-wall-modifying enzymes in plants [35]. However, the underlying mechanisms of cell wall changes that lead to softening of sprouts are complex and remain unknown. In a previous study, softening events of mung bean sprouts during storage were investigated transcriptionally, and the results revealed an increased expression of the pectin-degrading genes PG and PME [15]. Additionally, the texture of mung bean sprouts can be maintained by stabilizing the structure and content of pectin [15]. Another study on mung bean sprouts suggested that cell wall components play a role in changes in firmness. Increasing in the formation of cell wall components, including lignin, cellulose, and hemicellulose, which support elongation of mung bean sprouts during storage, is responsible for these textural changes [24].
3.3 Weight lossWeight loss is the preliminary indicator of quality deterioration of fresh produce after harvest, and higher weight loss of sprouts during storage has been described in previous studies [15, 24, 36, 37]. These phenomena are primarily the result of water loss caused by transpiration and respiration processes. A previous study of mung bean sprouts found that during postharvest storage, weight loss consistently increased from 0% (after harvest) to 27.3% (at 3 days after storage) together with an increase in respiration rate [15]. A progressive weight loss was also observed in other sprouts such as garlic sprouts, artichoke sprouts, and broccoli sprouts with extending the period of storage. The increased rate of dehydration and respiration was considered as a reason for their weight loss incidences [16, 36, 38].
3.4 Microbe attackSprouts contain a large amount of water and are rich in available nutrients. These factors provide favorable conditions that promote the rapid growth and survival of foodborne pathogens on sprouts [25]. This quality was assessed by monitoring the microbial load of mesophilic bacteria, total coliforms, yeasts, and molds and by detecting the presence of Salmonella [23]. A survey conducted on microbial contamination of sprouts found mesophilic (7.9 log CFU g−1), psychrotrophic (7.3 log CFU g−1), and Enterobacteriaceae (7.2 log CFU g−1) microorganisms [39]. After harvest, sprouts can also be contaminated, or the growth of pathogens can increase depending on storage temperature, relative humidity (RH), and the gas composition of the storage environment. Sprouts infected by certain pathogens with an increased log concentration in different conditions throughout the storage period are summarized in Table 1. An increase in pathogens like Salmonella spp., Escherichia coli O157:H7, and Listeria monocytogenes, are responsible for sprout-associated foodborne illnesses worldwide [40]. However, microbiological threats can be minimized through good agricultural practices (GAP) during sprouting as well as through good handling practices during harvest and postharvest processing [2].
| Sprouts | Storage conditions | Pathogens | log CFU g-1 | Ref. |
|---|---|---|---|---|
| Alfalfa | Refrigeration storage | Aerobic plate count, and coliforms | 7.17–7.61, 5.52–6.65 | [41] |
| 4 °C, 8 days in perforated packaging | Aerobic plate count, and molds and yeasts | 5.71, 4.19 | [42] | |
| 4 °C, 8 days in MAP | 7.32, 4.37 | |||
| 4 °C, 8 days in vacuum packaging | 5.68, 5.67 | |||
| Broccoli | 4–6 °C, 6 days | Aerobic bacteria, and yeasts and molds | 7.3, 8.05 | [43] |
| 4 °C, 12 days | Mesophilic bacteria, total coliforms, and yeasts and molds | 9.95, 9.95, 8.94 | [23] | |
| 10 °C,14 days | Enterobacteriaceae, aerobic mesophilic bacteria, aerobic psychrotrophic bacteria, and molds and yeasts | 9.60, 10.04, 10.19, 8.59 | [44] | |
| 5 °C,14 days | Enterobacteriaceae, aerobic mesophilic bacteria, aerobic psychrotrophic bacteria, and molds and yeasts | 9.15, 10.06, 9.58, 8.47 | ||
| Chickpea | 8 °C, 12 days | Aerobic plate count, coliform count, Staphylococci, and yeasts and molds | 8.9, 8.1, 4.9, 5.4 | [45] |
| Dew gram | 8 °C, 8 days | 8.9, 8.6, 4.6, 4.3 | ||
| Lotus | 4 °C, 8 days | E. coli O157:H7, Salmonella typhimurium, and L. monocytogenes | >10 | [26] |
| Mung bean | 4 °C, 8 days | Total plate count, lactic acid bacteria, yeasts and molds, total coliforms, and E. coli | 9.07, 7.93, 1.94,7.95, <1.00 | [46] |
| Rapeseed | Refrigeration storage | Aerobic plate count, and coliforms | 7.33–8.28, 6.44–6.99 | [41] |
| Radish | 10 °C,14 days | Enterobacteriaceae, aerobic mesophilic bacteria, aerobic psychrotrophic bacteria, and molds and yeasts | 9.47, 10.10, 10.23, 8.75 | [44] |
| 5 °C,14 days | Enterobacteriaceae, aerobic mesophilic bacteria, aerobic psychrotrophic bacteria, and molds and yeasts | 9.63, 9.93, 9.28, 8.60 | ||
| Soybean | 20 °C, 3 days | Aerobic bacteria | 9.78 | [47] |
| 4 °C, 5 days | Aerobic bacteria | 8.89 |
Sprouts are a rich source of various bioactive compounds. However, storage condition has a considerable effect on endogenous bioactive compounds of sprouts. For instance, refrigerated storage for more than 7 days revealed a significant decline in glucosinolate concentrations in Brassica and rocket sprouts [11, 23]. Level of toxic biogenic amines such as histamine, tyramine, tryptamine, putrescine, and cadaverine can be induced during sprouting time. Factors influencing the biogenic amine production in sprouts include the presence of microorganisms that can decarboxylate amino acids, and the favorable conditions of such microorganisms for the growth and production of their enzymes [18, 20]. The presence of individual amines during the germination of some leguminous seeds is well established. However, the profiling and quantitative evaluation of biogenic amines and their changes in stored sprouts are very limited. Vale et al. [23] only investigated the amount of biogenic amine content in four varieties of Brassica sprouts and indicated that increase in amine content was the result of microbial activity during storage. A comparative analysis of biogenic amines in sprouts stored under refrigeration and at room temperature has not been performed yet in any study. Since high content of biogenic amines can cause hazardous effects on human health, it is necessary to ensure the safety of sprouts. Therefore, it would be interesting to determine the influence of different storage conditions on the individual and total biogenic amine content of sprouts and their level of acceptance.
Sprouts are characterized by limited shelf life and are highly sensitive to harvest and postharvest handling practices. To prevent the occurrence of microbiological contamination of sprouts, the first step is to follow safety measures, including GAPs, and to carefully harvest using hygienic procedures and tools. All the strategies should be implemented, beginning with seed treatment, as the transmission pathway of contamination begins with seeds and spreads to the sprouts [48]. Extensive literature indicating the sources of contamination during preharvest and postharvest has been reviewed by Yang et al. [25] and Sikin et al. [49], and the effect of current measures on seed and sprout disinfection, including physical, biological, and chemical applications, were also examined.
Physical intervention methods include temperature control, light exposure, gamma irradiation, and treatment with ultrasound, plasma, or high hydrostatic pressure. Biological interventions are based on antagonistic microorganisms and antimicrobial metabolites. Chemical interventions include a wide range of chemical disinfectants and sanitizers, such as ozone and chlorine, as well as electrolyzed water. However, although various studies have focused only on the elimination of pathogen growth on sprouts, their consequences after harvest on sprout nutritional and sensory properties are limited. This review therefore presents information on postharvest interventions for ensuring microbial quality control in sprouts (Table 2).
The application of chlorine at 50–200 ppm concentration is widely used for postharvest disinfection. Chlorous acid, chlorine dioxide, and sodium chlorite are commonly applied to fresh postharvest produce. These substances provide outstanding disinfection effect and reduction of Salmonella spp., L. monocytogenes, S. typhimurium, and E. coli, without changes in visual quality parameters [50, 51, 52]. However, the primary obstacle to disinfecting sprouts using these substances is the inability of the antimicrobial solution to reach the location of the pathogen. For instance, in a previous study, active chlorine, even at 20,000 ppm concentration, failed to inactivate Salmonella in cowpea sprouts, primarily because of a lack of chlorine penetration into sprout tissues where many Salmonella were located [53]. Because chlorine-based sanitizers naturally produce polluting and carcinogenic compounds [25], highly concentrated chlorine may have undesirable effects on sprout quality and human health. In this regard, organic acids such as malic acid, lactic acid, and acetic acid have been introduced as alternatives, which have much greater potential for ensuring the quality and safety of sprouts [26, 54].
Typically, the so-called Hurdle technology (combination of preservation methods) can control microbial growth in sprouts, resulting in an extended shelf life. For example, a combined treatment of malic acid and ozone was more effective in inactivating Shigella flexneri with no adverse effects on the quality of radish and mung bean sprouts than a single disinfectant treatment [54]. However, treatment timing is a critical factor related to the elimination or elevation of microorganisms while using ozone. For instance, a 2 min treatment using water-containing ozone showed promising results when applied to alfalfa sprouts for controlling L. monocytogenes. However, a 5–20 min treatment resulted in the deterioration of the sensory quality of sprouts during subsequent storage at 4 °C for 7–11 days [55]. In the future, more factors, including effects on sprout quality, treatment time, and concentration, should therefore be considered to optimize sanitizer delivery to sprouts.
Emerging physical techniques, including electron beam or gamma ray irradiation, as well as combinations with ultrasound, blanching, and ascorbate dip, have proven to be potentially useful for ensuring the hygiene and safety of mung beans, chickpeas, and lucerne sprouts, along with extending their shelf lives [56]. Ultraviolet (UV) treatment is another effective physical treatment that is now exploited in the food industry as it does not leave any residues. However, owing to its low penetration power, it is commonly used only for surface decontamination of food material [57]. In a previous study, when UV treatment was used with a sanitizing mixture of aqueous chlorine dioxide and fumaric acid, its decontamination efficiency for inactivating pathogens on sprouts was significantly improved [58].
| Postharvest interventions | Sprouts | Experimental conditions | Storage conditions | Name of pathogens and reduction levels (log CFU g-1) | Effect on postharvest quality | Ref. |
|---|---|---|---|---|---|---|
| Chemical | Mung Bean | Chlorous acid (268 ppm) for 10 min at 22 °C | 4 °C for 9 days | S. typhimurium and L. monocytogenes (not detected) | No changes in visual quality (observation) | [50] |
| Mung bean | Organic acids (1% concentration, pH 1.52) | - | Total aerobic microorganism (3.4), and Enterobacteriaceae (6.3) | Caused degradation, color change, and unpleasant odor (observation) | [59] | |
| Lotus | Lactic acid (2%) | 4 °C | E. coli O157:H7, S. typhimurium, and L. monocytogenes (2.3) | Improved the color and safety | [26] | |
| Physical | Broccoli | Far-red light (730 nm) | 5 °C for 15 days | Psychrophilic bacteria (8.1), enterobacteria (5.8), mesophilic bacteria (8.0), and molds and yeasts (5.3) | Better quality | [60] |
| Chickpea, and dew gram | Radiation (1 and 2 kGy) | 8 °C for 16 days | Coliform count (2 and 4), yeasts and molds (1 and 1.5), and Staphylococci (1 and 1.5) | No negative effects on sensory and nutritional qualities | [45] | |
| Mung bean, dew gram, chickpea, and garden pea | Radiation (2-kGy) | 4 and 8 °C up to 12 days | Complete elimination of S. typhimurium (104), and L. monocytogenes (103) | No negative effects on texture, nutritional, and organoleptic qualities (unpublished data) | [61] | |
| Mung bean | Plasma-activated water for 30 min | 4 °C, 6 days | Total aerobic bacteria (2.32), and total yeasts and molds (2.84) | No significant changes in the total phenolic and flavonoid contents, and the sensory characteristics | [62] | |
| Hurdle | Radish, and mung bean | Malic acid (2%) + Ozone (2 ppm) | 28 °C for 10 days | Shigella spp. (4.4 and 4.8) | No significant changes in physicochemical properties | [54] |
| Broccoli | Lactic acid (2%, v/v) + sodium hypochlorite (4 mg/L) | 4–6 °C for 6 days | Listeria innocua (1.19) | No negative effects on the storage quality | [43] | |
| Mung Bean | Chlorine Dioxide (100 ppm) for 5 min at room temperature + MAP (CO2 gas packaging) | 5±2 °C for 7 days | S. typhimurium (>2), and L. monocytogenes (2.9) | Maintained quality and extend the shelf life | [51] | |
| Alfalfa | Chlorine (200 ppm) + Perforated packaging | 4 °C for 8 days | Total coliforms (5.7) | No changes in visual quality | [42] | |
| Bean | Sodium hypochlorite (100 mg/L) + phytic acid as secondary wash | 10 °C for 4 days | E. coli O157:H7 (4), and L. monocytogenes (4) | Retained the color of bean sprouts | [52] | |
| Soybean | Slightly acidic electrolyzed water + Fumaric acid + Ultrasound at 40 °C | 4 °C for 7 days | L. monocytogenes (4), and E. coli O157:H7 (4) | Maintained good quality until the end of storage with slight deterioration due to ultrasound usage. | [63] | |
| Mung bean and chickpea | Ultrasonication (4–10 min; 40–50 °C) + blanching (50–70 °C for 4–10 min using potable water) + ascorbate dip (0.25%, 5% and 1% up to 10 min at 4±1 °C) + gamma irradiation (1–2.5 kGy) | 4±1 °C for 35 days | Total aerobic plate count, yeast and mold count, and Staphylococcus (not detected (< 10)) | Retained nutritional, physico-chemical, and sensory attributes | [56] | |
| Lucerne | 4±1 °C for 21 days |
Short shelf life and susceptibility to spoilage are the key factors affecting sprout quality. Spoilage occurs sooner and more frequently in sprouts when proper storage is not provided by distributors or retailers. Table 3 provides a summary of implications of existing technologies on the quality and shelf life of sprouts.
4.2.1 Cold storageGenerally, low temperature storage can minimize postharvest quality degradation and extend shelf life by slowing the rate of respiration, senescence, and spoilage growth [64]. However, the selection of optimum storage temperatures broadly depends on respiration rates and organoleptic qualities. For example, in radish sprouts (without radicle), visual quality was better maintained when stored at 4 °C because of the lower constant respiration rate [65]. In contrast, the membrane structure of mung bean sprouts was heavily disrupted during 4 °C storage, which allowed eluted phenolics and oxidative enzymes to move from vacuoles to cytosol, eventually resulting in browning [28]. Thus, it has been a long-term challenge to select appropriate temperatures to extend the shelf life of sprouts while simultaneously overcoming these limitations.
The nutritional quality of bioactive phytochemical compounds was also better maintained when sprouts were stored at cold temperatures. For instance, the isoflavone content of 4-day-old soybean sprouts after cold storage (4 °C) lasting a week was generally equal or higher than that of fresh sprouts [66]. Additionally, storage temperature of 4–5 °C has been recommended to avoid extreme losses of bioactive compounds, and to maintain shelf life up to 14 days in broccoli and radish sprouts [44]. Genotypic variation and time of harvesting may also influence the accumulation of bioactive composition in sprouts. Total glucosinolate level (GLs) in 9-day-old Brassica oleracea sprouts (cultivars include red cabbage, broccoli, Galega kale, Penca cabbage, etc.) showed a significant reduction over 12 days when stored at 4 °C [23]. However, Force et al. [11] did not find any significant loss of GLs in 7-day-old sprouts of broccoli, kohl rabi, white radish, and rocket under the same conditions (4 °C for 3 weeks).
4.2.2 Modified atmosphere packagingModified atmosphere packaging (MAP) is one of the most effective technologies which can provide good quality and longer shelf life of vegetable crops by decreasing oxygen (O2) and increasing carbon dioxide (CO2) partial pressures in package headspace [67]. However, improper packaging may be ineffective and reduces the produce quality in terms of shelf life and visual quality. The design of a successful MAP system depends on the product weight and respiration rate, atmospheric composition, appropriate packaging film and its permeability, and storage temperature [68]. Packaging materials such as polypropylene, low-density polyethylene, and polyethylene have proven successful for extending the shelf life of sprouts. However, sprouted vegetables benefit more when they are stored in packaging film combined with cold temperature near their genotypic chilling tolerance. Kou et al. [69] observed that buckwheat sprouts (without radicle) stored in MAP at 5 and 10 °C had smaller microbial populations and less tissue electrolyte leakage than those stored at 15 and 20 °C. The balance between O2 and CO2 in packaging film is also critical for MAP storage, and an optimal ratio is required for each specific sprout. For instance, low O2 levels in package headspace may result in anaerobic conditions which may encourage the formation of ethanol and acetaldehyde which is responsible for off-odor development, and high CO2 levels cause irreversible membrane damage [70]. In these cases, high oxygen transmission rate (OTR) film has shown an amazing capacity to retard off-odor development by achieving desired CO2 and O2 equilibrium conditions inside packages of radish sprouts (without radicle) [64]. MAP combined with both active and passive packaging is useful to extend shelf life and preserve quality of postharvest fresh produces. A recent study found active packaging at 0 °C, with gas concentrations of 15% CO2 and 7.5% O2, was an optimum solution for extending shelf life based on the lowest count of aerobic mesophilic bacteria, prevention of off-flavor development, and discoloration of sprouts [38]. However, there still remain insufficient published studies on the use of optimum active packaging technologies and their impacts on biochemical composition, another important quality trait of sprouts.
4.2.3 Light exposure and irradiationPrevious research of strategies using postharvest exposure to visible spectrum lighting in young plants during their storage period is quite limited. Hasperué et al. [16] evaluated the effect of white-blue light-emitting diode (LED) on Brussels sprouts at room temperature, and they found that light-treated sprouts had decreased respiration rates after 0 to 5 days of storage, and maintained the lower rate, as well as the green color in tissues until 10 days. Light treatment preserved higher levels of glucosinolates and phenolic content in broccoli sprouts, whereas yellow LED light showed better performance than green and white LED light. This phenomenon was also demonstrated in cases of UV exposure, where moderate UV exposure showed potential benefits to the biosynthesis of health-promoting bioactive compounds. For instance, when harvested broccoli and radish sprouts were exposed to 15 Wm−2 UV-B and 9 Wm−2 UV-C individually, the UV-B exposed produce showed the highest total phenolic content and total antioxidant capacity after 10 days of storage. UV-B also increased glucosinolate content >30% [71]. Conversely, continuous light exposure during postharvest storage had a negative effect on the sensory quality and amount of bioactive compounds found in radish sprouts (without radicle) by increasing the O2 level in the package headspace [72]. Thus, postharvest light conditions (spectral composition and intensities) need to be carefully considered in order to maintain desired concentrations of bioactive compounds and consumer acceptability.
Light plays an important role in elucidating the molecular mechanism of secondary metabolites biosynthesis in sprouts. Lim et al. [73] specifically found the upregulation of chalcone synthase-encoding genes (CHS6, CHS7, and CHS8) in response to UV-B irradiation during the sprouting time of soybean sprouts. Involvement of light in defense mechanisms of soybean sprouts was previously studied by Dhakal et al. [74]. They suggested that light treatment increases the resistance against rotting disease caused by Pseudomonas putida 229. This developed resistance was observed by the accumulation level of isochorismate synthase and pathogenesis-related 1 gene expression that regulated the pathway of salicylic acid and jasmonic acid biosynthesis. However, further research is needed to understand the biosynthetic pathway and stress tolerance using genetic approaches during postharvest storage of sprouts.
Irradiation is another very effective method for reducing foodborne pathogen in sprouts. However, very few studies have found irradiation effective in maintaining the sensory and nutritional qualities of sprouts. In one case, gamma irradiation, even at low doses, appeared to be a useful technique for preserving the visual quality of garlic sprouts and for delaying the oxidation of total phenols, total ascorbic acid, and chlorophyll degradation during storage [36]. In contrast, UV-irradiation, another radiation source, did not have any impact on sprout quality and shelf life [75].
4.2.4 High pressure processingHigh pressure processing (HPP), as a novel and non-thermal physical processing technology, has shown significant potential to preserve bioactive substances in sprouts. In general, the conversion of predominant glucosinolates to isothiocyanates and sulforaphane is essential to enhance their bioactivity. HPP treatment at 600 MPa exhibited the highest conversion (85%) of this beneficial compound in broccoli sprouts stored at 6 °C for 12 days after undergoing pressurization [76]. HPP technology is frequently utilized to reduce microbial growth on sprouts. In practice, pressures up to 700 MPa, and treatment times varying from a few seconds to several minutes are used to inactivate microbial cells [77]. However, there are no prior reports on the effects of HPP on sensory quality and shelf life of sprouts in combination with inactivation of pathogenic and spoilage microorganisms. Additionally, HPP at 300 to 700 MPa has some serious limitations, such as the difficulty of controlling and managing such high operating pressures, high equipment cost, and safety [49].
4.2.5 Chemical treatmentSome preharvest applications, such as chitosan and calcium treatments, have shown promising results and increased defense functions against oxidative stress during the storage of sprouts. In broccoli sprouts, a 10 mM CaSO4 spray application dramatically enhanced antioxidant enzyme activities including superoxide dismutase, peroxidase, catalase, and glutathione peroxidase, and increased antioxidant capacity including phenolic compounds and ascorbic acid. Moreover, this treatment also led to an increase in glucosinolate content, especially glucoraphanin, during growth, and prevented its loss during storage [78]. Chitosan is characterized as a biodegradable and biocompatible polysaccharide extracted from natural resources, and is an alternative coating material for a wide range of food products. Supapvanich et al. [79] observed improved antioxidant and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities, and increased total phenols, flavonoids, and ascorbic acid contents, in chitosan-treated sunflower sprouts. The effect of chitosan treatment on transcription level of genes such as PAL, CHS, and isoflavone synthase which involved in phenylpropanoid pathway has been uncovered in soybean sprouts [80]. Chen et al. [80] concluded that chitosan treatment was ineffective for increasing isoflavone content in soybean sprouts because isoflavone biosynthetic gene expression level negatively responded to chitosan treatment in the high-isoflavone cultivar. However, a postharvest dip treatment using non-toxic adenosine triphosphate (ATP) and ascorbic acid was established as a potential inhibitor of PPO enzyme activity. Moreover, this did not adversely affect the taste and odor of stored mung bean sprouts [15, 81]. Nevertheless, postharvest dip/wash and drying processes profoundly reduced shelf life of these delicate vegetables due to mechanical damage [82].
| Postharvest technologies | Sprouts | Treatment conditions | Shelf life | Outcome | Ref. |
|---|---|---|---|---|---|
| Cold storage | Radish (without radicles) | 4 °C | 3 weeks | Maintained constant respiration rate and visual quality | [65] |
| 10 °C | 14 days | ||||
| Soybean | 4 °C | 4 days | Minimized postharvest morphological degradation | [83] | |
| Mung bean | 1 °C | 4 days | Hypocotyls remained whiter | [84] | |
| 6 °C | Cotyledon color was better | ||||
| Soybean | 4 °C | 7 days | Increased isoflavone content and reducing power | [66] | |
| Green pea, lentil, and mung bean | 4 °C | 7 days | Elevated starch digestibility and antioxidant activities | [85] | |
| Broccoli and radish | 5 °C | 14 days | Maintained nutritional quality and acceptability for consumers | [44] | |
| Broccoli, kohl rabi, white radish, and rocket | 4 °C | 3 weeks | Maintained the glucosinolate level | [11] | |
| MAP | Buckwheat (without radicle) | 16.6 pmol m-2 s-1 Pa-1 OTR film at 5 °C | 21 days | Freshest appearance with lowest tissue electrolyte leakage | [69] |
| Artichoke | 15% CO2 + 7.5% O2 at 0 °C±2 °C | 18 days | Avoided the development of off-flavor, prevented discoloration in sprouts, and gave a good appearance | [38] | |
| Quinoa | 5% O₂ + 20% CO₂ at 5 °C | 7 days | Helped to retain texture profile and odor | [37] | |
| Brussels | Polyvinylchloride film at 0 °C | 42 days | Minimized browning and losses in weight and firmness, maintained ascorbic acid and total flavonoid content while increasing the radical-scavenging activity | [32] | |
| Chickpea | Perforated polypropylene and low-density polyethylene at 10 °C | 7 days | Maintained color quality and removed off-odor | [86] | |
| Mung bean | Moderate vacuum packing (↓0.04 mPa) at 4 °C | 7 days | Restricted sprout elongation and maintained the firmness, slowing down the browning | [24] | |
| Radish (without radicle) | 29.5 pmol m-2 s-1 Pa-1 OTR packaging at 1 °C | 28 days | Lowest tissue electrolyte leakage, aerobic mesophilic bacteria, yeast and mold count, and off-odor score | [64] | |
| Light exposure and irradiation | Brussels | White-blue LED at 22 °C | 10 days | Lower respiration rate, remained greener, better visual quality | [16] |
| Broccoli | Far-red LED (730 nm) at 5 °C | 15 days | Increased the biosynthesis of phenolic compounds | [60] | |
| Broccoli | Continuous 35 µmol m−2 s−1 yellow illumination at 5 °C | 15 days | Enhanced and maintained the biosynthesis of glucosinolates and phenolic compounds | [87] | |
| Broccoli | 50% of 15 kJ m−2 UV-B was applied after harvest and on the first day of storage at 4 °C | 10 days | Highest total phenolic content and total antioxidant capacity, enhanced the glucosinolate content | [71] | |
| Garlic | Gamma irradiation (1.25 kGy) at 3±1 °C, 85% RH | 15 days | Maintained the external appearance, texture, and appeal, enhanced the content of total phenols, and also delayed the oxidation of total phenols, total ascorbic acid, and chlorophyll degradation | [36] | |
| Alfalfa | Gamma irradiation (1.71 and 2.57 kGy) at 5 °C, and then stored at 6 °C | 7 days | Increased carotenoid content compared to non-irradiated sprouts | [14] | |
| High pressure processing | Broccoli | 400−600 MPa for 3 min at 30 °C and stored at 6 °C | 12 days | Increased isothiocyanate formation | [76] |
| Brussels | 200 MPa for 3 min at 5 °C and stored at 4±2 °C | 4 days | Increased the essential amino acids (isoleucine, leucine, lysine, phenylalanine, and threonine) in lower amount | [88] | |
| Chemical treatment | Mung bean | 1mM ATP solution for 5 min, and stored at 20 °C | 3 days | Maintained the quality, delayed browning, and softening | [15] |
| Mung bean | 20 mM ascorbic acid for 2 h, and stored at 4 °C | 7 days | Reduced enzymatic browning of sprouts via PPO inhibition | [81] | |
| Mung bean | Ethanol vapor for 1 h and stored at 7±1 °C | 120 h | Suppressed the sprout length and weight, reduced the non-enzymatic browning, and maintained overall acceptability | [75] | |
| Broccoli | 10 mL of 10 mM CaSO4 every 12 h of cultivation period and after harvest, stored at 4 °C | 15 days | Enhanced antioxidant activities | [78] | |
| Sunflower | Before harvest, watered with 500 mL of 1% chitosan solution and after harvest, stored at 4±1 °C | 9 days | Induced ferric reducing antioxidant potential, DPPH free radical scavenging activity, and all bioactive compounds | [79] |
Postharvest deterioration of sprouts occurs sooner and more frequently due to its perishable nature and lack of postharvest handling technologies. Therefore, a balanced approach incorporating sanitation, temperature, moisture, and atmosphere should be implemented to slow the growth of spoilage microbes, optimize quality retention, and extend shelf life of sprouts. In this review, postharvest physical, biological, and nutritional quality characteristics and the technologies applied to sprouts according to their characteristics was extensively explored. Appropriate postharvest handling practices have shown great potential to maintain the key quality characteristics of sprouts, such as color, flavor, texture, and nutrients. However, pre- and postharvest chemical treatment, drying techniques after washing, photoperiodic intensity exposure, atmospheric composition, and harvest timing which impact quality and shelf life of sprouts, all demand further research. Additionally, the application of some novel and non-thermal techniques, such as ultrasound and cold plasma, on sprouts should be investigated.
Overall, this discussion recommended the following: (1) Harvesting of sprouts should be done early before they reach their maturity stage thereby increasing their shelf life in correspondence with optimized visual quality and bioactive compound contents. (2) Combinations of chemical, physical, and novel technologies could be utilized as effective alternatives to eliminate pathogenic microorganisms during postharvest handling, and (3) During storage and transportation, a combination of MAP (high OTR film packaging) with low temperatures (4 °C) could be applied to reduce respiration rates, and increase shelf life without compromising nutritional value and consumer acceptance.
To ensure high quality of sprouts, postharvest handling practices should be applied soon after harvest, and fresh consumption is necessary to minimize contamination and deterioration. This manuscript provides an optimistic view for researchers in order to modify existing postharvest preservation methods, which in turn can reduce contamination, and maintain the sensorial and nutritional quality of this popular vegetable category.
