Technical papers
Effects of the combination of phytic acid and vacuum packaging on storage quality of fresh-cut lettuce
2022 Volume 28 Issue 1 Pages 45-52
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2022 Volume 28 Issue 1 Pages 45-52
Fresh-cut lettuce is one of the most popular vegetables for healthy consumption. However, fresh-cut lettuce would spoil rapidly due to tissue damage. In order to improve the quality and extend the storage period of fresh-cut lettuce, this experiment investigated the effect of phytic acid (PA), vacuum packaging (VP), and combined treatment. The results showed that the PA + VP group inhibited the microbial growth and browning reaction of fresh-cut lettuce remarkably, and reduced the frangibility and the weight loss. The malondialdehyde (MDA), peroxidase (POD), and polyphenol oxidase (PPO) levels were decreased in PA + VP group. The combination of PA and VP can significantly inhibit the decline of total sugars, phenols, and vitamin C contents. Collectively, this study concluded that the combination of PA and VP was more effective in maintaining fresh-cut lettuce than alone treatment.
Lettuce (Lactuca sativa L.), being cultivated all year round worldwide, is one of the most popular leafy vegetables consumed either fresh or in salad mixes (Nicolle et al., 2004). Lettuce is rich in vitamins, minerals, and other phytochemicals essential to people's health, such as phenolic compounds and dietary fiber (Llorach et al., 2008). However, the damage of lettuce tissues would accelerate the quality defects such as enzymatic browning and microbial infection (Altunkaya and Gökmen, 2004; Złotek and Gawlik-Dziki, 2015). Enzymatic browning is the main problem in minimal processing and further storage of leafy vegetables leading to a shorter shelf-life (Chisari et al., 2010).
The common preservation methods of fresh-cut vegetables are physical preservation such as low temperature treatment, radiation treatment, ultra-high pressure treatment, vacuum treatment, and chemical preservation such as coating treatment and preservative treatment (Rico et al., 2007). Vegetables slathered with chemical preservatives would cause allergic reactions with cancerogenic potential in sensitive individuals (Allende et al., 2006). Therefore, natural preservatives are preferred by consumers and industry for preserving fresh vegetables and treating minimally processed ones. Phytic acid (PA), abundant in edible legumes, cereals, oilseeds, and other plants, is a secondary metabolite highly phosphorylated (Cheryan and Rackis, 1980), which is also applicable in the food industry as a natural preservative to inhibit the oxidation, browning, and senescence of fresh-cut vegetables (Fox and Eberl., 2002).
PA is an anti-browning and anti-senescence agent for many fresh-cut vegetables but has never been used for fresh-cut lettuce. Focusing on the physiological response to the combination treatment of exogenous Pa and vacuum packaging (VP) of fresh-cut lettuce (Lactuca sativa L. var. angustana Irish), this study investigated the changes in microbial growth, hardness, frangibility, weight loss rate, malondialdehyde ((POD), polyphenol oxidase (PPO) levels, total phenolic, content, and color parameters using PA, VP and combined treatment during 8 days of storage at 4 °C.
Study design Fresh lettuce (Lactuca sativa L. var. angustana Irish) from ‘Vegetable planting base of Changchun, China’ were fresh cutted into 4 × 3 × 0.5 cm pieces and randomly divided into 6 groups (150 g/group), including the control group (CK), low concentration Phytic acid group (0.5PA) (Yinuo Biological Technology Co., Ltd, Zhejiang, China), high concentration Phytic acid group (1PA), vacuum packing group (VP), 0.5PA + VP group, and 1PA + VP group. The CK group was dipped in 500 mL of deionized water for 10 min at 4 °C. The 0.5PA and 1PA groups were dipped in 500 mL of 0.5% and 1% PA (mg/mL, w/v) solution for 10 min at 4 °C, respectively. The pH value of 0.5% and 1% PA solution were 6.5 and 6.2. Three groups were drained on absorbent and aseptic paper The VP group was dipped in 500 mL of deionized water for 10 min at 4 °C and vacuum packaged with PET/PE material. The 0.5PA + VP group and 1PA + VP group were dipped in 500 mL of 0.5% and 1% PA (mg/mL, w/v) solution for 10 min at 4 °C, respectively. The pH value of 0.5% and 1% PA solution were 6.5 and 6.2. Three groups were drained on absorbent and aseptic paper and vacuum packaged with PET/PE material. Storage at 4 °C is terminated when 50% appears browning, rotting, or odor. All samples for each condition were taken for analysis at days 0, 2, 4, 6, and 8.
Weight loss measurement The weight loss was tested by regularly weighing the samples from day 0 after dipping and draining, and the weight loss rate is calculated according to the following formula: weight loss rate (%) = (original weight − tested weight) / original weight × 100%.
Bacterial colony count detection The bacterial colony count was detected at day 0 before dipping and at days 2, 4, 6, and 8 during storage. The samples (25.0 g) were placed in 225.0 mL phosphate buffer water and made into a 1: 10 sample homogenate, then inoculated in agar dishes after gradient dilution and incubated in a constant temperature incubator at 37.0 °C for 24–48 h.
Hardness and brittleness analysis The CT3 texture analyzer (Brookfield, Massachusetts, USA) and TA-44 probe were used for the test. The parameters are precompression speed of 2.0 mm/s, compression speed of 0.5 mm/s, upward velocity of 0.5 mm/s, trigger point load of 5.0 g, and probe test distance of 4.0 mm. In the TPA function curve, the force value at the highest peak in the first compression cycle is the hardness value, and the smaller peak occurring before hardness is the brittleness index.
Measurement of MDA, POD, and PPO levels The 2.0 g of sample was homogenized in 10.0 mL phosphate buffer solution. The levels of malondialdehyde (oxidase (PPO) were measured according to the recommendations provided on the kits (MDA No. A003-3-1; POD No. A084-3-1; PPO No. A003-3-1; Jiancheng Bioengineering Institute, Nanjing, China).
Total Sugar determination The sample (0.5 g) was grinded in 20 mL water and put in boiling water for 30 min, then centrifuged at 8 000 × g for 15 min to extract the supernatant. The 5.0 mL of anthrone-sulfuric acid solution was added, and the absorbance at the wavelength of 620 nm was measured (UV-2600, Shimadzu, Japan).
Total phenolics determination The sample (2 g) was placed in 10 mL of 70% methanol solution (containing 1% HCl) for grinding and sonication for 40 min, then centrifuged at 8 000 × g for 15 min at 4 °C to extract the supernatant. The 0.5 mL of Folin–ciocalteau was added, and the absorbance at the wavelength of 765 nm was measured (UV-2600, Shimadzu, Japan).
Vitamin C determination The sample (1.0 g) was placed in 5.0 mL of 0.05 mol/L oxalic acid (OA)-0.2 mmol/L ethylene diamine tetraacetic acid (EDTA), then centrifuged at 8 000 × g for 10 min at 4 °C to extracte the supernatant. The 2.0 mL of ammonium molybdate (5 g/100 mL) was added and placed in a constant temperature water bath at 80 °C for 10 min and the absorbance at the wavelength of 760 nm was measured (UV-2600, Shimadzu, Japan).
Colour of lettuce samples The color was determined on photosynthetic tissue with a Portable Chromaticity Meter (RT300, Tintometer, Salisbury, UK). All measurements were carried out on the surface area, initially, after cutting, and after different storage periods. The results were expressed as L*, a*, and b* parameters.
Statistical Analysis All determinations were triplicate. Analyses of variance (ANOVA) of the data were evaluated with the SPSS (19.0, SPSS Science, Chicago, IL). Duncan's Multiple Range Test was for determining the statistical significance of the differences between the means (p < 0.05). Data were reported as mean (SD).
PA combined with VP treatment inhibits the microbial growth of fresh-cut lettuce The total bacterial colony of fresh-cut lettuce was recorded during storage. As shown in Fig. 1, after 4 d of storage, the number of bacteria in the samples treated with PA, VP, or PA combined with VP were significantly decreased than the CK group (p < 0.05). The PA + VP treatment significantly reduced the number of bacteria compared with the other treatment groups on days 6–8 of storage (p < 0.05). It indicates that PA combined with VP treatment has synergistic inhibition on the growth of microorganisms.
Effect of PA on the total bacterial colony count of fresh-cut lettuce.
Effect of PA combined with VP on weight loss, hardness, and frangibility of fresh-cut lettuce As shown in Fig. 2, the weight loss in the groups of VP, 0.5PA + VP, and 1PA + VP was significantly decreased compared with the groups ofCK, 0.5PA, and 1PA groups (p < 0.05) after 2 d storage. However, the weight loss in the 1PA + VP groups was significantly decreased compared with the VP group (p < 0.05) on day 8, showing that the VP could reduce the weight loss rate of fresh-cut lettuce. In addition, the hardness and frangibility of fresh-cut lettuce decreased with longer storage time. However, PA treatment significantly inhibited this decrease (p < 0.05). 1% PA combined with VP treatment can enhance hardness and frangibility effectively, and reduce the weight loss.
Effect of PA on the weight loss rate (a), hardness (b), and frangibility (c) of fresh-cut lettuce.
Effect of PA combined with VP on color a*, b*, and L* value of fresh-cut lettuce The browning reaction is exacerbated for the fresh-cut. The sensory quality was investigated using the color a*, b*, and L* values, which could also show the browning of fresh-cut lettuce. As shown in Fig. 3, the value of a* and b* in the samples treated with PA or combined with V Pdecreased significantly, and L* was increased compared with the CK group after 4 d of storage (p < 0.05), the PA + VP treatment inhibited the browning reaction remarkably.
Effect of PA on the color a* (a), b* (b), and L* value (c) of fresh-cut lettuce.
PA combined with VP treatment inhibits the MDA, PPO, and POD levels of fresh-cut lettuce The damage of fresh-cut lettuce tissue cells intensified oxidative stress and enhanced the browning reaction. Among them, MDA, PPO, and POD were essential indicators of enzymatic browning investigated in this study. As shown in Fig. 4, the MDA, PPO, and POD levels in 0.5PA and 1PA groups were significantly decreased compared to the CK group (p < 0.05) after 4 d of storage. PA combined with VP treatment can decrease the levels of MDA, PPO, and POD.
Effect of PA on the MDA (a), PPO (b), and POD (c) of fresh-cut lettuce.
Effect of PA combined with VP on total sugars, phenol, and vitamin C contents of fresh-cut lettuce Total sugars, phenols, and vitamin C in fruits and vegetables are essential resistance-related metabolites whose level would decrease continuously after fresh-cut treatment. As shown in Fig. 5, the contents of total sugars, phenols, and vitamin C in 0.5PA and 1PA groups were significantly increased compared to the CK group (p < 0.05) after 4 d of storage. PA combined with VP treatment can significantly inhibit the decline of these indicators.
Effect of PA on the total sugars (a), phenol (b), and vitamin C contents (c) of fresh-cut lettuce.
The consumers' interest in healthy and convenient foods, especially fresh-cut fruits and vegetables, has increased recently (Sothornvit et al., 2007). Fresh-cut lettuce is a popular fresh vegetable rich in nutrients with a short shelf-life due to its high respiration rate (Alongi et al., 2018). However, few studies related to its preservation have been reported. Many studies have shown that PA is a natural preservative that inhibits oxidation, browning, and aging of fresh-cut vegetables (Fox and Eberl, 2002; Li et al., 2017). In this study, PA combined with VP treatment was used for the first time to examine the relevant physiological indicators of fresh-cut lettuce in preservation.
Firstly, fresh-cut lettuce loses its important protective layer due to peeling and cutting, and the nutrients that leach from the tissue promotes the growth and reproduction of microorganisms. The amount of bacteria is an essential indicator of microbial contamination degree of fresh-cut lettuce during storage (King et al., 1991; Sinigaglia et al., 1999). VP method could destroy the survival environment of aerobic bacteria (Xing et al., 2012). In addition, PA has a unique structure in that it has 12 replaceable protons (Oatway et al., 2001). The structure is responsible for its ability to “chelate” positively charged ions; this characteristic property means that it is to be effective at destroying biofilms (West et al., 2014). PA had a strong inhibitory effect on E. coli destroyed the integrity of the cell membrane (Kim et al., 2014). In this study, both PA and VP treatments had a synergistic inhibition of microbial growth. Secondly, the weight loss, hardness, and frangibility are important indicators affecting the quality of fresh-cut vegetables (Waghmare and Annapure, 2015). The increased weight loss and decreased hardness and frangibility in storage are mainly caused by respiration and the consumption of organic matter, while fresh-cut processing would also damage the tissue structure of lettuce, accelerating water loss (Altunkaya and Gökmen, 2008). In this study, PA combined with VP treatment can better inhibit the changes of cell structure, reduce nutrient consumption, enhance its hardness and frangibility, and reduce the weight loss. PA and pectin were expected to be present as negatively charged molecules to chelate minerals and form insoluble complexes at pH 6.0–7.0 of vegetable (Rousseau et al., 2020). Due to PA alleviating the changes of minerals and pectin, the apparent texture of fresh-cut lettuce was less damaged, which could maintain the hardness and frangibility of cell structure (Zhang et al., 2015).
The appearance is the primary perception for consumer acceptability (Sothornvit and Kiatchanapaibul, 2009). The sensory quality was investigated using the color a*, b*, and L* value, which was also fitted for showing the browning of fresh-cut lettuce. In this study, the PA + VP treatment inhibited the browning reaction remarkably.
Browning in fruits and vegetables can be essentially divided into non-enzymatic browning and enzymatic browning. Numerous studies have shown that enzymatic browning is the leading cause of browning in fruits and vegetables (Moon et al., 2020). Enzymatic browning means the oxidation of phenolic substances in fruit and vegetable tissues to quinones by the action of enzymes, which then polymerize to form brown substances, resulting in tissue discoloration. Phenolic substances, phenolic enzymes, and reactive oxygen species are the three necessary conditions for enzymatic browning (Manzocco et al., 2000). Peeling and cutting treatment of fresh-cut lettuce would damage the tissue cells were, intensify oxidative stress and enhance the browning reaction (Altunkaya and Gökmen, 2008), among which MDA, PPO, and POD are important indicators (Almeida and Nogueira, 1995). MDA is the end product of lipid peroxidation in cell membranes with the ability to react with various substances in the cell, resulting in severe damage to the membrane system and the enzyme system in the cell (Zeng et al., 2020). PPO, as the primary enzyme causing browning in fruits and vegetables can catalyze the combination of oxygen and polyphenols to form quinones (Almeida and Nogueira, 1995), which polymerize to form brown deposits. Thus PPO vitality directly affects the sensory quality of fruits and vegetables (Olmedo et al., 2018). POD activity can reflect the degree of oxidative stress in tissues through being spiked when plant tissues are stressed and mechanically damaged (Lamikanra et al., 2001). PPO and POD would enhance the browning of plant tissues (Kim et al., 2014). PA treatment inhibited 99.2%PPO in apple juice and significantly reduced the browning reaction (Du et al., 2012). PA delayed the senescence of Gynura bicolor D.C leaves effectively by decreasing the incidence of decay, maintaining low relative leakage, retarding the accumulation of MDA, inhibiting respiration rate and PPO activity, and enhancing the activities of POD and SOD. Therefore, PA treatment has the potential to extend the storage life of vegetables (Jiang et al., 2014). In this study, combination treatment of PA and VP could effectively inhibit MDA levels, PPO activity, and POD activity and delay the oxidation and browning reactions in fresh-cut lettuce.
In addition, sugars, phenols, and vitamin C substances are essential secondary metabolites for fruits and vegetables, which are resistance products of plant tissues (Alothman et al., 2009). The information of flavonoid concentrations and phenolic acid esters are sensitive to environmental factors that could be used to develop postharvest conditions, thus increasing the dietary benefits of leaf lettuce (Caldwell, 2003; Zhao et al., 2007). Oxidation of phenolic compounds often catalyzed by the polyphenol oxidase enzyme, forming melanins, results in browning of surface tissue. The fresh-cut treatment would affect the total sugars, total phenols, and vitamin C in fruits and vegetables, whose declination is probably due to cell membrane damage, higher oxidation rate, and other substances (Alothman et al., 2009; Sánchez et al., 2009), since polyphenolic compounds and vitamin C are the primary antioxidants in vegetables, as widely reported in the literature (Caldwell, 2003). Studies have shown that PA has strong antioxidant properties and can prevent the loss of total phenols and vitamin C effectively (Empson et al., 2006). In this study, PA combined with vacuum treatment can maintain high levels of total sugars, total phenols, and vitamin C in fresh-cut lettuce.
The combination treatment of PA and VP can improve the hardness, frangibility, color value, total sugar, total phenols, and vitamin C levels of fresh-cut lettuce, and reduced the weight loss, total bacterial colony, MDA levels, PPO activity, and POD activity. It was showed that the combination treatment of PA and VP could maintain the quality of fresh-cut lettuce and prolong its shelf-life.
Acknowledgements We would like to thank Qingdao yaoling technology Co., Ltd. for English language editing. This research was supported by the project of Jilin Province Science and Technology Agency (Grant No. 20190301081NY to Liu H.) and Jilin Engineering Normal University (Grant No. BSKJ201817 to Liu H.)
Conflict of interest There are no conflicts of interest to declare.
