| Litchi fruit is often processed into juice to reduce the profit loss because of fruit decay and price fall during the market saturation and this is of great significance for both litchi industry and farmers. However, there are several problems(e.g., browning, loss of nutrition and aroma, etc.) that could lead to the quality loss of litchi juice during shelf-life and consequently restrict the development of litch juice industry. In order to understand primary factors in the browning of litchi juice and aroma changes during storage,this study investigated the influence of storage conditions(packaging materials, storage temperature, oxygen concentration and illumination) on the quality parameters of litchi juice(e.g., L-ascorbic acid, glucose, fructose, sucrose, amino acids, total phenols, 5-HMF,p H, TSS, epicatechin, rutin and aroma). The change of different parameters during storage was analyzed using headspace GC-MS fingerprinting approach and partial least squares regression(PLSR) analysis, and thus provides a theoretical possibility of predicting quality changes of litchi juice during storage. Results of this study are as follows:(1) Qualities of litchi juice were affected by oxygen when samples were stored protected from light at 4°C, 25°C and 35°C in different types of containers(e.g., PET bottle,PLA bottle and multiply pocket). For storage in PET or PLA bottle, a fast degradation of L-ascorbic acid(L-AA) in litchi juice was observed as a result of both dissolved oxygen and diffuse oxygen, of which the later one is predominant. For storage in multiply pocket,the degradation of L-AA was relatively slow and mainly affected by the dissolved oxygen rather than diffuse oxygen. The degradation of L-AA was best described by a first-order kinetic model for all storage conditions except for the one in PLA bottle at 35°C which fit zero-order model only. Moreover, the higher was the storage temperature and diffuse oxygen, the faster was the L-AA degradation. At the beginning of storage, the dissolved oxygen concentration(DOC) was decreased sharply for all three types packaging. As storage time proceeded, DOC dropped smoothly and finally kept steady,except for the PLA bottle in which the DOC increased fast. During the storage, the hydrolysis of sucrose wasobserved as a function of temperature and a first-order kinetic model gave the best fit for it.At the beginning of storage, the concentration of both fructose and glucose displayed a fast increase along with the hydrolysis of sucrose and then decreased slowly. Total amino acids reduced fast when the storage temperature increased, especially for threonine, lysine,histidine and arginine. Along with the decrease in total phenol concentration, the content of5-HMF and BI increased continuously and all of them were best described by a zero-order kinetic model. High temperature and high diffuse oxygen content accelerated the reaction rate. For storage at 4°C, the browning of litchi juice was determined by the content of diffuse oxygen and mainly caused by L-AA degradation and phenols oxidation. For storage at 25°C and 35°C, the Maillard reaction in the system was the primary cause for the browning in addition to the L-AA degradation and phenols oxidation.(2) The influence of dissolved oxygen on the quality of litchi juice during storage was investigated at low DOC, medium DOC and high DOC respectively. For samples of low DOC or medium DOC, there was a sharp decrease in the concentration of dissolved oxygen during the first 3 days of storage, after which the concentration became almost stable over the subsequent weeks. The change of L-AA in low DOC or medium DOC sample during the first 3 days of storage was dominated by aerobic degradation and described well by a zero-order kinetic model with a reaction rate K of-0.33 and-0.45 respectively. The change of L-AA after that(3 days-17 weeks) followed a first-order model and the reaction rate K was-0.029 and-0.034 respectively. There were both aerobic L-AA degradation and anaerobic L-AA degradation during this period and the later one was the predominant pathway. At the end of storage, the content of L-AA was reduced by 46.6% and 58.5%respectively. For storage at high DOC, the decrease of DOC and headspace oxygen concentrations were very sharp at beginning and the aerobic degradation dominated the change of L-AA over the whole investigation(4 weeks) with no detection of L-AA in the end. The fractional conversion model described well the decreasing trend of L-AA and the reaction rate K was-0.97. For samples of low DOC and high DOC, changes of both dissolved oxygen and headspace oxygen concentrations followed a fractional conversion model. The decrease in sucrose concentration at all three DOC tests could be best modeled by a first-order model, wherein fructose and glucose concentrations increased at first andthen went down. A zero-order kinetic model gave the best fit for the change of 5-HMF and BI which both increased as a function of DOC. The increase in DOC accelerated the degradation of total phenols, wherein epicatechin content decreased in a zero-order kinetic model, while the change of rutin was relatively slight. Primary factors for browning of litchi juice under differernt DOC were L-AA degradation and phenols oxidation, while the Maillard reaction was of secondary importance.(3) The influence of light on qualities of litchi juice during storage was investigated at four different conditions: stored protected from light, under low luminous intensity, under high luminous intensity, and UV-light. The L-AA concentration of all stored litchi juice samples decreased continuously over the test periods and followed a first-order kinetic model. The total phenols decreased sharply at the beginning of storage(0-1week) and then became steady. Samples stored protected from light favored the retention of L-AA and total phenols in the juice, while the degradation of total phenols was deteriorated by the UV-light.The concentration of sucrose decreased in all test samples and was best modeled by a first-order model. The decrease in sucrose concentration was relatively slow for the juice stored protected from light. Along with the degradation in sucrose, the concentration of fructose and glucose increased at first and then went down slowly. Total amino acids reduced continuously, especially for leucine, threonine, lysine, histidine and isoleucine. The content of 5-HMF, BI and CIEb value increased continuously, wherein changes in the juice under UV-light illumination were the fastest and under high luminous intensity were the slowest. All of them were best described by a zero-order kinetic model. CIEL value decreased in a first-order model and the reaction rate of all four tests was similar. The browning of litchi juice under different illumination conditions was predominated by L-AA degradation and phenols oxidation, and the Maillard reaction was of secondary importance.(4) Changes of aroma components in litchi juices during storage were investigated and a higher number of volatiles were observed at higher storage temperatures and longer storage time for all three packages(PET bottle, PLA bottle and multiply pocket). When litchi juice was stored at 4°C, an obvious decrease in citronellol, geraniol and geranial was observed during shelf-life. For the sample stored at 25°C, linalool concentration also displayed a significant decrease while the α-terpineol and rose oxide showed an obviousincrease. When the sample was stored at 35°C, there was a decrease in nerol, isogeraniol and phenylethanol concentration but an increase in the nerolin concentration. The difference in DOC had a slight influence on the concentration of citronellol, geraniol,geranial, linalool, α-terpineol and rose oxide, but an obvious change was observed for each aroma component when the storage temperature was increasing. Similarly, the concentration of citronellol, geraniol, geranial, linalool, α-terpineol and rose oxide changed in a same way for the sample stored under fluorescent-light of different intensity and UV-light. |
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