Research On Technology Of Quick-freezing Of Citrus Reticulata

In this study, Satsuma mandarin, Shatang tangerine and Ponkan tangerine was selected as quick-freeze experimental materials that represent the main types of Citrus reticulata. According to the effect of freezing rate on its quality, the optimum freezing temperatures were identified. According to changes in quality and microorganism number of different cultivars during frozen storage, the optimum storage time were determined. Finally, different thawing methods was studied on quality of quick-frozen citrus segments, the optimum thawing methods were determined and the conclusions were as followed:(1) The optimal freezing temperature of Satsuma mandarin was -50 ℃, the corresponding freezing rate was 0.36℃/min, followed by freezing temperature-60℃ and-70 ℃, the effect on the color and texture were greater than on nutrition when it was frozen at-40 ℃, the effect on the nutrition were greater than on color and texture when citrus were frozen at-80 ℃. The optimal freezing temperature of Shatang tangerine was-40℃, the corresponding freezing rate was 0.37℃/min, followed by freezing temperature-50℃ and -60℃, and freezing had the same effect on color, texture and nutrition of Shatang tangerine. The optimal freezing temperature of Ponkan was -70 ℃, the corresponding freezing rate was 0.47℃/min, followed by freezing temperature-40 ℃ and-50℃, the effect on the color and texture were greater than on nutrition when it was frozen at -30℃.(2)By grey correlative degree analysis, the effect of freezing rate on citrus quality could be concluded as follows:there will be a greater impact on the quality of citrus segments when freeze rate was too slow or too fast, moderate freeze rate made high quality citrus segments. Moderate range of Satsuma mandarin was -50 ℃ to-70℃, moderate range of Shatang tangerine was -40℃to-60℃, moderate range of Ponkan was-40℃to-70℃.(3)Sensory quality of satsuma mandarin always kept with a high level until it decreased rapidly after 120 days of frozen storage; The total soluble solids(TSS), Vc, reducing sugar were decreased from the initial level after increased to a certain extent, titratable acid (TA) kept with a high level until it decreased rapidly at at 140 days later, and the TSS/acid ratio showed a slow upward trend; L* value and a value decreased by 5.69%,35.46% respectively during frozen storage, b value and c value were increased to the initial level after decreased to a certain extent during initial 60 days, the total chromatic aberration value (△E*)had remained stable after it increased 1.26 times at first 60 days; The hardness, gumminess and chewiness decreased by 39.73%,38.45%,39.67% respectively, elasticity and cohesiveness had no significant differences compared to the original; the response value of aroma decreased slowly at previous 120 days of frozen storage, and decreased rapidly at 160 day, that meaned the concentration of aroma have decreased rapidly after 120 day, the main ingredients of aroma have changed rapidly after 120 day.(4) Sensory quality of Shatang tangerine decreased slowly and kept with a high level during frozen storage; nutrition quality kept stably during storage; L* value,a value,b value and c value decreased by 18.19%,96.91%,36.13%,40.01%respectively during storage, △ E* has increased 13.27 times; The hardness decreased slowly by 24.53% during storage, cohesiveness first decreased then increased, and finally decreased by 11.11%, elasticity, gumminess, chewiness first decreased then increased, and finally decreased,which were no significant differences compared to the original level; the response value of aroma decreased slowly at previous 80 days, and decreased rapidly at 100 day, that meaned the concentration of aroma had decreased rapidly after 80 day, the main ingredients of aroma had changed rapidly after 80 day.(5)Sensory quality of Ponkan decreased slowly and kept with a high level during frozen storage; nutrition quality kept stably during storage; L* value, a value decreased by 12.19%,43.37%, respectively during storage, b value and c value always kept stably during storage; △E* had increased 9.15 times; The hardness, gumminess, chewiness, elasticity decreased by 26.883%、13.34%、19.57%、6.98%, respectively and the cohesiveness increased by 5.08%; the response value of aroma decreased slowly at previous 80 days, and decreased rapidly at 100 day, that meaned the concentration of aroma had decreased rapidly after 80 day, the main ingredients of aroma had changed rapidly after 80 day.(6)The total plate count of citrus segments met hygiene requirements at-18℃ storage for 180 days,which were less than 100cfu/g, and Escherichia coli was not founded during storage.(7)Overall, compared with the five thawing methods, thawing with refrigerator and microwave oven had a few impact on quality of citrus segments, but the former took a longer time of thawing, and the later had a maximum drip loss, thawing with running water and warm water had notable effect on quality of citrus segments, although they were thawed with a less time and minimal drip loss. Specifically, thawing with microwave oven have little effect on TSS, reducing sugar, TSS/acid ratio, aroma and color of Shatang tangerine, while thawing with refrigerator had a few effect on Vc, TA, texture and color of Ponkan. The effects of different thawing methods on quality were no significant differences among different cultivars, so the refrigerator or microwave oven were the good method to thaw the citrus segments depend on the specific circumstances.