Effect Of Precooling Process Parameters On Precooling Rate And Quality Of Vegetables In Cold Chain

Vegetables are an important source of nutrients for human beings,but they are usually prone to aging and decay due to their high water content and vigorous respiration,and are not easy to be stored at room temperature.Therefore,freshly harvested vegetables are in urgent need of precooling treatment to reduce their temperature,inhibit respiration and metabolism,slow down the consumption of nutrients,and extend the freshness period.Precooling,as the"first kilometer"of the whole cold chain of vegetables,has become a key measure to maintain the quality of postharved vegetables.The setting of precooling process parameters will affect the precooling effect of vegetables,and will also have different degrees of influence on the quality changes in the subsequent cold chain circulation process.If the precooling process parameters are not set reasonably,the precooling rate and nutritional quality of vegetables will be seriously reduced.In this study,four kinds of vegetables,including green pepper,broccoli,butterhead lettuce and tomato,were selected to study the effects of precooling process parameters,such as internal and external packaging,initial temperature,delayed cooling and fruit size,on the precooling rate and the quality of vegetables during cold chain.The main research results are as follows:(1)Study on the effect of different internal and external packaging on precooling rate and the quality of green pepper in cold chain circulation.Green peppers are packaged with plastic crate,corrugated carton,plastic crate+anti-fog film,corrugated carton+anti-fog film,respectively,then foced-air preecooled to 9?from the initial temperature(22?25?).After precooling,they are packed with plastic crate+anti-fog film,moved to 10?cold storage for 8-day refrigeration,for 2-day simulated transportation and 2-day sale,determining the precooling time,weightless loss,sensory quality,total soluble solids(TSS),vitamin C(VC),chlorophyll and total phenolic contents.The results showed that the precooling rate of the plastic crate package is faster than that of the carton,and the precooling rate of the film-free package is faster than that of the film-added package.The quality of green pepper packed with plastic crate is higher than that of carton,and the quality of pepper packed with anti-fog film is higher than that of film-free.The precooling time of green pepper packed in plastic crate,carton,plastic crate+anti-fog film,carton+anti-fog film were 55,60,85,135 min,the weight loss was 0.41%,0.39%,0.18%,0.17%,and the respiration intensity was 16.57,18.50,18.89,19.23 mg CO₂/(kg·h),respectively.The precooling rate of pepper packed in plastic crate was the fastest and the energy consumption was the least.In cold chain circulation,its quality declined slowly,and the sensory evaluation was 5.4 after 12 days of cold chain circulation at 10?,which still showed commercial property.The weight loss was 1.27%,the hardness was 20.1 N,the contents of TSS,VC,chlorophyll and total phenol were 3.85%,56.07 mg/100g,46.22 mg/kg and 0.94 OD₂₈₀/g,respectively.(2)Study on the effects of different initial product temperatures on the precooling rate and quality of broccoli in cold chain circulation.The broccoli with different initial temperatures(12?,20?,27?)was forced-air precooled to 3?.After precooling,the broccoli was packed with anti-fog film and moved to cold storage at 0?for 6-day refrigeration,and for 2-day simulated transportation and 2-day sales.The results showed that the higher the initial temperature of broccoli,the longer the precooling time,and the worse the quality of broccoli in cold chain circulation.The precooling time of broccoli at the initial temperature of 12?,20?and 27?was 35,70 and 100 min,respectively,and the weight loss after precooling was 1.95%,2.27%and 2.48%,respectively.Sensory evaluation of the three initial temperature of broccoli after 10 days of circulation were5.9,5.4 and 5.1,respectively,which still have commercial nature.The precooling rate of broccoli at the initial temperature of 12?was the fastest,and its quality was maintained the highest during cold chain circulation.After 10 days of circulation,the weight loss rate was 2.90%,and the contents of VC,chlorophyll and total phenol were 0.54 mg/g,0.28mg/g and 11.63 OD₂₈₀/g,respectively.Compared with the initial value,the retention rates were 62%,61%and 78%,respectively.Therefore,vegetables should be harvested in the day when the ambient temperature is lower,which is conducive to shorten the precooling time and maintain the nutritional quality of vegetable in the circulation.(3)Study on the effect of delayed cooling on precooling rate and the quality of head lettuce during cold chain circulation.Before precooling,lettuce was placed at room temperature(18?20?)for 1 h,5 h and 8 h,and then precooled to 3?.After precooling,lettuce was packed with anti-fog film and refrigerated in cold storage at 0?for 6 days,transported and sold for 2 days,respectively.The precooling time,respiration intensity,weight loss,VC,chlorophyll and total phenol were measured.Because the plastic crate is open,the temperature of the butterhead lettuce hasn't risen during the delay,so the precooling time is almost the same(100?105 min).However,in cold chain circulation,the longer the delayed cooling time,the faster the quality of lettuce decreased.After circulating at 0?for 10 days,the sensory evaluation of lettuce delayed for 1 h was 5.7,the weight loss was 1.38%,and the contents of TSS,VC,chlorophyll and total phenol were 3.12%,10.04 mg/100g,24.30 mg/kg and 0.83 OD₂₈₀/g,respectively.(4)Study on the precooling rate of different size of tomatoes and the quality changes during cold chain circulation.Large,medium and small tomatoes were forced-air precooled to 3°C from the initial temperature of 17?17.5°C,and then packed with anti-fog film.They were placed in cold storage at 0°C for 6 days and simulated transportation and sales for 2 days,respectively.The precooling time of large,medium and small tomatoes were 165,100 and 95 min,respectively,and the weight loss were 0.34%,0.45%and 0.64%,respectively,which showed significant differences.After cold chain circulation for 10 days,sensory evaluation of large,medium and small tomatoes were 7.2,6.9,6.5,weight loss was 0.52%,0.54%,0.77%,TSS content was 4.85%,4.77%,4.39%,VC content was 21.19,18.42,17.31 mg/100g,total phenolic content was 1.26,1.18,1.07OD₂₈₀/g,lycopene content was 1.48,1.42,1.20 mg/kg,?-carotene content was 0.49,0.48,0.47 mg/kg,respectively.The nutrient content of tomato varies with the size of fruit.Generally speaking,the larger the fruit,the higher the nutrient content.However,the decreasing trend of nutrients in different specifications of tomatoes during cold chain circulation was roughly the same.Due to the large variation in precooling rates,tomatoes of different sizes should be precooled separately.