| Lactuca sativa L.,also known as lettuce,which is an annual and biennial leaf vegetable that belongs to the genus lactuca in the Compositae family.Lettuce is popular among consumers for its crisp taste and simple preparation method.Lettuce is rich in protein,vitamins,cellulose,minerals,organic acids,phenols,terpenoids and other nutrients,which make it have anti-inflammatory and regulating the balance of sodium and potassium in the body and other health functions.At present,due to the increasing demand of Chinese consumers for safe,healthy and convenient food,fresh-cut lettuce shows a broad market prospect.Fresh-cutting usually cause tissue damage of lettuce,which lead to faster browning and shorter storage time.Therefore,it is urgent to find an efficient method to improve the storage quality of fresh-cut lettuce.At present,some achievements have been made in the application of ultrasound and ZnO NPs treatment in fruit and vegetable preservation,but there is little research on the use of ultrasound combined with ZnO NPs treatment to inhibit fruit and vegetable browning.In this study,with purple leaf "Jinnong" lettuce as experimental material,the optimal technological conditions of ultrasound combined with ZnO NPs treatment were studied,the effects of ultrasound combined with ZnO NPs treatment on physical and chemical properties,apparent quality,microstructure and aroma composition of fresh-cut lettuce were explored,and the fresh-cut lettuce treated with ultrasound combined with ZnO NPs treatment was used as raw material that develop a fast and convenient lettuce frozen instant food.Specific research results are as follows:Using weight loss rate,the a~* value and chlorophyll content as indexes,the optimal technological conditions of fresh-cut lettuce treated with ultrasound combined with ZnO NPs were obtained by single factor experiment and response surface optimization test.The results showed that the storage quality of fresh-cut lettuce treated with ultrasound combined with ZnO NPs was significantly improved,when the ultrasound power was 300 W,the ultrasound time was 10 min,the ultrasound temperature was 46℃,and the concentration of ZnO NPs solution was 0.07 g/L.On the 8th day of storage,the weight loss rate of fresh-cut lettuce was reduced to 0.23%.The a~* value and chlorophyll content were up to-8.99 and 2.34 mg/100 g,respectively,which were significantly different from the control group(P<0.05).The above results showed that ultrasound combined with ZnO NPs treatment could effectively maintain the apparent color of fresh-cut lettuce and inhibit water loss,which could be used as a new method of fresh-cut lettuce preservation for further study.The effects of single ultrasound treatment,single ZnO NPs treatment,ultrasound combined with ZnO NPs treatment and control treatment on appearance quality and main nutrients of fresh-cut lettuce were investigated.The results showed that on the 8th day of storage,compared with the control treatment,the weight loss rate and respiratory intensity of fresh-cut lettuce in the ultrasound combined with ZnO NPs treatment were reduced by 12.34% and18.63%,respectively;the contents of a~*,hardness,protopectin,chlorophyll,cellulose and ascorbic acid of fresh-cut lettuce in cooperative treatment group were the highest,which were 3.02,1.04,1.32,1.15,1.37 and 1.38 times of those in control group,respectively(P<0.05);the contents of titratable acid and soluble solid increased by 13.11% and 5.85% of those in control group,respectively.Slightly different,the contents of soluble sugar(4.46%)and soluble protein(1.77 mg/g)of fresh-cut lettuce in the cooperative treatment group reached the highest on the 4th and 6th days of storage,respectively.The results of scanning electron microscopy showed that ultrasound combined with ZnO NPs treatment could provide more exit and entry microchannels for freshcut lettuce cells and promote ZnO NPs to enter into fresh-cut lettuce cells more efficiently,providing a hypothesis for the mechanism of synergic treatment.The effects of single ultrasound treatment,single ZnO NPs treatment,ultrasound combined with ZnO NPs treatment and control treatment on physiological and biochemical reactions and flavor substances of fresh-cut lettuce were investigated.The result showed that the activity of polyphenol oxidase and peroxidase of fresh-cut lettuce were significantly inhibited by ultrasound combined with ZnO NPs treatment,and the enzyme activities were decreased by 33.98% and 24.77% compared with the control treatment on the eight day of storage(P<0.05);however,the activity of phenylalanine ammonia lyase,catalase and total phenol content in the cooperative treatment group reached the highest values on the 4th day of storage,which were 1.20,1.25 and1.15 times of those in the control group,respectively.Meanwhile,the free radical scavenging ability of 1,1-diphenyl-2-trinitrophenylhydrazine(DPPH),2,2-diazo-di-3-ethylbenzothiazol-6-sulfonic acid(ABTS)and total reducing ability of fresh-cut lettuce in cooperative treatment group were the highest,which were 1.69,1.33 and 1.76 times of those in control group on the 8th day of storage,respectively;the content of malondialdehyde and hydrogen peroxide was lower than those in control group significantly,only 72.49% and 82.02%(P<0.05).In addition,the heat-sensitive aroma substances of fresh-cut lettuce in cooperative treatment group were lost,but the contents of alene and nitrile substances increased,which provided more unique flavor for fresh-cut lettuce.Fresh-cut lettuce treated with ultrasound combined with ZnO NPs treatment was used as raw material to prepare a new type of frozen instant food through optimization of formula design.The optimal preparation conditions of lettuce products were studied by single factor experiment and orthogonal optimization experiment.The results showed that blanching time 10 min,salt water mass ratio 3:100(g/g),the frying temperature 160℃,the frying time 150 s,the sensory evaluation score of lettuce frozen instant food prepared under this condition is the highest,the quality is best,the taste is richest.Compared with the actual shelf life(140 days),the result obtained by using the shelf-life prediction model is 128 days.The error was less than 10% and indicated that the shelf-life prediction model has certain reference value.The optimal microwave reheating conditions of lettuce products were investigated by single factor experiment,the results showed that the sensory quality of lettuce products was the best when heated for 3 min under microwave medium-high fire condition,which provides a new idea for the development of lettuce convenient food. |
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