Research On Preservation Pretreatment And Quick-Frozen Food Preparation Of Fresh-Cut Lotus Root

Nelumbo nucifera Gaertn,also known as lotus root,is rich in carbohydrates,protein,ascorbic acid and others.In today’s promising fresh-cut fruit and vegetable market,fresh-cut lotus root is favored by domestic and foreign markets because of its natural,fresh,nutritious and convenient characteristics.However,it is prone to browning,reduced nutritional value and microbial spoilage during processing,which restricts product development and application.Therefore,it is very necessary to carry out research on the fresh-cut processing technology of lotus root.Ultrasonic（ultrasound,US）technology is used in the field of food processing,which can blunt enzymes,sterilize and improve quality.Ultrasound can also generate high-energy intermediate products such as O₂^-and H₂O₂,improve the antioxidant capacity of fruits and vegetables,and has the advantages of mild processing conditions,easy control and high safety.However,the fresh-keeping ability of single US treatment is low.The combined use of ultrasonic-heat（Heat+US）technology can cause an"additive effect"of mechanical and cavitation effects,exert a stronger fresh-keeping effect on fruits and vegetables,and can delay the quality deteriorates of lotus root to the greatest extent.Therefore,the application of this technology to deal with fresh-cut lotus roots aims to better maintain the original quality of lotus roots and prolong the shelf life.The main findings are as follows:The optimal process of Heat+US treatment for color protection and fresh-keeping of fresh-cut lotus root was studied.Through single factor test and response surface analysis method optimization test,the whiteness（W）of the lotus root surface was used as an index to compare different Heat+US treatment conditions on its effection of whiteness.The results show that the optimal process is:ultrasonic power 300 W,action time 10min,action temperature 50℃;under this process,the whiteness of the fresh-cut lotus root was effectively maintained,and the browning degree of the fresh-cut lotus root was 0.09 A₄₁₀nm,the weight loss rate was 7.00%,and the hardness was 2075.65 g on the 9th day of storage,which were significantly higher than those of the control（P<0.05）,and inhibited the microbial contamination of fresh-cut lotus roots during storage.The effects and possible mechanisms of Heat+US treatment on the physiological and biochemical functions of fresh-cut lotus root were studied.Through oxidase activity assay,antioxidant capacity analysis and scanning electron microscope observation,the results showed that Heat+US（power 300 W,time 10 min,temperature 50℃）treatment can significantly reduce polyphenol oxidase（PPO）and peroxidase（POD）activity,and increased phenylalanine ammonia lyase（PAL）activity and total phenolic content（P<0.05）;at the same time,Heat+US treatment could significantly increase 1,1-diphenyl-2-trinitrobenzene（DPPH）and 2,2’-diazo-bis-3-ethylbenzothiazoline-6-sulfonic acid（ABTS）scavenging ability of hydrazine,thus the antioxidant capacity of fresh-cut lotus root was improved.On the ninth day of storage under this condition,the total reducing power reached 39.25 m M TE g^-1 FW,which was 1.98 times higher than that of the control.There was a significant positive correlation（P<0.01）;scanning electron microscope observation showed that Heat+US treatment could generate more microchannels between cells of fresh-cut lotus root,which was helpful for improving the effect of preservation.The effects of Heat+US treatment on the main nutrient components and main flavor components during storage of fresh-cut lotus root were studied.The dialdehyde method was used to evaluate the damage of lotus root cells after treatment.The research results showed that the main flavor components such as esters,aldehydes,and ketones in the fresh-cut lotus root were less lost with Heat+US treatment,which was 92.34%of the control.On the 9th day of storage,the contents of protein and soluble solids were the highest,which were 0.60 mg/g FW and 5.88%,respectively,which were 1.13 and 1.02 times that of the control group（P<0.05）;not only that,the contents of ascorbic acid,pectin,titratable acid and starch were all significant higher than the control（P<0.05）;but Heat+US treatment had no significant effect on the soluble sugar content of fresh-cut lotus root（P>0.05）;at this time,the content of malondialdehyde（MDA）decreased by 38.78%compared with the control（P<0.05）,indicating that Heat+US treatment had less cell damage to fresh-cut lotus root.The development and application of quick-frozen microwave conditioning lotus root food was studied.Taking the lotus root pretreated by Heat+US as raw material and sensory score as the index,the seasoning process was optimized through single factor test and orthogonal test,and a hot and sour quick-frozen food was developed.The research results show that the optimal formula of seasoning liquid is:vinegar 8 m L,chili oil 3m L,soy sauce 3 m L and monosodium glutamate 0.16 g.In addition,the effect of different microwave reheating conditions on the quality of the above lotus roots was also studied.The results showed that the product had higher juice retention rate and better sensory quality when reheated under the conditions of microwave medium and high heat（490 W）for 3 min.This study can provide data basis and technical support for the processing of fresh-cut lotus root,and provide a certain theoretical basis for the preparation of lotus root conditioned food.