Study On Quality Control Of Fresh-cut Apples By Cold Plasma Combined With Honey Coating Treatment

In recent years,the market scale of prefabricated vegetables such as fresh-cut fruits and vegetables has developed rapidly.However,fresh cutting will destroy the integrity of fruit and vegetable tissues,leading to quality deterioration.Therefore,the development of safe and efficient fresh-keeping technology has important economic and practical significance.As a new nonthermal processing technology,cold plasma（CP）has been gradually applied to the field of food sterilization,but the research on the effect of sensory quality and physiological characteristics is relatively lacking.Honey has inherent antioxidant and antibacterial properties and is highly acceptable to the public,so it can become a natural edible coating.However,it is still unknown whether honey coating（HC）can improve the fresh-keeping effect of CP treatment.Therefore,fresh-cut apples are taken as this research object,aiming at the problems of quality deterioration during cold storage（4 ^oC）.To study the impact of CP treatment on the quality.And to explore the possibility of HC to improve the quality after CP treatment.Furthermore,to explore the inhibition mechanism of cold plasma combined with honey coating（CP/HC）treatment on polyphenol oxidase（PPO）.It aims to provide theoretical basis for fresh cut fruits and vegetables preservation in restaurants such as fresh fruit and tea stores and central kitchensThe effect of CP treatment on the quality of fresh-cut apples was studied.First,RegardedΔE as an important quality factor and response value,through single factor and response surface optimization,the optimal treatment conditions of CP were determined:power 51.5 W,time 9.9min.Under this condition,fresh-cut apples had no potential residue of nitrite.The microbial results showed that CP treatment effectively reduced the initial microbial number and growth rate on the surface of fresh-cut apples.The total number of colonies and the total number of molds and yeasts decreased by 1.97 lg CFU/g and 1.00 lg CFU/g respectively after 10 days at4 ^oC and did not exceed the microbial safety limit,ensuring the safety of fresh-cut apples during cold storage.In addition,CP treatment could effectively maintain the content of total phenol and ascorbic acid in fresh-cut apples.After 10 days,the retention rate reached 67.27%and70.48%,which improved the free radical scavenging ability of fresh-cut apples,accelerated the elimination of H₂O₂ in cells,and prevented ROS oxidation membrane lipid from forming malondialdehyde（MDA）.Further studies showed that CP treatment immediately reduced the PPO activity of fresh-cut apples.The inhibition rate of PPO activity was more than 35%on day0 and decreased after 2 days.CP treatment group could maintain a relatively fresh surface color within 2 days.The effect of CP treatment on improving the surface browning of fresh-cut apples was limited.To effectively solve this problem,we found that HC treatment had a continuous anti-browning effect,and the best treatment condition was 10%（w/w）jujube honey.Under these conditions,the effect of cold plasma combined with honey coating（CP/HC）on the quality improvement of fresh-cut apples was studied.The results showed that after 10 days theΔE was9.22 lower than that of the control,which effectively inhibited the surface browning degree of fresh-cut apples in the late cold storage period,and continuously inhibited the activities of enzymes related to the browning reaction.The highest inhibition rates of PPO and POD activities were 60.84%and 37.19%.The sensory evaluation results showed that the CP/HC treatment group still had bright surface color,higher hardness,and appropriate aroma and taste after 10 days,and the sensory acceptance was better than that of CP and HC treatment groups.In addition,compared with the control,CP/HC treatment reduced the respiration rate of fresh-cut apples by 46.67%,thus delaying the increase of weight loss rate,reducing the consumption of carbohydrates,and maintaining the content of soluble solids（TSS）and titratable acid（TA）.To further understand the anti-browning and quality control mechanism of CP/HC treatment,the inhibition mechanism of CP/HC treatment on PPO was explored.The results showed that compared to CP and HC treatment,CP/HC treatment had a better passivation effect on PPO activity,and the inhibition rate increased by more than 20%.Further exploration found that honey had a non-competitive inhibition on PPO,and honey could reduce brown products to form quinones in browning reaction and continue to reduce to colorless substrates.In addition,there was a strong positive correlation between the PPO activity inhibition rate and the free radical clearance rate,protein content,and total phenol content（correlation coefficient r>0.6）,indicating that the anti-browning ability of honey was closely related to its antioxidant substances and antioxidant capacity.After the extraction and reduction of polyphenols and protein substances in honey,it was confirmed that the protein and polyphenols were the important material basis for honey to play an anti-browning role.On the other hand,the experimental results of PPO molecular structure showed that the fluorescence intensity of PPO decreased with the prolongation of CP treatment time,and the maximum fluorescence emission wavelength shifted to 3.6 nm.The aromatic amino acids were modified by the active components of CP,and the fluorescence intensity quenching was closely related to the change of PPO activity（r=0.9751）,indicating that the change of PPO activity was related to the change of its tertiary structure.In addition,CP processing changes the secondary structure proportion of PPO,theα-spiral ratio decreased by 8.6%,theβ-Collapse and theβ-The corner ratio increased by 3.4%and 3.6%respectively,and the overall transition to irregularity and instability led to the decrease of enzyme activity,but CP treatment did not cause the decomposition of PPO subunit peptide chain.TEM images showed that PPO particles form large aggregates,which might prevent phenolic substrates from entering the PPO active center for the catalytic reaction.