Effect Of Pre-Cutting L-Cysteine Treatment On The Browning Of Fresh-Cut Apples

Fresh-cut apples are prone to browning,which leads to the decrease of nutritional value and commodity value.It is of great significance to explore a new method to efficiently prevent browning.L-cysteine is a highly effective antibrowning agent.Treatment with L-cysteine after cutting can inhibit PPO activity,improve antioxidant capacity and inhibit browning of freshcut apples.However,L-cysteine treatment after cutting can affect sensory quality of fresh-cut apples.But pre-cutting treatment has high safety and does not affect the product flavor.There is no report on the effect of pre-cutting L-cysteine treatment on the browning of fresh-cut apple.In this study,whole fruit was treated with L-cysteine before cutting(referred to as pre-cutting treatment),the optimal conditions of L-cysteine were screened by pre-cutting treatment,and the effects of pre-cutting L-cysteine treatment on the activity of enzymes related to browning,antioxidant capacity,contents of individual phenol and free amino acid,and gene expression of enzymes related to browning were analyzed.The mechanism inhibiting the browning of freshcut apple by L-cysteine treatment was preliminally investigated at the physiological and molecular levels.The results are as follows:(1)By comparing various physical and chemical methods,it was found that L-cysteine treatment before cutting had a better effect on inhibiting the browning of fresh-cut apples.The single factor experiment showed that the optimal conditions of pre-treatment with L-cysteine were as follows: soaking concentration 1%,soaking time 0.25 h.Under these conditions,freshcut apples treated with L-cysteine could delay browning and maintain good quality at room temperature for 48 h,and had no adverse effect on the flavor of fresh-cut apples,while freshcut apples in the untreated and water treated groups showed obvious browning at 2 h after cutting.(2)Pre-treatment with L-cysteine had significant effects on enzymes,phenols and antioxidants of fresh-cut apples.The results showed that pretreatment with L-cysteine significantly inhibited PPO activity and increased PAL activity in fresh-cut apple peel and flesh,resulting in the accumulation of total phenol and individual phenol,especially the contents of p-coumaric acid and hypericin in peel,and the contents of chlorogenic acid and epicatechin in flesh.Pre-treatment with L-cysteine could also increase SOD and CAT activities of fresh-cut apple peel and flesh,improve DPPH radical scavenging ability of fresh-cut apple,and reduce the degree of membrane lipid peroxidation.Moreover,the effect of pre-treatment with Lcysteine was independent of the p H of the solution.Therefore,pre-treatment with L-cysteine can inhibit the browning of fresh-cut apples by inhibiting PPO activity,increasing phenolic content and antioxidant capacity.(3)Pre-treatment with L-cysteine significantly increased the contents of various free amino acids.Among them,the contents of cysteine in peel increased most obviously,while the contents of cysteine,aspartic acid and glutamic acid in flesh increased significantly,and the increase rate was higher than that of proline,threonine,lysine,serine,leucine,glycine and alanine.The experiment of differential amino acid addition showed that the addition inhibited the browning of apple flesh in the untreated group.This showed that the reduction of free amino acid content in fresh-cut apples during storage had an important effect on the occurrence of browning.Therefore,pre-treatment with L-cysteine can also inhibit the browning of fresh-cut apples by promoting the synthesis of various free amino acids.(4)Quantitative PCR results showed that the expression of PPO gene(Md PPO6 and Md PPO10)in peel and flesh was significantly down-regulated after pretreatment with Lcysteine,which was consistent with the activity of PPO,indicating that L-cysteine could reduce the activity of PPO by inhibiting the expression of PPO related genes.The expressions of PAL,SOD and CAT genes(Md PAL,Md SOD,Md CAT6 and Md CAT13)in peel were also significantly up-regulated by L-cysteine treatment before cutting,but the up-regulated trend was not obvious in flesh.