Multi-Enzymes Treatment To Enhance The Flavor Of Strong Fragrant Rapeseed Oil

Rapeseed oil is one of most consumed edible oils in China due to its unique flavor and rich nutrition,which is highly favored by consumers.Presently,at least three types of commercial rapeseed oils,classified as strong,mellow and light fragrant rapeseed oils（SFRO,MFRO and LFRO）,are produced and consumed in China,based on odor attributes originated from pressing and refining steps.However,the boundaries of fragrant types and grades are unclear,making it difficult to to qualitatively and quantitatively distinguish these three type oil products.Among them,SFRO accounts for more than 30%of Chinese rapeseed oil market and shows a dark-brown color appearance and a unique flavor while containing high contents of tocopherols,sterols and other nutrients,which are the main factors attracting the product quality and consumers’preference.To comprehensively and systematically compare the physicochemical characteristics of SFRO,MFRO and LFRO,and enhance the flavor of strong fragrant rapeseed oil,the present study aimed to:1)determine and characterize the profiles of physical and chemical components,key aroma compounds,and nutrients and their concentrations in 24commercial rapeseed oils in Chinese markets to establish a multi-dimensional and quantitative method to distinguish these SFRO,MFRO and LFRO based on comparison of their aromas,physical/chemical/nutrient components,and tastes;2)enhance the flavor of SFROs by enzymatic treatment including amylase,cellulose,hemicellulose,neutral protease and their combinations,and 3)find the potential reaction process and mechanism to enhance the flavor of SFROs using monosaccharide-amino acid reaction to simulate the Maillard reaction process in vitro.The outputs of present study would provide a theoretical and practical reference for developing a series of strong fragrant rapeseed oil products.The main research results obtained are as follows:（1）The profiles of physical and chemical components,key aroma compounds,and nutrients and their concentrations in 24 commercial SFROs,MFROs and LFROs in Chinese markets were determined and characterized.The results showed that the acid value,peroxide value,and red value of SFROs were the highest,followed by MFROs,and LFROs were the lowest.A total of20 fatty acids were identified in all fragrant rapeseed oils,with high oleic acid content（57.27%-63.94%）,followed by linoleic acid（17.94%-25.27%）and linolenic acid（4.98%-9.05%）.The SFROs exhibited highest contents ofα-,γ-andδ-tocopherol（537.62-875.81 mg/kg）,followed by MFROs（444.85-647.03 mg/kg）and LFROs（l327.79-431.37 mg/kg）.The SFROs also contained the highest concentrations of total sterol of over 4444.28-5057.40 mg/kg and coenzyme Q₁₀ of244.45-559.74 mg/kg,followed by MFROs 3555.43-3964.89 mg/kg and 204.84-396.46 mg/kg,and LFROs 2488.58-3189.85 mg/kg and 73.51-233.52 mg/kg,respectively.The SFROs had a strong nutty,roasted fragrance flavor and slight bitter flavor.The MFROs showed a strong typical rapeseed flavor and a soft taste while the light fragrant rapeseed oil only showed a light taste and flavor.PCA and cluster analysis indicated that the flavor of LFROs mainly come from aldehydes and ketones,and that of MFROs from degradation products of glucosinolates,aldehydes,ketones,esters,alcohols,and acid compounds.The degradation products of glucosinolates,pyrazines,furans,phenols,ketones,and aldehydes contribute significantly to the SFROs.Among them,pyrazines could be classified as the key volatile components endowing the unique flavor of SFROs.（2）Effects of amylase,cellulose,hemicellulose,and neutral protease,and their combinations-treated rapeseeds on the physical/chemical quality and flavor of SFROs were well investigated.Results showed that the acid value,peroxide value and moisture of all the SFROs from the enzyme-treated rapeseeds met the criteria required by the Chinese National Standard Rapeseed Oil（GB/T 1536-2021）.The combined treatment of neutral protease and amylase resulted in the highest percentage of unsaturated fat acids（93.30%）.All experimental group contained high contents ofα-,γ-andδ-tocopherol（456.46-594.35 mg/kg）,total sterol（3273.23-4523.46 mg/kg）,and coenzyme Q₁₀（28.43-46.75 mg/kg）.Sensory evaluation results showed that the neutral protease+amylase treatment group showed a distinguishing strong nutty and a rich roasted fragrance and strong sweet flavor with highest overall score.The volatile compounds offering the flavor mainly were originated from Maillard reaction products such as pyrazines and furans with the highest contents,compared to those of other experimental groups.（3）The changes of monosaccharide and amino acid contents in the enzymatically-treated rapeseeds before and after roasting were determined,and on this basis,monosaccharide-amino acid reaction was further used to simulate the Maillard reaction in vitro to find the potential reaction process and mechanism for enhancing the flavor of SFROs.Results showed that monosaccharides such as glucose and xylose and amino acids including Leu,Lys,Val,Phe,Ala,Tyr and Arg released by enzymatical treatment are the main substrates of Maillard reaction.After roasting,the total amount of glucose and free amino acids in the neutral protease+amylase treated rapeseeds significantly decreased from 8.42 g/kg to 2.04 g/kg and from 128.91 mg/kg,to47.39 mg/kg,respectively.After roasted at 160℃and 20 min,the neutral protease+amylase treated rapeseeds had the darkest and reddest color（42.55 of L*value and 7.18 of a*value）,and the highest browning index of 1.96 and 5-hydroxymethylfurfural content of 1.36 mg/kg.In vitro simulation experiments of Maillard reaction showed that 57 and 47 volatile compounds were synthesized from the reaction of glucose,xylose and amino acids,which mainly consisted of pyrazines and furans and were consistent with SFROs from enzymatically treated rapeseeds.Among them,glucose-lysine reaction products contained the highest content（27.45%）of pyrazines,suggesting glucose and lysine in enzymatically treated rapeseeds were the key substrates for Maillard reaction during roasting.