Construction And Application Of Enzymatic Hydrolysis System For Improving Milk Fat Flavor

Dairy products are indispensable parts of people’s diet,which contain a variety of nutrients.A considerable number of consumers are worried about excessive intake of full-fat dairy products,which would lead to obesity,elevated cholesterol or other health problems,as long as they pay more attention to health.So they prefer to buy low-fat or skimmed dairy products.However,skimmed milk has disadvantages such as tastelessness and weak flavor.Therefore,it is urgent to develop flavor-enhanced low-fat dairy products.Enzymatic hydrolysis of light cream,butter or other substrates could significantly increase the content of fatty acids,methyl ketones and lactones,thus improving the flavor of dairy.In this paper,crude lipoxygenase(lipoxygenase,LOX)was extracted from soybean,and its enzymatic properties were preliminarily explored.Using whipped cream and butter as substrates,a crude enzyme hydrolysis system of soybean LOX was constructed.The influence of amount,enzymatic hydrolysis temperature and enzymatic hydrolysis time on enzymatic hydrolysis products was determined,and the preparation process of milk flavor with enhanced milk flavor was determined,which provided a new idea and theoretical reference for the development of flavorenhanced low-fat products and the application of soybean LOX in food industry.The main research contents are as follows:In this paper,soybean LOX crude enzyme was first prepared by water extraction.Taking the enzyme activity as indexes,the optimum extraction conditions was determined as follows:the raw material were soybeans without degreasing treatment,the ratio of material to liquid was 1:10(g:L),the extraction temperature was 4℃ and the extraction time was 1 h,the obtained LOX crude enzyme activity was 563 U·mL-1.The characterization of LOX crude enzyme were initially investigated,and the effects of temperature,pH,metal ions and substrate pro-solvent on the enzyme activity were studied.The stability of crude enzyme was also studied.The results showed that the optimum pH of LOX crude enzyme was 7 and the optimum temperature was 40℃.The enzyme activity was enhanced by adding 2%methanol,dimethyl sulfoxide and ethanol,and the enhancement effect of dimethyl sulfoxide was the most obvious,and the enzyme activity reached 121.5%of the original;the enzyme activity was inhibited by adding 6%methanol,dimethyl sulfoxide and ethanol,and the inhibition effect of ethanol was the most obvious,with the enzyme activity of only 17.7%.The addition of 0.1 mol·L-1 Na+,K+,Ca2+,Mg2+,Mn2+can improve the activity of LOX crude enzyme,Mg2+has the best improvement effect.and the enzyme activity reached 132.8%of the original;the addition of 1.0 mol·L-1 K+,Ca2+,Mg2+,Fe2+,Mn2+and 0.1 mol·L-1 Fe2+could inhibit the activity of LOX crude enzyme,and the inhibition effect of 1.0 mol·L-1 Ca2+was the most obvious with the enzyme activity of only 24.5%.The molecular weight of LOX crude enzyme was about 95 kDa.The stability of LOX crude enzyme was poor.The enzyme activity decreased significantly after seven days of storage,which was only 27.6%,and there was obvious precipitation and stratification in the solution.The enzymatic effects of self-made LOX crude enzyme and six commercial enzymes on butter and light cream were compared.The types,relative contents of volatile compounds after enzymatic hydrolysis and sensory evaluation were used as reference indicators.The results showed that the self-made LOX crude enzyme had a good effect on enhancing the milk flavor.Principal component analysis was performed on the volatile compounds of enzymatic hydrolysis products.There was significant different between samples prepared by LOX crude enzyme and by other commercial enzymes.The optimum conditions for enzymatic hydrolysis of butter with LOX crude enzyme were determined,taking sensory evaluation score,lipolysis rate,acid value of enzymatic products and relative contents of volatile components as reference indicators,:The amount of enzyme added was 10%.The enzymolysis temperature was 40℃ and the enzymolysis time was 1 h.The main volatile components increased from 40 to 48 after the hydrolysis of butter with LOX crude enzyme.The contents of saturated fatty acids increased,especially the content of hexanoic,octanoic and butanoic acids.The content of ketones increased significantly,such as 2-nonanone,and 2-heptanone was produced.The contents of esters such as δ-decalactone and butyl caprolactone increased significantly;these substances have a fresh and creamy aroma.The principal component analysis of volatile compounds under different enzymatic hydrolysis conditions showed that the samples with optimum enzymatic hydrolysis conditions were far away from other samples.The product after enzymatic hydrolysis was modified by esterification,and it was determined that the optimum addition amount of ethanol was 2%.The content of esters after esterification was greatly increased,and the sensory evaluation score was higher.The application of enzymatic hydrolysis products in food was explored.Taking the fat content as the reference index,the maximum addition amount of the butter enzymatic hydrolysis product in the flavored milk was determined to be 0.91 g·L-1.The enzyme hydrolysis product was added to skim milk to obtain flavor-enhanced low-fat milk,and the results of sensory evaluation showed that the milk had a significantly stronger milk flavor than commercially low-fat milk.When the hydrolysis product was applied to baking foods,the sensory evaluation score showed that the flavor-enhanced cookies got stronger sweet smell and higher popularity than the ordinary ones.