Chemical and biological modification of anhydrous milkfat.

摘要

Multiple fines of evidence indicate that saturated fatty acids and cholesterol raise plasma cholesterol levels. Several methods for improving the nutritional properties of milkfat have been developed over the past few years. In this study, two processes were developed to reduce the hypercholesterolemic effect of milkfat. The first process was based on converting the hypercholesterolemic myristic acid to myristoleic acid using the Δ9-desaturase system of hen hepatic microsomes. Desaturation of myristoyl-CoA to myristoleoyl-CoA was maximal at pH 7.4, linear with time up to 10min, and proportional to protein concentrations. The initial velocity was linear with substrate concentrations between 13 and to 200μM. The desaturation of myristoyl-CoA was stimulated by bovine serum albumin and reduced by cytoplasmic proteins. Direct desaturation of free myristic acid by hen liver microsomal Δ9-desaturase without prior activation to myristoyl-CoA by the addition of adenosine triphosphate (ATP) and CoA was observed when the incubation medium was mixed at mixing speeds greater than 250 rpm in the presence of fatty acid-binding proteins (FABP). Direct desaturation was linear with time, proportional to the microsomal protein concentrations, and maximal at pH 7.9. The greatest direct desaturation rate was observed at a mixing speed of 500 rpm in the presence of FABP. As the Δ9-desaturase is inactive on triacylglycerol substrates, the application of the desaturation process on milkfat necessitates a lipase-catalyzed hydrolysis of milkfat triacylglycerols to liberate the free fatty acids (FFAs) prior to their desaturation, followed by re-esterification the FFAs. They would not all return to their original positions in the triacylglycerol moiety, hence the reaction of the final material to milkfat would be somewhat tenuous.; The second process is based on reducing cholesterol and FFAs in anhydrous milkfat through fat blending after the cholesterol was removed. The removal of cholesterol is based on the specific affinity of β-cyclodextrin for free cholesterol and cholesterol esters. The process, optimized by response surface methodology, consists of a sequence of steps: refining, dilution with distilled water, heating, mixing, and centrifugation. The process is efficient since the cholesterol was almost completely (>98%) removed. Free fatty acid reduction was greater than 92.40%, which was more than 45% greater than the prior-art processes. Gerbil feeding studies showed that modified milkfat (i.e., cholesterol-reduced milkfat) reduced serum total cholesterol by 28% and triacylglycerols by 47%. Also, modified milk fat could be used for whole milk reconstitution and consequently the whole spectrum of dairy products such as ice cream, yogurt, and cheese.