Hydrolytic rancidity or lipolysis is caused by the release of free fatty acids from the glycerol backbone of triglycerides. The reaction is catalyzed by the lipase enzyme, which can be a native milk lipoprotein lipase or can originate from bacterial sources. Triglycerides are generally protected from lipase activity as long as the milk-fat globule remains intact. However, damage to the globule will lead to rapid lipolysis because lipase, which is situated on the surface of the globule, can access the triglycerides. Therefore, precautions must be taken to prevent damage to the milkfat globule until pasteurization, which denatures most types of lipase. This means that raw milk/cream must be pasteurized before or immediately after homogenization to assure denaturation of lipase. Likewise, it is strongly recommended never to recycle pasteurized milk/cream back into raw milk/cream storage, which is essentially an issue of rework handling. Cream from poor-quality raw milk can also develop rancid off-flavors during storage because some bacterial lipases are quite heat stable and do not denature during pasteurization.
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