Hydrolytic attack, bacterial

Lipolysis is important from the standpoint of the qnality of milk and dairy products. The natural lipoprotein lipase in milk is the main agent in the lipolysis of raw milk. In dairy products, hydrolytic rancidity is primarily cansed by heat-stable lipases prodnced by psychrotrophic bacteria or by residnal native lipase that survived heat treatment. The milk lipase and most bacterial lipases preferentially attack the FA residnes in the terminal positions. Becanse milk fat contains relatively high proportions of short-chain FA, which are esterified in the in-3-position, lipolysis increases the level of these FA in free form. The short-chain FA are solnble in water and they may be present in dissociated form (salts) or associated form (acid form), depending on the pH of the aqueous phase of the milk product. The characteristic lipolytic flavors occur when the acids are present in the acid form. Some FA have low flavor thresholds, and even very small amonnts canse an unpleasant, rancid flavor of milk and dairy products. Only in some types of cheese (e.g., mold cheeses) is moderate lipolysis desirable for specific flavor. 

The following polyols are well established in the urethane adhesive field (i) polyesters, often preferred due to their combination of cohesive and adhesive properties (ii) polyethers, selected when hydrolytic stability and resistance to bacterial attack are predominant requirements (iii) poly(caprolactone) glycol (see Fig. 8.2), which offers a compromise in properties between the polyesters and polyethers and (iv) poly(butadiene)-based glycol (see Fig. 8.2). 

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