A variety of mechanisms is responsible for product off-flavors.

Off-flavors can occur in just about any type of product. For instance, because milk is rather bland-tasting, extremely low levels of certain organic chemicals can lead to the onset of off-flavors. Yet another concern regarding dairy products involves lactose, protein and milkfat. While these may be good nutrients for human consumption, they also are good nutrients for bacteria.

When off-flavors appear in a product, we should address several issues. It is important to determine the mechanisms of off-flavor formation. It is also important to find out whether it is an isolated problem caused by a single case of product abuse by a customer, or if an off-flavor is the result of processing, ingredient, packaging or microbial problems involving an entire production run. We must find out whether it requires product to be recalled.

More significantly, does the off-flavor indicate a problem that may evolve into a public health threat? It will be important to see if the same type of off-flavor has occurred in previous products and whether future events can be avoided.

A variety of sources and classes of off-flavors occurs in products. It is important to identify the chemicals involved in the off-flavors and their mechanism of formation. In this manner, the source of the off-flavor can be eliminated. For instance, light-induced off-flavors are the most common milk flavor defect. They have two distinct components. Initially, a burnt active sunlight flavor evolves and predominates for two to three days. Degrading sulfur-containing amino acids found in whey proteins are probably responsible for this light-activated flavor.

And milk isn't the only dairy product that's susceptible to this flavor. Vanilla ice cream may develop a strong putrid flavor when it is distributed.

On another front, the presence of iron, copper and nickel can significantly accelerate lipid oxidation reactions and generate strong oxidation off-flavors. And acid, malty, fruity, bitter, stale, putrid and unclean off-flavors have been linked to contamination of a product by bacteria. Various off-flavors are produced as metabolites during a bacteria's growth phase. Contamination of milk after it is pasteurized can cause off-flavors too.

When you overheat milk, it can produce cooked off-flavors. These are caused by methyl ketones produced from the thermal degradation of ketoacids in milkfat triglycerides and by different sulfur compounds. When dairy processors forget to flush processing lines that have been cleaned with sanitizers, it's not unusual for small amounts of sanitizer to contaminate the first few bottles of milk filled on a processing line. This occurs because sanitizers are powerful oxidizing agents.

Further information. Amy Skovsende, Dairy Management Inc., 10255 W. Higgins Rd., Suite 900, Rosemont, IL 60018; phone: [phone]; fax: [phone]; URL: www.extraordinarydairy.com.