When I was asked to write on "Aspects of Shelf-life," I was uncertain how to proceed. The factors that limit shelf-life and means to extend shelf-life are at once complex and simple. So, I will begin and end this piece with the same advice: "To maximize shelf-life, pay close attention to the quality of the incoming milk and other ingredients, product formulation, the selection of starter culture, the sanitation program in your plant and maintain the appropriate low-storage temperature."

What is shelf-life?

Shelf-life is generally defined as the length of time a product remains acceptable for consumption by consumers. For some dairy products, shelf-life is mandated by regulation at the state or even municipal level. However, in most cases, shelf-life is determined by the manufacturer and is generally indicated to the consumer as a "sell by" or "best if used by" date.

What limits the shelf-life?

In fresh products, such as cultured dairy products, changes brought about by the microorganisms and by chemically induced changes. Both of these changes are affected by several factors. These factors can be intrinsic (internal) to the food, such as pH, chemical composition, water activity, presence of inhibitors and oxidative state (Eh) and those that are extrinsic (or outside the food), such as temperature, gaseous atmosphere (primarily level of oxygen) and relative humidity. An additional factor, particularly important in fresh products, is post-pasteurization contamination with spoilage microorganisms, which can occur during manufacture, transport and filling operations. The combination of level of post-pasteurization contamination, intrinsic factors present in the food, the extrinsic conditions to which the food is exposed and the manner in which product is physically handled will ultimately dictate final product shelf-life.

Chemical changes in cultured dairy products

A variety of chemical changes may occur during refrigerated storage of cultured dairy products. Many will impact flavor. For example, fats may become oxidized (when ions of copper or iron are present) or rancid (through the action of lipase enzymes). When exposed to light, products can develop a light-induced flavor. Proteases (enzymes that degrade protein) from the milk or produced by starter culture may lead to production of bitter peptides. Other changes can influence the finished product texture. For example, syneresis or "wheying off" in products such as yogurt, buttermilk and sour cream is caused by contraction of the casein gel. This can result from improper formulation (i.e., too low a milk solids not fat level or inadequate stabilization), improper heat treatment or by continued development of lactic acid by the starter culture (a microbiological issue).

Microbiological changes in cultured products

Microbiological issues in cultured products are of two general types, those related to starter culture and those that result from spoilage microorganisms. The primary issue with starter culture is metabolic activity after the completion of fermentation. This is often termed "post-fermentation acidification." This situation leads to an increase in lactic acid contained in the product which in turn results in a decrease in pH leading to a change in flavor and syneresis.

The second category of microbial problems are those brought about by post-pasteurization contaminants. Common defects associated with these microorganisms include slime formation, changes in color, production of gas, development of "unclean" or "yeasty" flavors, and the visible presence of mold on the surface of products. In addition, some spoilage microorganisms produce diacetyl reductase, an enzyme that converts the buttery-flavored diacetyl to a flavorless compound.

Extending/maximizing shelf-life of cultured products

To maximize shelf-life in cultured dairy products, begin, as with all dairy foods, with selection of good quality milk. It is axiomatic that you "can't make milk any better than it was when it arrived at the plant." Problems with raw milk quality, while important, are beyond the scope of this brief piece. However, assuming good milk quality and adequate pasteurization conditions, there are several strategies that can be employed.

To deal with chemical limitations to shelf-life, minimize the time between milk reception and pasteurization. Doing so will limit growth of psychrotrophic bacteria, which can produce proteases and lipases that will degrade quality even after they are destroyed by pasteurization. To prevent oxidation, remove sources of iron or copper. While this has been accomplished in most plants, bad welds or inadequate repairs can occur. Light-induced flavor can be avoided by packaging in light-blocking materials and minimizing exposure to light during storage.

Discussions with professionals in the cultured products industry reveal the biggest challenge to extending shelf-life is control of microbiological issues. While judicious selection of starter culture systems and proper cooling after manufacture and during storage will minimize problems with post-fermentation acidification, this issue persists.

All cultured dairy products, with the exception of cottage cheese, have a low pH, which retards the growth of spoilage bacteria and selects for growth of yeasts and molds which are more acid-tolerant. Control of post-pasteurization contamination begins with sanitation. Have an excellent environmental sanitation program to avoid contamination of the product following fermentation. Aerosols, produced by spraying floors while the vat is emptying or near the filler when it is operating, can lead to increased contamination with spoilage organisms.

In addition to control of temperature, which is of primary importance, the yeasts and molds that are the primary actors in limiting shelf-life in most cultured products can be limited by modification of intrinsic or extrinsic parameters. For example, where permissible, the addition of the preservatives potassium sorbate or natamycin (intrinsic factors) can limit growth. Many companies are now modifying the gaseous atmosphere (extrinsic factor) by flushing packages with carbon dioxide or nitrogen to limit growth of spoilage organisms including yeasts, molds and pseudomonads.

There are a variety of chemical and microbiological issues to consider when discussing shelf-life. Details related to starter culture selection, formulation, processing, fermentation, and other specifics vary from product to product. Again, my advice is: "To maximize shelf-life, pay close attention to the quality of the incoming milk and other ingredients, product formulation, the selection of starter culture, the sanitation program in your plant and maintain the appropriate low-storage temperature."