The concern about shelf life is a reality for every food product. Most companies would like to add some time to the current shelf life while maintaining channel flexibility and acceptable sensory properties.

Extending shelf life also means reducing food waste, which is a goal globally. Cultured dairy products such as yogurt, sour cream, kefir, quark and cottage cheese typically have a pH below 4.6. This low pH provides a level of safety against pathogen survival or growth, but it doesn’t protect against yeast and mold growth.

Yeast and molds will grow at refrigeration temperatures. They can assimilate antimicrobial organic acids such as lactic acid. They can tolerate high salt concentrations and create loss of product quality through proteolytic and lipolytic activity.

Yeast and mold prevention

The first lesson in preventing yeast and mold growth is to make a product that has very low numbers of yeast and mold. Good Manufacturing Practices (GMPs) are key.

Yogurts produced under GMP conditions should contain < 10 yeast colony-forming units (CFU)/gram, while visible spoilage will be present at >105-106 CFU/gram. Mold levels should be <1 spore/gram, but levels of 1-5/grams can create issues by the end of shelf life.

How do yeast and mold get into cultured dairy products in the first place? Unfortunately, yeast and molds are everywhere in the environment, and mold spores are especially hardy and prolific. In a dairy plant, yeast and molds can originate from packaging material, ingredients such as fruit, water used as an ingredient or in processing, employees and lack of a good cold chain, just to name a few sources.

Ingredients that slow growth

Consumers could also contaminate packages after they are open. So manufacturers can also use ingredients that are designed to slow down growth after the product is packaged. But some of the most effective ingredients would not be considered clean label by most consumers.

Sorbate ingredients such as potassium sorbate have been used quite effectively for a long time in cultured dairy products. Sorbate is fungistatic, meaning it inhibits growth. Natamycin is a polyene macrolide antibiotic produced by some Streptomyces spp. It is fungicidal, meaning it kills fungi versus just their slowing growth. Both ingredients are effective at low levels of addition.

Many companies have introduced clean-label ingredients that use bioprotection to extend shelf life. These microbial food cultures are fungistatic, so they delay growth of unwanted contaminants such as yeast and mold. Lactic acid bacteria have GRAS (generally recognized as safe) status and are currently being used to help extend the shelf life of cultured dairy products. They are added as a part of product fermentation, as live cultures or fermentates.

Most of the cultures with high fungal activity are from the genus Lactobacillus, but Lactococcus, Leuconostoc, Pedicoccus and Propionobacterium cultures are also effective. For the cultures to work, they need to be a part of the fermentation process and stay alive during the product shelf life. They work through mechanisms that produce natural organic compounds such as lactate, acetate, peptides and diacetyl; by competing for nutrients and space with fungal contaminants; and through cell-to-cell communication. Live cultures can be added to cottage cheese dressing (which is not fermented) and will still exert their competition over spoilage organisms by their sheer numbers and by producing some inhibitory metabolites. 

Fermentates are a “pre-fermented” ingredient wherein the primary and secondary metabolites have already been produced. There are no viable starter organisms present. They should be selected to have minimum impact on sensory characteristics over the shelf life of the finished product.

How much more shelf life is attainable can be determined by testing these ingredients over a series of production runs under controlled conditions and storing them under specific temperatures and times. Some of these bioprotectant ingredients do contribute flavor, so it is important to evaluate not only microbial changes during shelf-life studies, but also flavor changes.