Making Fluid Milk Taste Better Longer

According to USDA, fluid milk accounted for 18.1percent (17.4 billion lbs.) of edible food lost by retailers, foodservice and consumers in 1995. (See Kantor et al., 1997 for more information). One major factor contributing to dairy product loss and reduced quality is spoilage of products by bacteria. Spoilage presents the dairy industry with a two-pronged problem: Direct economic losses from products removed from the distribution chain and long-term loss of consumers who have had an unpleasant encounter with dairy products and later refuse to willingly choose milk as a beverage.

The Milk Quality Improvement Program (MQIP) is a dairy farmer-funded research program at Cornell University (Ithaca, N.Y.) dedicated to improving dairy product quality with the ultimate goal of increasing consumer demand for and consumption of these products. This article describes recent program efforts to reduce bacterial spoilage of commercial fluid milk products, with the ultimate goal of extending the length of time that products taste good.

In the past decade, dairy plants in the Northeast region of the United States have made significant strides in extending the length of time that fluid products taste fresh by developing and implementing effective strategies for reducing or preventing entry of spoilage bacteria into pasteurized milk in the processing plant (See Carey et al., 2005). Elimination of these so-called “post-pasteurization” contaminants that can be present in a dairy processing plant is essential for extending the shelf-life of fluid products beyond 14 days. However, elimination of post-pasteurization contaminants from the processing environment has uncovered a new microbial hurdle for further extension of fluid product shelf-life.

A 2003 shelf-life study from the MQIP concluded that the most predominant organisms in New York State (NYS) fluid milk products with shelf-lives longer than 14 days were gram-positive spore-forming bacteria. Bacteria that can form spores (“sporeformers”) are capable of surviving extreme changes in environmental conditions, including exposure to acid, drying, heat, etc. One can think of a spore as an emergency survival kit for a bacterium. Fortunately, not all bacteria are capable of forming spores, so traditional processing strategies (e.g., milk pasteurization) will destroy bacteria of public health significance that may be present in raw milk. However, elimination of spore-forming bacteria capable of causing food product spoilage is technically challenging, and will require development and implementation of novel intervention strategies.

The presence of spore-forming bacteria in fluid milk products with shelf-lives less than 14 days has likely been previously masked by the presence of high numbers of typical “post-pasteurization” bacterial contaminants that are generally capable of growing quickly to high numbers in fluid milk products. Typical sources of post-pasteurization bacterial contaminants include improperly cleaned and sanitized milk pasteurization and packaging equipment. Growth of these organisms in milk products can result in production of consumer-detectable, objectionable off-flavors. Unfortunately, elimination of typical post-pasteurization contaminants by implementation of effective processing plant control measures has not resulted in elimination of all bacteria capable of spoiling pasteurized milk products. The recently identified group of Gram-positive, spore-forming organisms present in high numbers in pasteurized fluid milk products at more than 17 days post-processing is a significant threat to further fluid milk shelf-life extension due to the ability of these microbes to survive conventional high temperature short time (HTST) pasteurization processes typically used for manufacturing fluid milk products, as well as their ability to increase in numbers in milk products held at refrigeration temperatures (40 to 45ºF).

In March 2005, the MQIP initiated a study to better understand the microbial ecology and sources of spore-forming bacterial spoilers in the dairy processing environment. Two commercial NYS fluid milk processing plants with product shelf lives of 18 and 21 days were selected for the study. Both plants, which had successfully eliminated post-pasteurization contamination problems, were experiencing high bacterial numbers in fluid milk products approaching code date, thus preventing extension of product shelf-life beyond 18 or 21 days. The predominant organisms identified in the packaged milks on final day of shelf life were identified as gram-positive spore-forming Bacillus and Paenibacillus spp. These spore-forming microorganisms are commonly found in the farm environment. High numbers of sporeformers in raw milk are strongly correlated with soiled teats and dirty milking environments. Our current work points to raw milk as the probable source of these heat-resistant, cold tolerant bacteria in pasteurized milk products. This observation suggests the need to increase the focus on raw milk quality from the farm to the dairy plant for further extension of fluid product shelf-lives.