Thermal processing, such as pasteurization, has been the standard method to extend the shelf-life of dairy products. However, the high temperatures (70–120°C) employed can damage and/or cause structural modifications to the proteins within the product, leading to noticeable changes in sensory characteristics.
The study examined a low temperature, short time (LTST) process for pasteurization, in which low heat and variable pressure were utilized to aid in the pasteurization of a fluid milk product.
Purdue associate professor in the department of food science, Bruce Applegate, Ph.D., and collaborators from Purdue University and the University of Tennessee published their findings in the journal SpringerPlus, where they show that increasing the temperature of milk by 10°C for less than a second eliminates more than 99% of the bacteria left behind after pasteurization.
"It's an add-on to pasteurization, but it can add shelf life of up to five, six or seven weeks to cold milk," Applegate said.
Low heat + low pressure = longer shelf life
To employ the LTST method, scientists sprayed tiny droplets of pasteurized milk, inoculated with Lactobacillus and Pseudomonas bacteria, through a heated, pressurized chamber. While in the chamber, temperatures were rapidly raised and lowered approximately 10°C but still below the 70°C threshold needed for pasteurization.
The treatment lowered bacterial levels of L. fermentum and P. fluorescens Migula, below detection limits, and extended the milk’s shelf life to up to 63 days.
"With the treatment, you're taking out almost everything," Applegate said. "Whatever does survive is at such a low level that it takes much longer for it to multiply to a point at which it damages the quality of the milk."
No detectable sensory differences
One hundred panelists were surveyed for a sensory evaluation of two pasteurized milk samples: one using traditional pasteurization and the second using the LTST pasteurization method. None of the panelists were able to detect any difference in color, aroma, taste, or aftertaste between the products, and in some cases even preferred samples of LTST pasteurized fluid milk.
The LTST method of pasteurization is also promising in regards to its source of energy used.
Phillip Myer, an assistant professor of animal science at the University of Tennessee and a co-author of the paper, said the process uses the heat already necessary for pasteurization to rapidly heat milk droplets.
"The process significantly reduces the amount of bacteria present, and it doesn't add any extra energy to the system," Myer said.
Myer also said the technology has the potential to reduce waste and allow milk to reach distant locations due to the longer shelf life the LTST process provides.
Not quite ready for industry applications
While the LTST pasteurization process demonstrated reduction in microbial load, prolonged shelf life, and minimal-to-no loss of sensorial properties, there are still several parameters of the process that need to be examined, the study stated.
For example, the study authors seek to use more thermally robust model microorganisms will help to better assess process efficacy.
“Systematic means to better limit product contamination, beyond technological parameters, would facilitate the application of LTST amendment beyond the laboratory to potential industry applications,” the study said.
Currently, FDA regulations require that raw milk be pasteurized. To that end, the MST unit can be connected in-line with a standard pasteurizer to enhance product shelf-life via greater log reduction of spoilage organisms.