Dairy products have been a key component of a healthy diet and a source of many nutrients. The industry has employed time-tested and validated thermal pasteurization and sterilization technologies to ensure consumer confidence. However, changes in consumers’ health and wellness-oriented lifestyles and their perceptions about nutrition have redefined the desired attributes in the processed foods they would like to have.
Assurance of microbiological safety is no longer sufficient. Consumers would like to retain attributes like freshness, nutrients and few preservatives. Food engineers and scientists have been evaluating various alternative preservation methods such as high-pressure processing (HPP), that not only ensure microbiological safety but also preserve product quality and nutrition. This article provides an overview of the basic principles and potential applications of high-pressure treatment of various dairy foods.
High-pressure processing explained
HPP can be industrially implemented as a batch, semi-continuous or continuous process. High pressure pasteurization typically involves application of elevated pressures (60,000 to 87,000 psi) at nearambient or chilled temperatures to a prepackaged food. Several studies show the microbiological quality of pressure-treated milk is comparable to that of thermally pasteurized milk (72C for 15 seconds). The treatment could effectively inactivate a variety of food pathogenic and spoilage vegetative bacteria, yeast and viruses. Gram-negative microorganisms showed to be more sensitive to HPP than Grampositive organisms.
In a 2004 report, the National Advisory Committee on Microbiological Criteria for Foods recommended that federal regulators redefine pasteurization as any process treatment (not just heat treatment alone) or combination thereof, that is applied to food to reduce the most resistant microorganisms of public health significance to a level that is not likely to present a public health risk under normal conditions of distribution and storage.
It is worth noting that like conventional thermal pasteurization, pasteurized food products via HPP still require additional barriers (such as refrigerated storage) for product stability during subsequent handling and distribution. Like heat, pressure is a physical process that can be used to preserve both liquid and solid foods, and is readily accepted by the consumers. Pressure-pasteurized value-added products, including deli meats, sea food, fruit juices, vegetable products, guacamole, salads and salsa are readily available in supermarkets in North America, Europe and Asia-Pacific.
There is no well-established correlation between pressure and heat resistance of microorganisms. However, like thermal processing, spores also exhibit high resistance to pressure treatment at ambient temperatures. The combination of pressure-heat treatment (400- 600 MPa, 90-120C) over a short holding time (3-10 minutes) is needed to inactivate bacterial spores. The process is referred to as pressure-assisted thermal processing (PATP). Note that rapid temperature increase during pressurization and subsequent cooling (to initial temperature) upon decompression is a unique PATP benefit.
Recently, a novel pressure-ohmic thermal sterilization (POTS) technology has been developed at The Ohio State University that combines beneficial effects of pressure treatment along with ohmic heating in sterilization of food material.
In 2009, the Food and Drug Administration issued no objection to an industrial petition for preserving a lowacid mashed potato product by PATP. As of today, there are no commercial lowacid products distributed using this preservation approach.
In a more recent development, it may be possible to pasteurize or sterilize liquid foods through a “continuous” high-pressure homogenization process. In this process, liquid foods are pressurized to modest pressures (about 300 MPa, with or without heat addition) and then depressurized using principles of homogenization and subsequently aseptically packaged. At the moment, there are no commercial high-pressure homogenized products in the market.
