Continuous-flow high-pressure throttling inactivates bacteria in soymilk.

There has been a long-felt need for a nonthermal process that can effectively and economically destroy microorganisms in a continuously or pulsed flowing liquid. It might also be a process that improves the functional properties of complex fluids, such as food products, without the use of heat.

High-pressure throttling (HPT) uses high pressure to continuously throttle liquids from an elevated pressure, typically at least 200 MPa, through one or a multiplicity of constrictions to a much reduced pressure, typically atmospheric pressure, in order to inactivate microorganisms and improve the functional properties of the liquid.

Researchers at the University of Georgia have learned how to inactivate C. sporogenes PA 3679 ATCC 7955 in soymilk using continuous-flow HPT. During this process, the temperature of soymilk increases due to instantaneous pressure release. Additional heat was supplied by a heat exchanger in order to achieve a set temperature.

The soymilk was immediately cooled to less than 40 C after a short preset hold time. The researchers reported a significant increase in the heat resistance in C. sporogenes spores when heated in soymilk compared to 0.1% peptone water. Continuous-flow HPT from 207 MPa or 276 MPa to atmospheric pressure reduced the microbial populations in inoculated soymilk up to 6 log cycles when the holding times were 10.4, 15.6 and 20.8 seconds and the process temperatures were 85, 121, 133 and 145 C, respectively.

The sporicidal effect of the process increased as the operating pressure, time and temperature increased. More injured spores were found at 207 MPa than at 276 MPa, indicating that lower pressure caused cell injury, whereas high pressure caused cell death.

High hydrostatic pressure could be used to satisfy consumer demand for fresh-like, minimally processed shelf-stable products. Many other alternative nonthermal food processing techniques are being studied and used but among all, high-pressure processing has been the most promising, according to the scientists. The HPT system commercially pasteurizes the soymilk in seconds without any viable microorganisms. Time and temperature are key factors in inactivating microorganisms even under high pressure.

Further information. Rakesh K. Singh, Department of Food Science and Technology, University of Georgia, Athens, GA 30602; phone: [phone]; email: [e-mail].