High pressures extend shelf life of soymilk.

Consumers demand additive-free, shelf-stable foods that offer good nutritional benefits and sensory quality. High-pressure processing (HPP) is an alternative to conventional thermal processing. Using HPP, scientists at Iowa State University investigated how processing parameters, pressure levels, dwell times and initial temperatures would impact the microbial attributes of soymilk and protein stability during the refrigerated aerobic and anaerobic storage of the product. Their work indicates that HPP can be effective in extending the shelf life of refrigerated soymilk without employing sterilization temperatures.

The researchers processed soymilk samples at 400 MPa, 500 MPa and 600 MPa for 1 minute and 5 minutes at 25 C and 75 C. They determined protein stability, total bacterial counts, psychrotroph counts, Enterobacteriaceae counts and injured cell counts throughout 28 days of aerobic and anaerobic storage of the samples at 4.4 C.

There was a significant difference between the untreated and treated samples for all the parameters tested. Psychrotroph counts and Enterobacteriaceae counts declined more after the treatments than did the total bacterial counts. The investigators did not observe any growth of Enterobacteriaceae throughout the storage period.

There was no significant difference between the 1-minute and 5-minute dwell times, except for the injured cell count and Enterobacteriaceae count during anaerobic storage, and for total bacterial counts during aerobic storage. When compared to HPP at 25 C, HPP at 75 C had a significant impact on protein stability and microbial counts except for Enterobacteriaceae during anaerobic storage. Pressures of 500 MPa and 600 MPa had a significant impact on protein stability, injured cell count, Enterobacteriaceae and total bacterial counts during aerobic storage.

The untreated soymilk reached the spoilage level of [10.sup.6] CFU per mL after seven days in storage. Pressurized samples underwent an extension of their shelf life by one to three weeks, depending on the treatment. The most effective treatment involved applying HPP at 75 C, which reduced the microbial population and extended the protein stability of soymilk.

Further information. Aubrey Mendonca, Department of Food Science and Human Nutrition, 3399 Food Sciences Building, Iowa State University, Ames, IA 50011; phone: 515-294-2950; fax: 515-294-8181; email: amendon@iastate.edu.

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