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High pressure and heat optimize rheology of yogurt model.

High-pressure homogenization (HPH) is a nonthermal technology that has been used in making dairy products to reduce the ripening times of cheese or to improve the shelf life of a product by decreasing its microbial load.

Scientists at the University of Tennessee wanted to see what effect HPH alone and in combination with a thermal treatment would have on the manufacture of a model yogurt. Their research suggests that HPH in combination with a thermal treatment can be used to improve the rheological properties and stability of yogurt. This would reduce the need for additives.

In experiments, raw skim milk was subjected to HPH--ranging from 0 MPa to 350 MPa--and to a thermal process, at 90 C for 5 minutes, in four treatments: HPH; HPH followed by a thermal treatment; a thermal treatment followed by HPH; and a thermal treatment alone.

After all of the treatments, the researchers acidified milk using 3% glucono-d-lactone, an acidifier, to obtain a model yogurt. The scientists monitored certain rheological properties of the samples--G' and yield stress. They checked G' five minutes after adding glucono-d-lactone and kept on checking it for up to 110 minutes. Then they checked for yield stress for up to 20 minutes.

The investigators kept another set of samples at room temperature for 3 hours after glucono-d-lactone had been added in order to evaluate their whey-holding capacity. When HPH was applied, the yogurts showed lower G' values--about 18 Pa. Contrary to what was expected by the scientists, the models exhibited higher yield stress--about 78 Pa.

When the pressure increased up to 350 MPa, the whey-holding capacity after 25 minutes was 43%, compared to 22% at lower pressures and 25% when the treatments were combined. For those times when HPH was combined with a thermal treatment, increasing the pressure increased the G' values, making them 15 times higher than they were with just HPH.

When a thermal treatment was followed by HPH, higher G' values, about 296 Pa, were obtained. But again, contrary to what was expected, there was a lower yield stress, about 53 Pa.

Further information. Federico Harte, Department of Biosystems Engineering and Soil Science, The University of Tennessee, 2506 E. J. Chapman Dr., Knoxville, TN 37996; phone: 865-974-7265; fax: 865-974-4514; email:
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Publication:Emerging Food R&D Report
Date:Jun 1, 2009
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