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Inactivate plasmin using sonication.

Plasmin is the major native milk protease. Plasmin, plasminogen and plasminogen activators are associated with the casein micelles and milk fat globule membranes in milk. The activity of this enzyme is controlled by certain inhibitors, such as plasmin inhibitors or inhibitors of plasminogen activators, that reduce the activity of plasminogen activators so that plasminogen cannot be converted into plasmin. Most plasmin inhibitors are present in the serum phase of milk.

Plasmin is very heat-stable and causes the breakdown of milk protein in a wide variety of dairy products. This protein breakdown is desirable for some dairy processes and products, such as the ripening of cheddar and Swiss cheeses. But it is detrimental for others, such as the age gelation that occurs in ultra-high-temperature (UHT) milk. Plasmin survives the direct and indirect heating of pasteurization and UHT processes. Scientists at Purdue University wanted to learn how to better control plasmin activity. For example, a decrease in plasmin activity will delay the onset of age gelation in UHT milk.

Sonication is able to inactivate enzymes. However, its use to inactivate plasmin has not been fully investigated. The researchers' objective was to determine the effect of sonication on plasmin activity in a buffer system.

Solutions containing 5 mU per mL of plasmin in a modified tris buffer (pH 7.6, 0.1M NaCl) were subjected to sonication treatment using a commercial sonication system with 20 kHz frequency, a constant duty cycle, a 3-mm ultra-high-intensity tapered microtip and a variable amplitude capability. Plasmin activity was quantified by using colorimetric analysis with a p-nitroanaline-based substrate that is specific for plasmin activity.

The results showed that a 30-second treatment at 116-micron ultrasound amplitude decreased plasmin activity by 10%. An additional 60-second treatment at this amplitude decreased plasmin activity by 20% compared to a control sample that was not treated by ultrasound. A 15-second treatment at a 305-micron ultrasound amplitude decreased plasmin activity by 55%.

So it appears that ultrasound can decrease plasmin activity, and that sonication has potential as an alternative way to control plasmin activity in UHT milk.

Further information. Lisa Mauer, Whistler Center for Carbohydrate Research, Department of Food Science, Purdue University, 745 Agriculture Mall Dr., West Lafayette, IN 47907; phone: 765-494-9111; fax: 765-494-7953; email:
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Publication:Emerging Food R&D Report
Date:Feb 1, 2004
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