Examine the impact of high pressure on texture.

Improving food quality involves inactivating the microorganisms that cause food spoilage. Currently, the most widely used method for inactivating these bacteria, yeasts and molds is thermal processing. However, high pressures can also be effective. In addition, the impact of high-pressure processing on food itself is an area of commercial interest. High pressure may denature proteins, solidify lipids and break down biomembranes. It can alter the structure of proteins and muscle, as well as affect the gelatinization of starch.

Although high pressure has been commercialized as a preservation technique, it has potential for use as a tool for manipulating the texture of foodstuffs, according to an article recently published in Chemistry and Society.

Covalent bonds are little affected by pressure. The main effect of high pressure is to bring about changes in hydrophobic and electrostatic interactions. This has consequences for the secondary structures in proteins. Treating proteins with high pressures may cause key conformational changes, which impact functionality.

At sufficiently high pressures, proteins will unfold completely or partially, and at high concentrations they will precipitate or aggregate into a gel network. Combining high-pressure and variable-temperature treatments may produce unusual gels and control their properties over a very broad range.

Egg white can be gelled under high pressure just as easily as by the application of heat. However, the nature of the gel formed by using the two techniques is very different. The order and extent of bond breaking and formation are very different. Protein systems will only produce gels at high concentrations. At lower concentrations, the protein remains soluble but has different functionality. For example, whey protein is a very poor foaming agent, but pressure treatment of lactoglobulin, a major constituent of whey, makes it far more surface active and improves foam stability. Studies with gluten and soya have also shown the potential of high-pressure processing to generate a range of high-quality gluten-based vegetarian products.

High-pressure treatment may also improve the properties of dairy products, such as yogurt and cottage cheese, which are acid-induced gels. About 80% of milk proteins are caseins. These exist in large supramolecular aggregates-micelles-large enough to scatter light, giving milk its opaque appearance. High-pressure processing fragments the micelles, so products made from pressure-treated milk have increased rigidity, strength and resistance to whey separation.

Fluid yogurts produced by stirring gels made from pressure-processed milk are more viscous than their conventionally produced counterparts, giving a thicker, creamier product. This quality can be enhanced by adding fruit flavors or pieces, as with traditional yogurts.

Some research has entailed the pressure treatment of meat and fish. Pressures of 300 MPa or more cause the meat or fish to take on a cooked appearance, although the taste is unaffected. At lower pressures, however, enzymes can be activated to improve tenderness. In beef, pressures as low as 80 MPa to 100 MPa induce changes that improve color stability. Unlike fresh meats, the appearance of cured meats is not affected by pressure treatment, and the quality may be improved.

Further information. Stephanie Grant, U.K. High Pressure Club for Food Processing, Department of Food Science, The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX, Northern Ireland, U.K.; phone: [phone]; fax: [phone]; email: [e-mail].