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Updated June 23, 2009




High-pressure processing (HPP) has been applied to various food products over the last decade, including seafood, meat products, and vegetables, mainly for the purposes of inactivating microorganisms and enzymes. This technology could also benefit crops such as soybeans. In 2007, Iowa retained its title as the nation’s top corn and soybean producer. The state’s farmers produced 439 million bushels of soybeans, making Iowa State University a logical place to perform a comprehensive study of the potential benefits of this technology in manufacturing soy-based products.

The vast majority of soybean protein production is used for feed while a low but steadily increasing percentage is used in food applications. Soy protein ingredients, such as soy protein isolate and concentrate, are among the most important soy-based food products in terms of quantity produced. They are obtained from defatted meal, a co-product of the soybean crushing industry. Hexane-extraction is currently the most cost-effective process to extract oil from oilseeds, reducing the oil content of soybeans from ~ 20% to less than 1%. A green alternative to this solvent-based process is enzyme-assisted aqueous extraction processing (EAEP). During EAEP the extracted oil is not released as free oil, but trapped in an emulsion stabilized by both protein and phospholipids. We identified strategies to free the sequestered oil, but the stability of this emulsion is an advantage that could benefit many food applications. HPP further enhanced the stability of this emulsion while extending its shelf life when compared to thermal-treated emulsions over 42 days of refrigerated storage.

Soymilk and tofu consumption have been increasing in Western countries in recent years, while being popular for many centuries in Asian countries. The renewed consumer interest in these products is due to the soy protein health claim approved in 1999 by the U.S. Food and Drug Administration and the abundant body of literature on the health benefits of soy protein and isoflavone consumption. Thermal treatment is applied to soymilk to assure its safety and increase its shelf life. Thermal treatment also decreases antinutritional trypsin inhibitors content by 90%, and inactivates lipoxygenase, an enzyme responsible for soymilk off-flavor. However, the thermal process can decrease the nutritional quality of soymilk as well as add undesirable off-flavors, therefore identification of an alternative process might benefit soymilk quality.

We first studied how processing conditions affected both lipoxygenase and trypsin inhibitors. While soymilk lipoxygenase can be inactivated by pressures greater than 400 MPa, trypsin inhibitors required a combination of high pressure and mild thermal treatment, i.e., ~ 65°C, to be destroyed. These processing conditions, however, use lower temperatures or shorter treatment times than conventional heat treatments. Using these conditions, soymilk vitamin B6 content was retained as well.

We also determined that 600 MPa applied at 25 and 75°C for short treatment times, 1 and 5 min, increased the refrigerated shelf life of soymilk up to 4 weeks. This is a major advantage for this highly perishable product. Injury of spoilage microorganism occured during treatment and recovery was observed during refrigerated storage of soymilk. Salmonella can contaminate soymilk and tofu. We are investigating the effect of pre-stresses, such as acid environment and starvation, on the survivability of Salmonella during HPP of inoculated buffer and soy-based products. Acid–adaptation of Salmonella did not modify their resistance to pressure. Starvation of Salmonella before HPP, however, increased the pressure-resistance of the cells compared to un-starved cells. These data will provide important information regarding the safety of pressurized products.

The content and profile of isoflavones, key soy compounds for health, are maintained after treatment up to 800 MPa at 25°C. HPP treatment at initial temperature of 75°C changes the isoflavone profile, which has been related to the adiabatic heating during treatment. Clear understanding of the health benefit of each isoflavone isoform will determine if pressure-induced changes in soymilk isoflavone profile might have health benefits. Pressurization of soybeans was, however, inefficient in increasing extractability of isoflavones into water during soymilk production.

As expected, soy proteins are affected by pressure treatment and this can lead to an undesirable change in soymilk appearance. By adequately adjusting the pH and soybean-to-water ratio, the appearance and viscosity of soymilk can be maintained while its emulsification properties are enhanced.

Soymilk with reduced trypsin inhibitors content, inactivated lipoxygenase, extended shelf life, enhanced isoflavone and vitamin B6 contents, viscosity similar to fresh soymilk, and improved functionality can therefore be obtained with HPP. Studies are currently being performed to determine one of the most important questions yet to be addressed: does pressurized soymilk and soymilk from pressurized beans taste good? The answer to this question, in addition to the advantages already identified, will determine if HPP can become an attractive alternative to thermal treatment in soymilk production.

The pressure-induced changes in soymilk protein significantly impact the textural properties of tofu. HPP could be used as a soymilk pretreatment before thermal production of tofu or as an alternative to thermal treatment for tofu making. Either way, use of HPP allows production of tofu with unique properties.

Tofu obtained under 600 MPa applied at an initial temperature of 25°C for 5 min from raw soymilk with 0.3% coagulant.
Tofu obtained under 600 MPa applied at an initial temperature of 25°C for 5 min from raw soymilk with 0.3% coagulant.

Work on HPP at Iowa State is not limited to soybeans and soy products. HPP has also been applied to meat products. From a food safety approach, HPP treatment opportunities include treating turkey breast roll with lauric arginate to deactivate Listeria monocytogenes. In collaboration with the University of Nebraska, HPP is also being applied as a culinary tool to develop meat entrees. Bovine muscle has improved texture, increased juiciness and excellent flavor when treated with moderate pressures. The potential health benefits of applying HPP to meat are also being investigated. This work has identified several bioactive peptides in pressurized meat. Specifically, peptides with sequence homologies to known angiotensin converting enzyme (ACE) inhibitors were found.

In addition to Iowa’s rank in crop production, the state is also first in egg production; Iowa's egg producers have 57 million layers producing around 14.25 billion eggs per year. Egg products can also benefit from HPP treatment and ongoing studies are identifying the effects of HPP on egg white proteins and whether bioactive peptides can be obtained from pressurized phosvitin.

The increasing availability of commercial food products using HPP technology demonstrates its potential to improve both their quality and safety. Greater understanding of non-thermal processing technology’s effect on food components and microorganisms will result in its even wider application. The green, nutritional and preservative-free aspects of HPP are a perfect fit for soy products, which the consumption increases in Western countries primarily among the consumers who are seeking for healthier diets.

Collaborations: ENITIAA, France; The Ohio State University; Illinois State University; and University of Nebraska-Lincoln.
Funding: National Research Initiative Grant from the USDA Cooperative State Research, Education, and Extension service improving food quality and value (71.1); USDA Special Research Grant; Center for Crops Utilization Research, Iowa State University; and Iowa Egg Council.

S. Jung
Department of Food Science and Human Nutrition and the Center for Crops Utilization Research, Iowa State University, Ames, IA, 50011-1061