Functionality Effects of Nonthermal Processes on Foods

The Nonthermal Processing Division of IFT held its annual workshop in Montreal October 12–14, 2010, hosted by McGill University and chaired by Professor Hosahali S. Ramaswamy ([email protected]), who was ably assisted by his graduate students and colleagues. As has been true of past workshops by this division, the papers and posters provide a valuable snapshot of developments in this area of food science and engineering. The primary objective of nonthermal processes is to reduce pathogenic and spoilage microorganisms and inactivate harmful enzymes, but a few of the presentations described the effect of high pressure on the functionality of foods.

Effects of High Pressure Processing
A paper by Haihong Wang ([email protected]) and co-workers from Alberta Agriculture and Rural Development investigated the effect of hydrostatic pressure on beef muscles. Fresh post-rigor semitendinosus (ST) muscles were cut into steaks, individually vacuum packaged, and immediately subjected to high pressure processing (HPP) at 0.1 (control), 50, 100, 150, 200, 250, 300, or 400 MPa for 5 min at 8°C. Instrumental color, drip loss, and thiobarbituric acid reactive substance (TBARS) values were measured on fresh steaks after HPP treatment. The pH, water binding capacity (WBC), total and sarcoplasmic protein solubility, and Warner-Bratzler shear force (WBSF) were measured on thawed steaks. The pH of HPP-treated muscles increased significantly (p < 0.05) with the increase of pressure. The drip loss was highest for 200 MPa samples with a value of 2.13% as compared to the control value of 1.21%. Lightness (L*) and redness (a*) values remained unchanged (p > 0.05) when pressure was below 150 MPa; however, the L* value increased significantly and a* value decreased significantly for the steaks treated with pressure higher than 200 MPa. WBC decreased significantly from about 20% for samples treated with pressure below 100 MPa to -3.42% for 400 MPa samples. Total protein and sarcoplasmic protein solubility were not affected by pressure treatments below 200 MPa but decreased significantly when higher pressures were applied. Neither WBSF nor oxidative stability (TBARS) was influenced by pressure treatment up to 400 MPa.

Jimmy Yao and his colleagues studied the effect of HPP on consumer sensory attributes and physical property of three deli meats during storage. The researchers investigated the following consumer sensory attributes: appearance, flavor, texture, packaged appearance, and overall acceptability. The evaluation was carried out using more than 60 consumer panelists at each storage time. The physical properties (pH, purge, color, and texture profile analysis) were also measured against the controls at each storage time. The researchers found that there were no practical sensory and physical property difference between HPP-treated samples and controls.

Chairman of the workshop, Hosahalli Ramaswamy, and his colleagues from McGill studied the inactivation of avidin activity in eggs by HPP and thermal processing methods. An egg primarily contains proteins called albumen, ovalbumin, conalbumin, ovomucid, lyzozyme, ovomucin, ovoglobulins, and avidin. Avidin is a water-soluble glycoprotein that binds vitamin biotin and forms avidin biotin complex, which is not absorbed by the human body. Deficiency of biotin can cause skin-and thyroid-related diseases. Avidin is highly stable to thermal treatment and is an anti-nutrient in human food. HPP causes inactivation of microorganisms and breaking of strong non-covalent bonds, leading to a number of changes in protein structure and interactions. It can be used for effecting desirable changes in functional properties of proteins. This study focused on comparative analysis of the effect of the combination of high pressure and high temperature treatment (HPHT) vs thermal treatment on biotin binding activity of avidin. HPHT was found to cause much faster inactivation of biotin binding activity of avidin than thermal treatment alone. The increase in treatment intensity of applied pressure was inversely proportional to residual avidin activity. The decimal reduction time (D value) associated with HPHT treatment was found to be 26 min (700 MPa/100°C) as compared to thermal treatment (100°C) of 111 min. The corresponding z_T values were 40°C, 37°C, 31°C, and 46°C at 700, 600, 500, and 0.1MPa, respectively.

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Another group from McGill compared the kinetics of thermal and high pressure destruction of lycopene in watermelon juice. Carotenoids have antioxidant activity and free-radical scavenging properties. Lycopene is one of the carotenoid compounds in watermelon juice. Several researchers have reported an association between dietary lycopene consumption and lower incidence of diseases such as prostate and oral cancers and reduction of risks of cardiovascular disease. The presence of large unsaturated trans carbon-carbon bonds in this antioxidant makes it chemically susceptible to heat degradation. For the study, the fruit juice was filtered with cheese cloth and its high pH (5.6-5.8) was reduced to 4.2, using acetic acid. The juice was subjected to different thermal (70oC, 80oC, and 90oC) and high pressure (400 MPa, 500 MPa, 600 MPa) treatments. Residual lycopene content was measured in duplicates. During heating or high pressure treatment the degradation of total amount of lycopene fit a first-order reaction model (R²≥0.92). Apparently the higher the temperature or the higher pressure, the faster the relative degradation of lycopene. The D value of thermally treated lycopene ranges from 14.7 hr to 91.8 hr whereas that of high pressure from 60.6 hr to 258 hr with z_T and z_P values of 25.4oC and 309±24 MPa, respectively. The result confirmed that the high pressure destruction of lycopene was slower than the thermal destruction rate. This gives an opportunity to preserve watermelon juice with high lycopene content.

High Pressure Starch Gelatinization
Led by Patricia Lebail ([email protected]), a group from Nantes, France, studied the impact of botanic origin on high pressure gelatinization of selected starches. Starch gelatinization during thermal processing encompasses a chain of different phenomena, including the progressive swelling of the starch granules and hydration of the semi-crystallized biopolymers (amylase and amylopectin). HPP can also provoke starch gelatinization. However, while all starches gelatinize with a hydrothermal process, not all starches gelatinize during HPP.

Selected starches with different botanic origin were considered for this study: potato starch, broad bean starch, tapioca starch, and bean starch. Starch suspensions in excess of water were prepared and installed in plastic pouches. The high pressure treatment took place at 500 MPa for 5 min. The degree of gelatinization was investigated using DSC, x-ray diffraction, and NMR. Results showed that starch gelatinization was possible for broad bean, tapioca, and bean starches whereas potato starch suspension was not affected by HPP. An x-ray scattering plot confirmed that potato starch was not affected and kept its crystalline structure after HPP. Further work is needed to study the impact of other processing parameters on the degree of starch gelatinization during HPP.

B.K. Simpson led a group from McGill that studied controlling histamine formation in fresh tuna flesh by HPP. Tuna is highly perishable and has been implicated in histamine poisoning because of high histidine levels in the muscle. The histamine is formed from histidine by enzymatic decarboxylation facilitated by microbial and endogenous decarboxylases. Demands for fresh, additive-free, and safe seafood products have fueled research to discover novel methods to prolong the shelf life of fresh products with minimal loss of quality. HPP was investigated for its effects on the quality and shelf life of fresh tuna meat by subjecting the test samples to various pressure treatments (150 MPa, 200 MPa, 220 MPa), holding times (15 min, 30 min), and temperatures (< 20°C). The research group analyzed pressurized and unpressurized samples for quality changes during chilled storage.

The pressure-treated samples lost their glossiness, and their redness decreased with pressure and holding time. HPP above 150 MPa resulted in increased firmness and drip loss. However, proteolytic activity did not change significantly during storage, and the total volatile bases (TVB) levels increased during storage. Furthermore, the researchers observed no consistent pattern for thiobarbituric acid (TBA) levels although the levels were low and indicative of high quality products. Histamine formation was inhibited by pressure treatment at 220 MPa for 30 min while other pressure levels appeared to enhance histamine formation.

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The study also investigated high pressure effects on histidine decarboxylase activity and established that high pressure treatments above 400 MPa achieved irreversible inactivation of the enzyme. Overall, the study showed that a pressure level of 220 MPa for 30 min was optimal in controlling proteolysis, texture degradation, and histamine formation without promoting lipid oxidation in fresh tuna fish.

HPP Effects on Mango Juice
Ramaswamy and Hiremath examined quality changes in high-pressure processed mango juice during storage. Fresh mango juice pressure treated at 400 MPa to achieve different process lethalities (0-10D based on Escherichia coli O157:H7 destruction rate) were stored at 4°C, 12°C, and 20°C for select times (up to 60 days), and the associated quality changes were evaluated. All samples were analyzed for their color, pH, acidity, °Brix, and microbial growth. The researchers observed decreases in pH and b values and increases in acidity and microbial growth in some pressure-treated samples at 400 MPa. Microbial growth and quality changes during storage were modeled assuming a first-order kinetic process. Decimal multiplication times (Dm) were used for microbial growth and conventional decimal reduction times (Dr) for quality factors. Temperature sensitivity was evaluated using the Z value approach. The results showed that the Dm and Dr values increased with a decrease in storage temperature or an increase in applied process lethality. The results indicated that while these pressure-processed samples were microbiologically safe, they were not stable with respect to product quality. Hence, additional treatments carried out at 550 MPa for 1 min and similar storage tests were carried out. Test samples that were pressure treated at 550 MPa maintained their superior quality throughout the 60-day test period and showed no microbial growth (at 4°C). High pressure pasteurization therefore has potential as an alternative method for the preservation of mango fruit juice.

J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries, Oak Park, Ill. 
[email protected]