Lipoxygenases heat inactivation

FIGURE 8.11 First-order reaction rate constants k for heat inactivation, plotted against mass fraction of water w. AP alkaline phosphatase, in skim milk, 80°C. Ec killing of Eschericia coli, in skim milk, 63°C. Ch chymosin, in whey, 80°C. Li lipoxygenase, in sucrose/calcium alginate, 72°C. 

In the case of pasteurized products, fermentation may be interrupted as soon as the pH value reaches <4.1. For that reason the goods are blanched, canned, topped with fermentation liquid or salt and sometimes herbs, spices and/or vine containing brine, and pasteurized. In order to achieve less heat treated and therefore better-quality products, as well as to save energy, the traditional pasteurization methods of trial and error should be replaced by evaluated and defined pasteurization processes using P-value calculation. However, the application is stiU difficult, due to the lack of information especially about the heat inactivation kinetics of quality-relevant enzymes like pecti-nolytic enzymes or lipoxygenases (Buckenhueskes, Gierschner Hammes, 1988). 

Peroxide value is the measure of reactive oxygen content of a fat in terms of milliequivalents per 1000-g fat, following AOCS method Cd 8-53 or AOAC Method 965.33 (109). Elevated peroxide values indicate that lipid oxidation has taken place (110). Free fatty acids can serve as substrates for lipoxygenase and peroxidase (111), both of which are inactivated during heating (112). Once the cell... 

FIGURE 7.9 Heating times needed (t ) for inactivation (reduction of activity to about 1 %) of the enzymes chymosin (Ch), lipoxygenase (LP), acid phosphatase (AP), and plasmin (PI) and for 30% of ovalbumin (OA) and /1-lactoglobulin (LG) to become insoluble. Because of the narrow temperature intervals involved, also plots versus T, rather than fT, can be approximately linear. 

Soymilk. In the traditional process, soybeans are soaked in water, ground into a slurry, cooked, and filtered to remove the insoluble cell wall and hull fractions. A number of modifications have been made in the process since the 1960s, including heat treatment before or during grinding to inactivate the enzyme lipoxygenase and thus prevent formation of grassy and beany flavors. The soymilks are available in plain and flavored, eg, vanilla and chocolate, forms (90,91). 

Lopez et al. [120] stndied the inactivation of three food quality enzymes—peroxidase, PPO, and lipoxygenase— by MTS. They revealed that a synergistic effect of temperatnre, pressure, and ultrasound results in rednced enzyme resistance and heat treatment for inactivation. Ultrasonic wave amplitude was found to be more effective on enzyme destruction than static pressure at constant temperature. This combined treatment was reported to be very useful in solving problems caused by thermostable enzymes. 

Soybean type-I lipoxygenase is particularly stable. Other lipoxygenases are less stable and activity is lost during purification. Heat treatment is often used in the food industry to cause inactivation so as to prevent off-flavours. The purified lipoxygenases are generally unstable at 70 °C but higher temperatures may be necessary for inactivating the enzymes in foodstuffs. 

To prevent flavor deterioration due to the action of lipoxygenases and other enzymes, unprocessed plant foods such as beans and peas are blanched before canning or freezing to inactivate these enzymes. These heat treatments must be carried out under mild conditions to avoid undesirable changes in texture and loss of protein bioavailability. 

Toasting of soybeans by heating at 100-1 lO C to inactivate lipoxygenase and antinutritional factors (e.g. trypsin inhibitor) is important to improve the quality of soybean protein products used for either animal feed or human consumption. However, this thermal enzyme inactivation is not carried out in the conventional processing of soybean oil. Therefore, the soybean flakes must be solvent extracted without delay to minimize free fatty acid and peroxide formation in the extracted crude oil and to produce a finished oil of improved oxidative and flavor stability. 

Gill lipoxygenase can be thermally inactivated above 60°C with a resulting improvement in shelf life stability of fish. However, heating inaeases non-enzymatic oxidation also, and this may exceed the oxidation due to lipoxygenase. 

In comparison to other cereals, oats contain a significant level of lipase. Its high activity is released once the oat kernel is disintegrated, crushed or squeezed. Linoleic acid is released from the acyl lipids that are present. It is then converted into hydroxy fatty acids by lipoxygenase and hydroperoxidase enzymes, giving rise to off-flavors (Fig. 15.11). All these enzymes are inactivated by heat treatment and thus quality deterioration can be avoided (cf. 15.3.2.2.2). 

Soybeans are swollen and ground in the presence of a 10-fold excess of water. Heating the suspension close to its boiling point for 15-20 min pasteurizes the suspension and inactivates lipoxygenase enzyme and proteinase inhibitors. A soy milk preparation enriched with calcium and vitamins is of importance in infant nutrition as a replacement for cow s milk, which close to 7% of infants in the USA are unable to tolerate. 

Application of Kinetic Data to Thermal Processing. In most studies on ttiermal inactivation of indicator enzymes including peroxidase, lipoxygenase, and LAHase, reaction rate constants and thermodynamic parameters have been determined on the assumption that thermal inactivation of the enzymes follows first order reaction kinetics (22). However, a deviation from first order kinetics is generally observed fipm the residual activity curve. This deviation has been explained by several mechanisms, including the formation of enzyme aggregate with different heat stabilities, the presence of heat stable and labile enzymes, and the series type inactivation kinetics. 

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