Food Processing Technologies by Amit K. Jaiswal

Author:Amit K. Jaiswal
Language: eng
Format: epub
Publisher: CRC Press

15.3.2 Effects on Enzymes and Lipid Oxidation

The most-recognized enzyme which adversely affects soy products quality is LOX. LOX (linoleate:oxygen reductase; EC1.13.11.12) is a nonheme iron-containing dioxygenases which catalyzes the oxidation of cis,cis-1,4-pentadiene to pentadienyl, which, upon abstraction of hydrogen, results in a pentadienyl radical intermediate. Pentadienyl radical may react with oxygen to form peroxyl radical isomers and fatty acid hydroperoxides. Hydroperoxides as well as decomposed products are potentially reactive substances that may cause deterioration of food proteins and formation of volatile compounds, such as aldehydes and ketones, in the presence of oxygen, light, enzymes, and high temperatures (Kumar et al. 2003; Huang et al. 2006; Ying-Qiu et al. 2008). These compounds negatively affect sensory perception by the off-flavor developed which compromises soy milk global quality.

In soy milk production, LOX inactivation is achieved by thermal treatments. Some studies (Kwok and Niranjan 1995; Wang et al. 2008) showed that heat treatment for 10 min from 80°C to 100°C was effective for that purpose. Poliseli-Scopel et al. (2012) used a soy milk procedure which guarantied LOX inactivation previously to the final treatment of soy milk to get a better understanding about changes caused by UHPH or heat treatment applied. Total inactivation of LOX was explained by the conditions of soy milk elaboration in which soybeans were ground at 80°C for 20 min.

Wang et al. (2008) studied the kinetics of LOX inactivation on soy milk by high hydrostatic pressure (HHP) processing from 0.1 to 650 MPa at temperatures from 5°C to 60°C. Results indicated that LOX inactivation was irreversible and followed a first-order reaction at all pressure and temperature combinations tested. According to those results, the rate constants of LOX inactivation increased with increasing pressure at constant temperature. LOX inactivation rate at high temperature decreased as pressure increased, whereas the highest sensitivity of the LOX inactivation rate to pressure was observed at about 30°C. Van der Ven et al. (2005) also applied HHP treatment on soy milk for LOX inactivation. HHP conditions applied were as follows: 500 MPa at 20°C, 600 MPa at 60°C, and 800 MPa at 20°C. The holding times of all treatments were 0, 1, and 2 min, except for 800-MPa conditions in which the holding time was 2 min. Results showed that at 500 MPa, an increase in LOX inactivation was observed as holding time increased.

Lipid oxidation may also be catalyzed by oxygen, light, metals, and high temperatures. Determination of hydroperoxide index allows the evaluation of the initial stages of oxidation and its evolution through time. Poliseli-Scopel et al. (2012) studied the effect of UHPH-treated soy milk at 200 and 300 MPa at inlet temperatures of 55°C, 65°C, and 75°C on primary oxidation. These UHPH treatments were compared with soy milk treated by pasteurization and UHT at day 1 and 15-days cold storage (4°C). UHPH-treated soy milk at day 1 showed significantly lower hydroperoxide values than at day 15, with concentration ranged between 0.2 and 0.4 meq/L in all samples (Table 15.4). After 15 days of storage, hydroperoxide index was higher in all treated samples.

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