effect of soy protein isolate and batter temperature on flow properties of batter and quality of deep-fryied chicken nugget

introduction: batter coating is a new technology to reduce fat content of fried foods. the ingredients and flowproperties of batter are the most important parameters to determine the performance of batter coating and quality of the final product. among batter additives, proteins can be used as an important and effective component, because of their emulsifying properties, water absorption, and barrier properties. previous studies have shown that soya protein isolate (spi) is a very good hydrophilic protein which could be used to control the viscosity of batter. the batter temperature has direct effect on its flow behavior's properties and has a major impact on the quality of final coated product. therefore, the aim of the present study was to evaluate the effects of temperature (5, 25 and cº45) and spi content (2 and 4%), on flow properties of the batter and quality of deep-fried chicken nuggets. materials and methods:raw materials such as fresh chicken breasts, onion, salt, hot pepper, wheat flour, baking powder, and 100% pure sunflower oil were purchased from local markets. spi (92% protein) were obtained from golharkhorasanrazavi company. the batter formulations consisted of wheat flour, salt (1.5% w/w, db), baking powder (0.5% w/w, db) and spi (2 and 4% w/v, db). for all samples, water/dry mix proportion had always been 5:3. rheological properties of the batters were carried out using a bohlin rotational viscometer. for each test, shear rate increased from 0 to 300 s−1, followed by a logarithmically decrease from 300 to 0 s−1. the flow behavior index (n) and consistency coefficient (k) values were computed by fitting the power law model, and time dependency properties of the batters were obtained from the area between the upward and downward curves. temperature dependency of consistency coefficients were assessed by fitting the arrhenius model. the batter coating on the chicken substrate was appliedat three temperatures (5, 25 and 45 cº), and batter pickups (%) were calculated by the weight difference between the chicken nuggets after coating to the weight of chicken nuggets before coating. coated samples were stored at -18 cº for two weeks. after thawing, chicken nuggets were fried at 180°c for 3 min in sunflower oil using a programmable deep fat fryer. the influence of spi concentrations and batter temperature on color and crust adhesion of deep-fat fried chicken nuggets was measured with image processing. in brief, images were taken by a digital camera under controlled conditions, and then analyzed by image j software. the percentage of adhesion (cra), obtained by calculating the ratio of the pixels corresponding to the perimeter of the substrate where coating is adhered to the pixels corresponding to the total perimeter of the substrate. the color of deep-fat fried chicken nuggets were examined in terms of l*, a* and b*values. oil and moisture content of the chicken nuggets was determined by standard techniques. results & discussion: results showed thatall samples had a shear-thinning behaviour at all batter temperatures (n≥0.539). power law model was adequately suitable to describe the flow behavior of the batters (r2≥0. 994). the activation energy was between 5.106 to 5.630 (kj/mol).addition of soy protein isolates into the batter, enhanced the time dependency, whereas, the increase of temperature decreased the hysteresis area. the relative increase in apparent viscosity with increasing spi might be attributed to the high capacity of spi to absorb free water. however, decrease in apparent viscosity and time dependency of batters with increasing the temperature could be due to the intermolecular interactions in batter system. an increase in temperature weakens the intermolecular interactions and increases the mobility of macromolecules which reduces the batter resistance to shear force. the batter temperature and formulation had significant effect on the batter pickup. this effect was mainly due to the changes occur on flow properties of batter. using spi increased the a* and decreased the l* of the final product. this could be due to the active participation of spi in the millard reaction. the data obtained for crust adhesion revealed that the increase in spi concentration lead to a decrease in batter adhesion to the crust. in other words, the ability of soya proteins to form film during early time of frying reduces the moisture migration from the substrate to oil, which creates a positive pressure in the inner layer of crust and reduces the adhesion. conclusion: chicken nuggets coated with batter containing spi had higher moisture and lower oil content than the control sample. finally, the change in batter temperature had no significant effect on the crust adhesion and oil and water content in the final product.