Effect of winterization process on properties of chicken skin fat

Chicken fat is obtained from chicken skin and abdominal fats. With about 30 % saturated fatty acids (mainly palmitic and stearic acid), 30-45 % oleic acid and 15-30 % linoleic acid, chicken fat has a similar fatty acid composition to the rice bran oil. It contains lower saturated fatty acids, cholesterol, and higher polyunsaturated fatty acids (PUFA) content than butter, tallow and lard. It is mainly used in animal feed industry and soap making. However, it has good potential for application in food industry. With a melting point of about 25 °C, it has a semi-solid state and is fluid at room temperature. However, it solidifies in cold seasons which makes its discharge from packaging or tanks difficult. Accordingly, the aim of this research was to produce a cold-stable chicken fat using winterization method. Winterization of oil is a process that uses cold temperatures to separate high-melting triacylglycerol and waxes from oil. Winterization is a type of fractionation (also known as fractionate crystallization), the general process of separating the triacylglycerol found in fats and oils, using the difference in their melting points, solubility, and volatility. Chicken fat was extracted from chicken skin by dry rendering. In this research, the effect of winterization temperature (10, 15, 20, 25 and 30 °C) and time (16 and 24 hours) on physicochemical properties of chicken fat (yield, cold test, cloud point, iodine value, slip melting point, oxidative stability, fatty acid composition and cholesterol content) was studied. Experiments of the effect of winterization time and temperature were performed using a factorial design. Data were analyzed for ANOVA (One-Way) and comparison of means (Duncan's multiple range test) at p<0.05 using SPSS software. Physicochemical tests were run at least in three replications. Statistical analysis showed that winterization time had not a significant effect on cold stability of the winterized fat (P<0.05). However, the effect of winterization temperature was significant (P<0.05). Results showed that with increase of winterization temperature from 10 to 30 °C, cold stability of the product decreased (from 4.25 to 0.45 h), but cloud point increased (from 1.0 to 8.5 °C) and the yield of winterized oil increased (from 74.4 to 98.1 %) (P<0.05). With the increase of winterization temperature, stearic and linoleic acid content decreased slightly (P<0.05). Furthermore, stearic and linoleic acid content of the winterized samples was lower than those of the chicken fat (P<0.05). Cholesterol content of chicken fat samples winterized at 10-20 °C was not significantly different from that of the chicken fat (P>0.05). However, when fat samples were winterized at 25 or 30 °C, lower cholesterol content was remained in the winterized fat samples (P>0.05). Winterization had not any effect on iodine value and oil stability index of chicken fat (P>0.05). However, iodine value of stearin samples obtained from winterization was lower than chicken fat and winterized chicken fat (P<0.05). The stearin fractions obtained in this study had slip melting point between 42-50 °C. Application of higher winterization temperature resulted in lower yield of stearin but a higher melting point. Stearin samples had higher saturated fatty acid content and lower iodine value than chicken fat or winterized chicken fat (P<0.05). Generally, none of the winterized samples could pass the cold stability test (which should be higher than 5.5 hours at 0 °C). However, winterization could enhance the cold stability of chicken fat, significantly (P<0.05). Though a cold-stable liquid oil was not obtained in this study, the improvement in cold stability of chicken fat may result in delayed solidification of chicken fat. The effect of winterization temperature on cholesterol content of chicken fat was more complicated than that of the other properties. This means, winterization of chicken fat may or may not be accompanied with cholesterol decrease. When choosing the winterization temperature, the yield of the winterized oil should be considered, as well. Application of lower winterization temperature will result in more cold-stable oil, but at a lower yield. Optimization of the winterization condition for production of a cold-stable oil with the highest yield is suggested for further study. In this study, chicken fat stearin was obtained as a by-product which can be used as a hard stock for formulation of margarines and shortenings.