Parameters screening of Qula hot-air drying and evaluation of color quality

Abstract: A crude cheese, Qula, which is mainly made up of casein, is a common milk product with local characteristics in northwestern China, such as Gansu, Xinjiang and Tibet. It is made from yak milk with a traditional method: defatting, acidifying, and drying in air (at room temperature). Qula is the raw materials of yak milk casein and collected by factory for further processing. Yak milk casein has been widely used in various fields, such as glues, papermaking, leather, food additives and pharmaceutical applications. However, the traditional process, especially for drying under the sun, can cause severe browning, yellowing and poor quality in Qula. Therefore, hot-air drying method which is based on the evaluation of color quality is adopted in Qula processing. Qula samples were from Gannan city in Gansu Province (Experimental group, 60%-62% moisture was conducted; Control group, 10%-12% moisture was conducted). L*, a*, b* values of dried products were taken as target parameters, and the three factors were scored by the comprehensive evaluation using the data standardization method. According to the central composite experimental design, the response surface methodology (RSM) was adopted to discuss the influence caused by drying temperature, material thickness and wind velocity based on the single factor test of temperature (30, 35, 40, 45, 50 and 55℃), material thickness (6, 8, 10, 12, 14 and 16 mm) and wind velocity (20, 40, 60, 80 and 100 m/min). In addition, the quadratic polynomial regression model was established by SAS software to optimize the drying techniques. The peroxide value (POV), thiobarbituric acid reactive substances (TBARS) and 5-hydroxymethylfurfural (HMF) were measured which are the important indictors of fat oxidation and furanic compound forms as an intermediate product in Maillard reaction. The results showed that the order of significance among three factors were: temperature>wind velocity> material thickness. The optimal parameters of hot-air drying were obtained with a temperature of 41℃, material thickness of 10 mm and wind velocity of 60 m/min, then the largest comprehensive score of 0.9012 was obtained. L* value of Qula by hot-air drying increased by 7.18 (P<0.05), a* and b* values reduced by 1.41 and 10.29, respectively (P<0.01), and the POV, TBARS value and the content of 5-HMF reduced by 1.15 g/100 g, 0.84 mg/kg and 1.973 mg/L, respectively (P<0.01). All in all, the damages which were caused by far oxidation and Maillard reaction can be reduced by this method; meanwhile, the color quality of Qula can be improved dramatically.