Objective Sheepskin is rich in collagen and has great potential for food applications. However, research on its pretreatment, such as degreasing and unhairing, remains limited in the food industry. Although the leather industry has well-established techniques for degreasing and unhairing, the chemicals used do not meet the requirements of food processing and cannot be directly applied to food production. Sheepskin contains 15%-30% fat and is therefore susceptible to off-flavors and oxidative rancidity, which limits its high-value utilization as food ingredients. This study aimed to develop a food-safe degreasing process for sheepskin and to further investigate the effect of the degree of degreasing on the efficiency of subsequent enzymatic unhairing.

Methods Sodium carbonate and sodium dodecyl benzene sulfonate (SDBS) were selected as degreasing agents. Single-factor experiments and response surface methodology (RSM) were conducted to optimize the degreasing conditions, including temperature, time, and the dosages of sodium carbonate and SDBS. Moreover, sheepskins were processed into three degreasing groups (non-degreased, once-degreased, and twice-degreased, using the optimized degreasing conditions), and these three groups were used for enzymatic unhairing. The effects of degreasing on unhairing efficiency and collagen fiber morphology were evaluated by measuring the unhairing rate, observing histologically stained tissue sections, and quantifying hydroxyproline (Hyp) in the unhairing float.

Results Single-factor experiments showed that the removal rate of fat from sheepskin increased initially and then decreased with the rise in degreasing temperature and dosage of sodium carbonate. The optimal conditions for degreasing temperature and dosage of sodium carbonate were 45 ℃ and 3%, respectively. When the SDBS dosage reached 1.5%, the addition of more SDBS led to only marginal increases in the degreasing rate. The degreasing rate increased over time and plateaued after 120 min. Based on these results, a quadratic regression model was established using RSM to describe the relationship between the degreasing rate and the process conditions. The analysis of variance results indicated that the model was highly significant (P＜0.0001), with a lack of fit that was not significant, and a high coefficient of determination (R² = 0.9754), suggesting an excellent fit. The influence of each factor on the degreasing rate of sheepskin was ranked as follows: degreasing time ＞ degreasing temperature＞sodium carbonate dosage＞SDBS dosage. Significant interactions were observed among degreasing temperature, degreasing time, and sodium carbonate dosage. The optimum conditions for the degreasing process were determined by solving the model, as follows: 140 min, 44.0 ℃, 3.1% sodium carbonate, and 1.7% SDBS. Under these conditions, the degreasing rate of sheepskin in triplicate validation experiments was (56.46±1.26)%, which was close to the predicted value of 56.67% (a relative error of 0.37%). This confirmed the predictive reliability of the model. The results of the enzymatic unhairing experiments showed that the degreasing treatment greatly enhanced the efficiency of subsequent unhairing. A high degree of degreasing can reduce the time required for the complete unhairing of sheepskin. The twice-degreased group (with a degreasing rate of 63.36%) reached a 100% unhairing rate after 3 h of enzymatic treatment. The once-degreased group (with a degreasing rate of 56.67%) required 7 h for complete unhairing, while the non-degreased group reached only a 32.43% unhairing rate after 7 h. Histological observations revealed that degreasing improved the initial dispersion of collagen fibers, facilitating protease penetration and uniform distribution in the sheepskin, thereby reducing enzymatic unhairing time. Moreover, the twice-degreased group had a much lower Hyp mass concentration of unhairing float (77.12 mg/L after 3 h of unhairing) than the once-degreased group (111.94 mg/L after 7 h of unhairing) when achieving complete unhairing. These results indicated that deep degreasing could shorten the unhairing time and reduce the extent of collagen damage.

Conclusion This study developed a safe and efficient degreasing process for sheepskin and clarified its positive impact on enzymatic unhairing. These findings provide practical technical support for the high-value utilization of sheepskin in the food industry.