In this study, three different probiotics (Lactobacillus plantarum LP1, Lactobacillus fermentum LF1, and Bacillus subtilis BS1) were used to ferment chicken liver. Among these, Lactobacillus plantarum LP1 was found to have the strongest growth and acid production capacity and thus was selected as the starter for further fermentation. Single-factor experiments were carried out with five variables, initial fermentation pH, glucose addition, inoculum size, fermentation time and solid-to-liquid ratio. On this basis, the fermentation process was optimized by response surface methodology using three variables at three levels each. The results showed that the influence of three variables on the absorbance at 280 nm (A_{280 nm}) of fermented chicken liver was in decreasing order as follows: initial pH of fermentation medium > solid-to-liquid ratio > inoculum size. The optimal fermentation conditions were determined as initial pH of fermentation medium, solid-to-liquid ratio for 5.57, 1:3.55 (m/V) and 1.5%, respectively. The A_{280 nm} of fermented chicken liver obtained under the optimized conditions was 1.537. The total contents of free amino acids and essential amino acids in chicken liver increased significantly after optimization (P < 0.05), indicating that fermentation can promote the decomposition of chicken liver protein and thereby improve the nutritional value of chicken liver.

Animal-derived processing by-products are a treasure house of bioactive compounds with huge output and low price. Their efficient exploitation is of great significance. Deep eutectic solvents are a new type of designable “green solvents” and are formed by mixing hydrogen bond donors with hydrogen bond acceptors in appropriate proportions. They possess great advantages, such as easy availability of raw materials, simple preparation, designable structure, and biodegradability and are therefore extensively used in the fields of chemistry, food and medicine. This paper briefly reviews the application of deep eutectic solvents in the extraction of bioactive components from animal-derived processing by-products and gives an outlook on future prospects, hoping to provide new strategies the extraction of bioactive components from animal-derived processing by-products.