Process parameter optimization and equipment development of thermophilic aerobic fermentation of dead pigs

Abstract: Carcass of dead pigs can be converted into dry granular materials rapidly by aerobic fermentation at 50 to 70 ℃ after adding fermentation strains into the mixture of pig carcass and auxiliary materials such as wheat bran. In order to optimize the fermentation process parameters and improve the quality of fermentation, a laboratory rapid aerobic fermentation system together with heating device was adopted in this study to process the stillbirth pigs. In the experiments, ventilation rate and fermentation temperature were selected as experimental factors and the mass fraction of total free amino acid (FAA), moisture content, particle size distribution and pH value of the products were selected as experimental indexes. The fermentation temperature was set at 50, 60 and 70 ℃ and the ventilation rate was set at 8, 9 and 10 L/(L?min). One pig stillbirth (1.5-2.2 kg) was selected in each experiment. The frozen pig carcass were divided into blocks of about 5 cm with a chainsaw, and then ground in a meat mincer. The carcass pieces were weighed and mixed with appropriate amount of bran to achieve a weight ratio of the bran to the carcass of 1:1.5 (dry-based ratio under bran moisture content of 9% and carcass moisture content of 75%, equivalent to a wet base ratio of about 1:5.5). After mixing, fermentation strains were added to attain strain weight: pig weight of 1:25 (dry-based ratio, or 1:100 for wet-based ratio assuming the strain moisture content of 10% and carcass moisture content 75%). The moisture content of mixed materials was about 65%, the pH value was about 6.55, and the mass fraction of FAA was 6.0 g/kg. In the controlled experiments with different fermentation temperatures, the ventilation rate was 8 L/(L·min). In order to keep the moisture content of fermentation materials within the appropriate range to maintain microbial activities during the fermentation process, the moisture content was adjusted to 65% by adding water after every 24 hours of fermentation. The results showed that the moisture content of fermented materials decreased by 8-11, 24-27 and 40-50 percentage at 50, 60 and 70 ℃, respectively. The mass fractions of FAA in fermented materials after 3 days of fermentation were 18.51, 16.62 and 9.42 g/kg, and the passing rates of 4.75 mm braided sieve were 57.40%, 83.95% and 83.44% respectively. In the control experiments with different ventilation rates, the fermentation temperatures were 60 ℃. The results showed that when the ventilation rate was 8, 9 and 10 L/(L·min), the moisture contents of the fermentation materials after fermentation for 3 days were 11%, 8.5% and 5%, the mass fraction of FAA was 13.18, 16.62, and 21.41 g/kg, respectively. The 4.75 mm woven sieve passing rate was more than 70%, which could reach 84.4% when the ventilation rate was 10 L/(L·min). Comprehensive analysis showed that the treatment effect was best when the fermentation temperature was 60 ℃ and the ventilation rate was 10 L/(L·min). Based on the above results of processing technology, a thermophilic aerobic fermentation equipment with a processing capacity of 150 kg per batch for dead pigs was designed and developed. This equipment was tested under a ventilation rate of 10 L/(L·min) and the fermentation temperature of 60 ℃. After 3 days of fermentation, the FAA mass fraction in the products was 20.74 g/kg with the moisture content of about 14% and the pH value of 5.4. Moreover, 88.89% of the products could pass the braided sieve of 4.75 mm diameter and Escherichia coli were not detected in the products. The energy consumption of the equipment was 2.37 kW?h per kg of dead pigs. The experimental results showed that the equipment could transform the dead pigs in the frozen state into dry particle materials in 3 days, and the treated products could meet the requirements of subsequent organic fertilizer production.