Effects of heating duration on the digestibility of fish protein powders at 99 °C in vitro using yellowtail Seriola quinqueradiata digestive enzymes

The effects of heating duration on digestibility of the muscle powder of three fish (pollock [WP], yellowtail [YT], mackerel [CM]) and the whole-body powder from mackerel (WM) were assessed in vitro using a digestive enzyme complex (EC) extracted from yellowtail pyloric caeca. This research will contribute to establishing a theoretical basis for feed processing conditions for yellowtail. Transcriptomic analysis showed that the most dominantly expressed genes in the pyloric caeca examined were those for trypsin and that their activity was well maintained in the EC. To assess heating effects, the powders were heated at 99 °C for 0, 3, 20, 72 and 240 min, followed by hydrolysis with the EC. After 3 min of heating the WP, YT and CM, the average production of essential amino acids (EAAs) decreased to 81.3%, 72.0% and 66.9%, respectively, compared to that detected in controls without heating. The production further decreased with increased heating duration from 3 to 240 min, but much of the decrease was within the first 3 min in WP, YT and CM. The decreasing rate of EAA production was higher in WM than CM, and increased with heating time. These results suggest that WP is the most suitable feed material for yellowtail of the four samples tested and that the shortening of heating time and removal of viscera are also important factors.