Yeast as a novel protein source- Effect of species and autolysis on protein and amino acid digestibility in Atlantic salmon (Salmo salar)

Yeasts are gaining increasing attention as alternative protein sources in fish feeds. The nutritional value of yeast depends on cultivation conditions, yeast species and processing conditions used after harvesting. The objective of the current study was to evaluate the effect of autolysis on apparent digestibility coefficients (ADCs) of crude protein and amino acids (AA) of different yeasts species in Atlantic salmon (Salmo salar). Three yeast species (i.e. Cyberlindnera jadinii, Blastobotrys adeninivorans and Wickerhamomyces anomalus) produced from hydrolysates of pre-treated wood and chicken products were used. After harvesting, each yeast was either directly heatinactivated with spray-drying or autolyzed at 50 degrees C for 16 h followed by spray-drying. The treatments consisted of a high-quality fishmeal-based reference diet and six test diets containing 30% of each of the yeast product and 70% of the reference diet. The results showed that protein and AA digestibility differed among the yeast species and that the effect of autolysis on nutrient digestibility was inconsistent among the three yeast species. The ADCs of protein in inactivated yeasts were 63%, 72%, 66% in C. jadinii, B. adeninivorans and W. anomalus, respectively. Autolysis increased the ADCs of protein by 12% and 9% in C. jadinii and W. anomalus, respectively, while it remained unchanged for B. adeninivorans. The ADCs of lysine were 67%, 79% and 72% in inactivated C. jadinii, B. adeninivorans and W. anomalus, respectively. Autolysis improved the ADCs of lysine by 15%, 7% and 13% in C. jadinii, B. adeninivorans and W. anomalus, respectively. The ADCs of methionine in inactivated yeasts was 47% in C. jadinii, 81% in B. adeninivorans and 74% in W. anomalus. After autolysing the yeasts, the ADC of methionine improved by 26% and 4% in C. jadinii and B. adeninivorans, respectively, while it slightly reduced by 2% in W. anomalus. Data from regression analyses showed that digesta viscosity, digesta dry matter and nitrogen solubility are important determinants of protein digestibility of yeasts in fish. In addition, cell wall porosity as demonstrated by nitrogen solubility test, had a larger impact on nutrient digestibility of yeasts compared to the cell wall thickness. In conclusion, the digestibility of protein and AA of yeasts in Atlantic salmon depends on type of yeasts and down-stream processing applied after harvesting. Also, the particular in vitro digestibility method used in the current study did not adequately reflect the protein digestibility of yeasts in Atlantic salmon.