Effects of alcohol extraction and heat treatment on the utilisation of soya bean protein by growing rats and pigs

Three experiments were conducted to determine the effects of alcohol extraction and heat treatment on the nutritional quality of soya bean protein. In experiment 1, 144 weanling rats were used to determine the effects of different alcohol and heat treatments of defatted raw soya bean flakes (SF) on protein utilisation. Treatments were: (1) solvent extraction (water or 50% alcohol/water mixtures of methanol, ethanol and n‐propanol), and (2) heat treatment (no heat, heating before extraction and heating after extraction). Solvent extraction consisted of soaking the SF in the solvents for 48 h. Heat treatment was autoclaving for 15 min at 121°C and 1.1 kg cm−2 steam pressure. Regardless of solvent used to extract the SF, rats fed SF that had been autoclaved grew faster (5.3 versus 3.4 g day−1)and were more efficient (gain:feed, 0.35 versus 0.27) than rats fed SF that had not been autoclaved (P < 0.001). Soya bean flakes that had been autoclaved before extraction supported rates and efficiencies of gain that were 16% and 12% greater (P < 0.005 and P < 0.001, respectively) than SF autoclaved after extraction. Alcohol‐extracted SF' supported greater rates (P < 0.03) and efficiencies (P < 0.002) of gain than water‐extracted SF. In experiment 2, 720 weanling pigs were used in a growth assay and, in experiment 3, 30 pigs were used in a nitrogen balance study. Treatments used in both experiments were: (1) defatted raw SF, (2) autoclaved SF, (3) ethanol‐extracted SF, (4) SF autoclaved before extraction with ethanol, and (5) SF autoclaved after extraction with ethanol. Ethanol extraction of the unheated SF resulted in an improved growth rate (74 versus 15 g day−l, P < 0.005) and nitrogen retention(51.5 versus 37.3%; P < 0.001) when compared with the raw unextracted SF. However, ethanol extraction in addition to autoclaving did not improve protein quality over autoclaving only (P > 0.05). Alcohol extraction improves the utilisation of protein from defatted raw SF by rats and pigs, but changes in protein utilisation due to alcohol extraction of properly heated SF are minimal.