The effect of heat on amino acids for growing pigs

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Abstract
Three experiments were conducted to examine the effect of heating field peas (Pisum sativum cultivar Dundale) on (1) proximate analysis and total amino acid composition, (2) ileal and faecal digestibilities of amino acids, and (3) digestible energy content. Alternative techniques for assessing ileal and faecal digestibilities and digestible energy respectively, were also investigated. Forced-air dehydrators were used to heat field peas at temperatures of 110°, 135°, 150° or 165°. In the first experiment the apparent ileal and faecal digestibilities of amino acids and the faecal digestibility of energy in the raw and heated field peas were determined using pigs fitted with ‘T’ -shaped cannulas. In the second, apparent ileal digestibility of amino acids and the faecal digestibility of energy were determined using the direct ileal and rectal sampling technique. This involved a single collection of digesta and faeces from the digestive tract of the pig while it was anaesthetized. The faecal digestibilities of amino acids and energy were determined using total faeces collection in the third experiment. In all experiments the respective field-pea treatments comprised 400 g/kg sugar-based diets and were the only source of amino acids. Heat significantly decreased the lysine (14.6–8.7 g/kg; P < 0.001), cystine (3.2–2.6 g/kg; P < 0.01) and arginine (16.7–14.5 g/kg; P < 0.05) contents of the heated peas. The ‘reactive’ lysine content of the field peas, as measured using the Silcock technique, was decreased by 0.11 and 0.30 with the application of heat at 150° and 165° respectively. Heat treatments did not alter the ileal digestibility of most amino acids. Only aspartic acid (0.72–0.58), glutamic acid (0.80–0.65) and the basic amino acids, lysine (0.79–0.56) and arginine (0.85–0.75), showed a significant linear decrease (P < 0.05) in ileal digestibility over the heat treatments, determined using the ileal cannulation procedure. Heating significantly (P < 0.05) decreased faecal digestibility for all amino acids. Faecal digestibility was consistently greater than ileal digestibility for the raw field peas; however, this difference decreased with heat application until faecal digestibility was equal or less than ileal digestibility at the 165° treatment. Heat linearly depressed digestible energy, diet dry-matter digestibility and diet energy digestibility. Losses in lysine, cystine and arginine are likely to be due to early and advanced Maillard reactions. Considerable binding of the remaining lysine also occurred as indicated by a decline in Silcock-reactive lysine. The results indicate that the direct ileal sampling technique is a viable alternative to the cannulation procedure for amino acids, but further method development is required to decrease the variability associated with measurements. The estimation of faecal digestibility using indigestible markers and the partial faeces collection technique was as efficient as total faeces collection. In general, ileal digestibility of amino acids showed little response to heating, however, any changes that were observed were greatest for lysine. In contrast, faecal digestibility of all amino acids was greatly reduced with increasing heat application. This response appeared to be largely due to the effect of heating on microbial degradation and synthesis of amino acids in the hind-gut, rather than a reflection of the changes within the protein induced by heating. This variable response makes faecal digestibility an unreliable estimator of amino acid ileal digestibility.
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