Nutritional effects of heat treatment of feeds - Effects on digestion of protein, amino acids and starch in mink, rainbow trout and dairy cows / Ernæringsmessige konsekvenser av varmebehandling av fôr - Effekter på fordøyelse av protein, aminosyrer og stivelse hos mink, regnbueørret og melkekyr
The objective of the present work was to study effects of different heat treatment methods on digestibility of protein, amino acids and starch in monogastric animals represented by the mink and rainbow trout, and on rumen degradation and intestinal digestibility of protein, amino acids and starch in dairy cows. 1) Autoclaving (120˚C and 130˚C for 30 min.) of soybean meal and fish meal was conducted. True digestibility of protein and individual amino acids in mink and in situ digestibility in dairy cows was measured (Paper I). 2) Effect of extrusion temperatures (100, 125 and 150˚C) on digestibility of protein, individual amino acids and starch in a wheat/fish meal based diet for mink and rainbow trout were investigated (Papers II and III). 3) The digestibility of barley, oats, wheat, wheat bran, maize, sorghum, peas and soybeans was evaluated in dairy cows after milling, after pelleting (81˚C) and after expander processing at 110 and 130˚C. Effective degradation of crude protein and starch in the rumen, as well as total tract indigested protein and starch fractions, were determined by in situ methods (Paper IV). 4) The effect of treatment temperature, treatment time and glucose addition on the digestibility of protein, individual amino acids and starch in barley and peas was studied in situ in dairy cows. Ground barley and peas were heat treated at 100, 125 and 150˚C for 5, 15 and 30 min with and without addition of glucose (Papers V and VI).
Main findings were: 1) Feed content of arginine, lysine and cysteine decreased after autoclaving at 130˚C of soybean meal. True digestibility of cysteine, aspartic acid and lysine in mink was most reduced in soybean meal, while true digestibility of aspartic acid and cysteine was most reduced after autoclaving of fish meal. Rumen degradation of methionine and glutamic acid was most reduced in soybean meal, while glutamic acid and histidine were the most affected amino acids in fish meal. Total tract digestibility of protein and amino acids was not affected in dairy cows. 2) Feed content of amino acids was not affected by extrusion. Extrusion decreased slightly true digestibility of amino acids in mink, and increased starch digestibility, whereas apparent digestibility in rainbow trout was unaffected. 3) Generally pelleting at 81˚C decreased slightly rumen protein degradation, but had no effect on rumen starch degradation. Expander treatment was effective in protecting proteins from rumen degradation in all feeds evaluated, except maize, while expander treatment increased starch degradation of maize, sorghum, peas and oats. 4) The content of cysteine, lysine and arginine decreased after the hardest treatment (150˚C/30 min) in both barley and peas. In barley, rumen degradation of lysine, methionine, alanine and valine was most affected after heat treatment. In peas, cysteine was the most protected amino acid against rumen degradation. Addition of glucose decreased rumen degradation of protein in heat treated peas. Rumen degradation of starch was decreased by heat treatment up to 125˚C, but was increased after treatment above that temperature in both barley and peas.
It was concluded that cysteine is likely to be the most affected amino acid during heat treatment of protein-rich feeds, both concerning the content in feeds and digestibility in monogastric and ruminant animals. Lysine is particularly affected after harsh heat treatment (≥130˚C/30 min) and in grains with high levels of carbohydrates. Extrusion at temperatures up to 150˚C is a lenient processing method concerning protein and amino acid digestibility in monogastrics. It was also concluded that it is possible to protect starch against rumen degradation by heat treatment, but processing conditions seem to be of great importance. Optimal protection of starch may require; low moisture levels (<25%), medium treatment temperatures (not above 125˚C), and no shear or mechanical treatment during the heating.
