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ESTIMATION OF PROTEIN QUALITY IN HEATTREATED SOYBEAN MEAL AND SOY PASS® Megan L. Proctor, Hardy M. Edwards III, Robert N. Corley III, Michael R. Murphy, and Catherine E. McDonald TAKE HOME MESSAGES ♦ As expected, heat treatment reduced the solubility of protein and estimated availability of amino acids in soybean meal. ♦ Soy Pass® had a protein solubility and estimated availability of amino acids equivalent to that of autoclaving soybean meal for 39 and 36 minutes, respectively. ♦ Both measures of protein quality suggest that the ruminal degradabilty of Soy Pass® has been reduced without adversely affecting postruminal amino acid availability to the cow. INTRODUCTION Increasing the rumen-undegradable protein of soybean meal without making it unavailable to the cow has been the goal of many different treatments. Heat treatment first reduces the solubility of protein in soybean meal and then, if prolonged or excessive, reduces the availability of its amino acids to the animal. Protein solubility is often assessed using a dilute KOH solution. Amino acid availability is sometimes evaluated by measuring the ability of a protein to support the growth of rats or poultry; however, in vitro methods are also available to estimate it. Soy Pass® is a treated soybean meal marketed by LignoTech USA, Inc. of Rothschild, WI, as an economical and palatable source of rumen-undegradable yet digestible protein for lactating dairy cows. Our objective was to compare the protein solubility and estimated availability of amino acids of this product to that of soybean meal which had been heat-treated for various lengths of time. MATERIALS AND METHODS Soybean meal from the University of Illinois feed mill was spread thinly in pans and heated (autoclaved) at 248 ºF (120 ºC) for 0, 10, 20, 40, or 60 minutes. Crude protein solubility of the control and treated soybean meals was determined after stirring a sample in a 0.2 percent KOH solution for 20 minutes. The nitrogen contents of the meals and solutions were measured using the Kjeldahl procedure. A dye-binding technique was used to estimate the availability of amino acids in control and treated soybean meals. Orange G (1-phenylazo-2-naphthol-6, 8-disulfonic acid sodium salt), under acidic conditions, binds to free amino groups. If the lysine in soybean meal is made unavailable by Maillard reactions at the epsilon amino group during processing, then less Orange G is expected to bind to the feed. Solubility and availability of amino acids in Soy Pass® were determined using the same techniques applied to the control and treated soybean meals. RESULTS AND DISCUSSION As expected, protein solubility and availability of amino acids (dye-binding capacity) in soybean meal decreased as time of heat treatment (autoclaving) increased (Figure 1). These results were similar to those reported in the literature and suggested that our treatments provided a good baseline with which to compare the Soy Pass® product. The protein solubility and dye-binding capacity of Soy Pass® were 24.2 percent and 41.8 milligrams/gram of soybean meal, which were equivalent to autoclaving soybean meal for 39 and 36 minutes, respectively. Depression of growth and feed efficiency in nonruminants is not usually seen until soybean meal has been autoclaved for at least 60 minutes; therefore, it does not appear that postruminal amino acid availability is an issue with Soy Pass®. 80 60 40 20 0 0 20 40 60 Autoclave time, min CP Solubility, % Dye, mg/g SBM Figure 1. Solubility of crude protein (CP) and dye-binding capacity of heat-treated soybean meals.