Download ESTIMATION OF PROTEIN QUALITY IN HEAT-

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.