Neither cottonseed oil nor glycerol creates firmer pork fat

Lee Johnston, WCROC

Cassio Villela, Ryan Cox, Gerald Shurson, Kaitlyn Compart, and Pedro Urriola, U of MN Dept of Animal Science

March 2015
Feed ingredient prices often drive pork producers to include very high levels (30 to 50 percent) of Dried Distillers Grains with Solubles (DDGS) in diets for growing-finishing pigs.  DDGS is the material remaining from production of corn-based ethanol and is commonly fed to livestock.

Such high inclusion rates of DDGS create very soft fat in the resulting pork carcasses due to the high concentration of unsaturated fatty acids in DDGS.  Soft pork fat causes great concern for processing and marketing of pork.  Pork processors find it difficult to slice bellies for bacon when the fat is soft.  In addition, shelf-life is reduced in pork products with soft fat and their visual appearance to consumers is less desirable.  Pork producers want to continue using DDGS in diets when it is priced competitively but they also want to produce pork carcasses with the more desireable firm fat.  So, we have been looking for solutions to this challenge.

Earlier research conducted at the West Central Research and Outreach Center (WCROC) indicated that crude glycerol fed in corn-soybean meal based diets improved firmness of pork bellies.  Pork bellies are high in fat and are processed for bacon.  Crude glycerol is a by-product of biodiesel production and is sometimes fed to livestock as an energy source.  Since glycerol seemed to increase fat firmness in corn-soybean meal based diets, we wanted to try using it in the high DDGS-containing diets being fed today.

Feeding cottonseed oil is a novel approach to correct DDGS-induced soft pork fat.  Cottonseed oil contains relatively high levels of cycloporpene fatty acids.  Cyclopropene fatty acids are part of the cottonseed oil and can decrease the conversion of saturated fatty acids to unsaturated fatty acids in the body.  Unsaturated fatty acids are the primary cause for soft fat in pork carcasses.  By decreasing this conversion, we may be able to improve the firmness of fat in pigs fed high livels of DDGS.  Over fifty years ago, prominent researchers recognized that cottonseed products (oil and meal) made pork fat hard.  For the past 50 years, this knowledge has been forgotten.  Cottonseed oil is not a common ingredient for swine diets.

So, we decided to conduct a study to determine if adding glycerol or cottonseed oil to a corn-soybean meal diet with high levels of DDGS would improve firmness of pork fat.  Financial support of the Minnesota Pork Board and the Agricultrual Utilization Research Institute (AURI) made this study possible.

Procedure

Two hundred sixteen grower pigs were housed in the grower-finisher swine research unit at the WCROC.  Beginning weight for the pigs was about 53 pounds.  Pens of pigs (nine pigs/pen) were assigned randomly to one of three dietary treatments resulting in eight pens per treatment.  Pigs were kept on their experimental diets for 108 days and were marketed at a body weight of about 265 pounds.

Experimental diets consisted of: 1) a corn-soybean meal diet with 40 percent DDGS (CONTROL); 2) the control diet plus five percent cottonseed oil (COTTON); or 3) the control diet plus eight percent crude glycerol added for the last six weeks before harvest (GLYCEROL).  Pigs had unlimited access to feed and water throughout the experiement.

Two pigs from each pen (47 pigs in total) were selected for in-depth evlaution of fat firmness.  Bellies from the right side of the carcasses were retrieved at harvest.  Bellies were subjected to a belly flop test as a measure of fat firmness.  In a belly flop test, bellies are placed perpendicularly on an elevated stick.  The distance between the two drooping ends of the belly are measured.  Firmer bellies would be more rigid and therefore have a longer distance between the belly ends compared with a softer belly.  Belly flop is measued in degrees.  So, the larger the number, the firmer the belly.

Results

Neither cottonseed oil nor glycerol diets improved belly firmness as measured by the belly flop angle (see table below).  The belly firmness measurement for cottonseed oil was numerically higher than the control diet but the difference was so small and variable that we would not expect this result consistently.  The belly flop measurement for glycerol-fed pigs was very similar to that of the control pigs.  Similarly, subjective firmness scores of belly fat were not different when comparing cottonseed oil or glycerol diets with the control diet.

Iodine value is a common measure to indicate how much unsaturated fat is present in a sample.  Unsaturated fat is soft fat.  As the Iodine Value number gets higher, the fat gets softer.  Feeding cottonseed oil increased the Iodine Value compared with control and glycerol-fed pigs.  We expected that feeding cottonseed oil would decrease the Iodine Value number but the opposite occurred.  The cottonseed oil diet had higher fat content than the other diets and this may be why the Iodine Value rose for these pigs.  We hoped that the cyclopropene fatty acids in the cottonseed oil would be able to overcome the problem with soft fat but our data suggest the cottonseed oil was not effective.  Likewise, feeding glycerol was not effective in making bellies and pork fat firmer.

In conclusion, neither cottonseed oil nor glycerol additions to diets containing 40 percent DDGS increased belly firmness or reduced Iodine Value.  Our results are disappointing in that we did not find a solution to the problem with soft fat caused by feeding DDGS.  However, we did learn that cottonseed oil and glycerol are not good solutions.  This means pork producers and other researchers can spend their time and effort on potential solutions other than cottonseed oil or glycerol for this problem in the future.

Table 1: Effect of feeding cottonseed oil or glycerol on firmness of pork belly fat
Dietary treatments(1)
ItemCONTROLCOTTONGLYCEROLPooled SEP - Value
Belly firmness, degrees(2)6.218.576.060.950.16
Subjective firmness scores(3)4.674.224.450.170.22
Iodine value71.1(a)78.6(b)70.6(a)0.720.01

(1) CONTROL = corn-soybean meal with 40% DDGS basal diet; COTTON = control diet + 5% cottonseed soil; GLYCEROL = Control diet + 8% glycerol fed during the last 6 weeks of the experiment.

(2) Belly firmness - a larger number indicates a firmer belly.

(3) Subjective firmness scores from: 1 = very soft to 5 = very firm.

(a), (b) Means within a row with different superscripts differ (P<0.05).