Pork producers and packers are concerned about carcass fat quality, but definitive guidelines remain elusive.
A wide variety of factors impact carcass fat quality and, in turn, pork quality, but the main focus should be on consumer acceptance of the end product, agreed packers and producers attending a National Pork Board-sponsored symposium in Des Moines, IA, recently.
Fat quality is defined in terms of a carcass' physical, chemical and sensory characteristics and is typically measured by iodine value.
Quality, it seems, is in the eye of the end user. Soft fat has typically been undesirable to the pork export market, but domestic customers may be a bit more tolerant.
Foremost in the fat quality discussion is bacon. U.S. consumers buy more than 700 million lb. of bacon each year — a $2 billion industry.
Dana Hansen, North Carolina State University, says one survey indicates 50% of U.S. consumers have bacon in their freezer at all times. “But fat quality can have a negative impact when you see it in the bacon package,” he says. Firm fat ensures slice definition of vacuum-packaged bacon. Soft fat can cause processing challenges. Soft bacon slices flatten together with no slice definition and have a very oily or wet appearance in the package, Hansen explains.
Even though bacon slice yields are not affected by fat firmness, packers say soft fat leads to processing and storage challenges. “Generally speaking, poor fat quality (soft fat) leads to reduced product workability,” he continues.
Soft fat can also decrease shelf life because of an increased lipid oxidation rate, which may lead to rancidity, depending on the length and type of storage conditions.
Iodine value (IV) is the most common measurement associated with pork fat quality. IV measures the mass of iodine (grams) that is absorbed by 100 grams of an unsaturated fat. Double bonds of an unsaturated fat react with the iodine solution. The higher the iodine number, the more unsaturated fatty acid bonds are present in the fat.
The firmness of fat in pig carcasses is determined by the ratio of unsaturated fatty acids compared to saturated fatty acids. The higher the IV number, the greater the proportion of unsaturated fatty acids compared to saturated fatty acids. A high number means more soft fat, a low number means fat is firmer.
“Current pork fat quality standards are not well-defined,” says Jerry Shurson, University of Minnesota. The Danish Meat Research Institute recommends a maximum IV of 70, generally the maximum standard in Europe. Research conducted in 1997 suggests that acceptable pork fat quality can be achieved at an IV at or below 74.
IV is commonly measured on backfat, belly fat or jowl fat. “Various adipose tissue sites are affected differently by dietary fatty acid composition,” Shurson says. The different locations measured on the carcass can yield very different IV numbers, leading to some inconsistency in how IV is reported.
Research shows that IV even varies within backfat layers. Outer backfat layers tend to have higher IV levels than middle backfat layers, and middle is higher than the inner backfat layer. Jowl IV tends to register higher than backfat IV, as a whole.
IV can also be calculated using equations that include various fatty acids, or it can be measured directly in the laboratory. Different values are obtained depending on the equation used vs. determining it directly, Shurson says.
The debate is about whether IV is the best measure of pork fat quality, and if it is, the ideal IV and where it should be measured remains unresolved.
Pigs really are what they eat, with resulting body fat bearing a close resemblance to the types of fats consumed. Dietary changes can have the greatest impact on managing fat firmness, but non-nutritional factors also play a big role.
Genetics, growth rate, age, body weight, health status, gender and again, where fat is measured are all factors. Gilts, typically leaner than barrows, generally have softer fat. Research confirms a 1-2 IV unit difference between barrows and gilts.
“The pig is what it eats as long as it is growing and healthy, but it is important to realize that stress can also impact carcass quality,” says Mickey LaTour, Purdue University, who has studied how stress, health and housing affect fat quality.
“Physiological stress can lead to cortisol release and drives a metabolic effect on the animal,” LaTour explains. In a 2009 study, he found that grow-finish pigs subjected to higher temperatures and reduced space had poorer fat quality, even though all pigs were on the same diet. “We found that when pigs are heat stressed and crowded, that added 3-4 points to the iodine value score,” LaTour says. Researchers speculate that a recovery period, diet or management changes could help reduce the impact the stress has on fat quality, but more research is needed.
The amount of distillers dried grains with solubles (DDGS) continues to increase as pork producers seek cost-effective diet alternatives. In 2008, an estimated 3 million metric tons of DDGS were fed. The 2009 total is expected to be higher. Whether or not DDGS leads to less desirable carcass fat remains unanswered.
In 2009, the use of DDGS in swine diets saved as much as $30/ton in feed costs, Shurson says. About 85% of the DDGS fed is used in grow-finish diets, fed at levels up to 40% of the diet. The feed cost savings is $3 to $6/market hog.
Shurson says as the ethanol industry evolves, there is a greater variety of byproducts available, such as crude glycerol from biodiesel production and steep water from wet-milling ethanol production. “We need to understand the impact of some of those co-products on pork quality,” he relates.
Shurson says both fat content and energy content can be quite variable with ethanol co-products. “Depending on what our targets are for acceptable pork fat quality, some co-products are more acceptable as feed than others,” he says.
In 2009, Shurson and University of Illinois colleague Hans Stein summarized about 25 studies focused on the impact of feeding different levels of DDGS to grow-finish pigs.
“In general, feeding distillers grains doesn't have much impact on performance of grower-finisher pigs,” Shurson says. “More specifically, it seemed that feeding around 30% or less DDGS didn't impact carcass backfat thickness, belly thickness or belly fat color. But we did see a reduction in dressing percent about half of the time. Usually, when we see a reduction in yield, it is a very small amount.”
Shurson says research indicates IV increased by approximately two units for every 10% of DDGS added to the diet. Diets containing up to 30% DDGS produced loin muscle characteristics that met current National Pork Board target values.
“Depending on what your pork fat quality standards are, this would suggest greasy appearance is the biggest negative, but overall, feeding DDGS doesn't seem to have a negative impact on pork fat,” Shurson says.
Recent University of Minnesota research looked at consumer taste panel evaluations of loins and bacon from pigs fed up to 30% DDGS. Cooking loss was not negatively impacted by the DDGS, and taste panels could not tell the difference in loins across treatments.
Bacon was another matter. The taste panels rated flavor, tenderness and fattiness of bacon from pigs fed 30% DDGS more positively vs. control pigs and pigs fed lower levels of DDGS. The Minnesota research indicated pork loins from pigs fed up to 30% DDGS could be stored up to 28 days without significant effect on lipid oxidation.
Shurson believes a “best-of-both-worlds” strategy will work. “We did a study to find out what happened if we took DDGS out of the diet prior to harvest and found that withdrawing distillers helps correct some of the IV targets,” he explains.
An IV of less than 70 can be achieved by feeding 15% dietary DDGS throughout the grow-finish period with no withdrawal, or by feeding 30% dietary DDGS with a three-week withdrawal, pre-harvest.
“Research has shown linear reductions in linoleic acid and IV content of belly fat over time when DDGS was removed or reduced in the diet,” he says.
In addition to level and length of time DDGS is fed, swine nutritionists remind producers that negative effects may also be affected by other ingredients in the diet. The complete diet needs to be evaluated without focusing too much on individual ingredients, according to Kevin Touchette, Cargill Animal Nutrition.
Wayne Cast, swine nutrition specialist with Value-Added Science & Technologies (VAST), says the economic benefit from feeding DDGS is huge, and any discussion about restricting IV through dietary measures, such as limiting DDGS, should be approached cautiously.
“Restrictions on IV could potentially increase feed cost. Making changes that reduce one IV unit can add $0.29 to $1.08/pig, depending on the price of DDGS and the nutrient contents of the ration,” he explains. VAST's services include evaluating quality and cost of DDGS sources in rations.
The Fat Quality Symposium was a first step in bringing pork industry stakeholders together to discuss the various factors that impact pork quality, says Steve Larsen, National Pork Board director of pork safety.
During a packer round table discussion, Dennis Seman, applied principle scientist for Oscar Mayer-Kraft, summed up the challenges and goals associated with fat quality:
“We want every consumer to have a good experience when they buy our product. We want consistency from pork producers and we also have to please the consumer. We're all in this together.”
Lora Berg is a freelance writer from Lakeville, MN.