Discussions about fat quality in pork carcasses has heated up in recent months, while pork producers cast about trying to find lower-cost substitutes for the dietary staple ingredient in swine diets — corn.

The primary alternative, distiller’s dried grains with solubles (DDGS), is the go-to ingredient, perhaps ironically, provided by their biggest competitor for the nation’s corn supply — the ethanol industry. Other alternative feed ingredients include animal-vegetable (oil) blends from the restaurant industry, bakery meal/byproducts, glycerol and pet foods.

As corn prices rose, producers and swine nutritionists began to push the envelope, testing the upper limits of DDGS in diets, but not without some pushback from packers.

Currently, the most common measure of fat quality is iodine value (IV). “While not the perfect measurement of fat quality, we still come back to it because it’s the best measure we have right now,” comments Ken Prusa, professor in charge of the Sensory Evaluation Unit at the Iowa State University Nutrition Wellness Research Center. “It’s a controversial measure because it can’t be obtained at typical line speeds and not all packers collect the fat samples at the same location in the carcass.”

Still, some packers have put producers on notice that excessive IV levels will be met with price penalties.

Why all the hubbub about iodine value? Basically, IV levels are a measure of quality, and high levels result in soft, hard-to-slice bellies. It takes a No. 1 belly to produce bacon. A downgrade to a No. 2 belly means it cannot be used to produce bacon.

“If you take a belly down to its trim value, you’re looking at up to $25/head loss, on average,  and that’s significant for a plant slaughtering millions of hogs a year,” Prusa emphasizes. “Some No. 2 bellies are inevitable due to thin bellies, inspector trimmings and things like that.”

There are different ways to obtain IV. “You can actually extract fat from the belly, jowl or backfat, send it to a laboratory and they will provide a direct IV,” he continues. “There are also ways to calculate an iodine value using a fatty acid profile. With the lab IV, you get a number. But with the fatty acid profile, you can actually see how dietary factors affect the various fatty acids. There is also a standardized equation that allows you to calculate IV from the fatty acid profile.”

Iodine Value Trends

Working closely with packers and producers, some trends related to iodine values have emerged. “The general rule of thumb from work done in conjunction with Kansas State University is each 1% increase in DDGS in the diet will increase the IV level by 0.33 units,” Prusa notes. “So, if you increase DDGS by 10%, IV will go up 3-4 IV units. Of course, other factors come into play — genetics, gender, season, etc. But if you are just talking about DDGS levels, you can get a pretty good estimate of what IV levels will be.

“Usually, with a strictly corn-soy diet (no DDGS), we expect normal pig fat to be between 68-70 IV. If there is no fat in the diet and the pig has to manufacture his own fat, you will see an IV down around 65. But the pig is kind of lazy. It’s not going to make its own fat if there’s fat available in the diet. Whatever it eats, specifically polyunsaturated fat, it is going to be deposited in its carcass tissues,” he continues.

The IV of some common fats and oils are:

Coconut oil – 8-10

Beef Tallow – 38-55

Lard – 46-70

Choice white grease – 68-70

Corn oil – 111-130

Soybean oil – 137-143

Manhaden oil – 139-173

“We know that if we can keep IV in the low 70s, slaughtering at 280-285 lb., we can produce very good bellies from those pigs. When IV creeps up to the high 70s, low 80s, we know there will be issues with soft bellies and possibly some concerns about off flavors. When we make bratwurst or summer sausage and other products with high fat content, they will be more susceptible to rancidity and lipid (fat) oxidation,” Prusa explains.

Sensory panel members are trained in the ISU Sensory Laboratory using oxidized oil samples to identify the “painty, rancid aroma” you might smell if you opened a bag of stale potato chips.

“It’s probably not a huge problem in vacuum-packaged products because oxygen is required for the reaction to take place,” he says. A larger problem would be the more open, shingled bacon slices, which may be thawed and refrozen, commonly used in the fast-food and foodservice industries.

Sources of Variation

Variation in IV levels is normal in pigs — higher in the jowl, for example, than the belly or loin. Even at various points of the belly, IV can range from 67 up to 72, according to some studies.

Some packers collect fat samples from the jowl, which consistently has one to two points higher IV. “The jowl is a less valuable part of the carcass and easy to get at, but we feel the belly is the best place to take the fat sample because that’s where the potential for a problem lies,” Prusa says. To convert jowl IV to belly IV, multiply jowl IV x 0.98236 (i.e. 70 x 0.98236 = 68.77).

IV in gilts tends to run 2-3 points higher than barrows. Feeding Paylean (Elanco Animal Health) tends to push IV levels 1.5 to 2 points higher. By comparison, a gilt fed Paylean (without supplemental fat) would run about five IV points higher than a barrow not fed Paylean.

When DDGS are fed at high levels to very lean pigs, their fat layers are less pronounced. Sometimes the fat and lean layers tend to separate, which makes it very difficult to slice bellies, butts and the blade ends of the loin, he relates.

“All of these things have to be taken into account to make sure you hit an IV target in the low 70s,” Prusa emphasizes. “It’s even an issue in rendering. We’ve heard soap manufacturers have had to change their processing methods because the soap is softer and not setting up as well. It can even change the consistency of crayons. There are so many different things that are affected when we feed pigs differently, things we never think of, so if we don’t look at the entire package, we might miss something.”

What Lies Ahead?

Prusa recognizes it is impractical to ask producers to avoid all feed ingredients that are high in unsaturated fats. “Not feeding DDGS is probably not viable. My recommendation to producers is to feed no more than 20-25% DDGS, then in the last two finishing diets — the last 40-42 days — drop DDGS levels to 5%. We have quite a lot of data on that scenario and it seems to work out quite well,” he says.

Some producers and packers have tracked fat quality on pigs fed higher levels coupled with a withdrawal period before slaughter. “It takes a while for the fat to react. It’s not going to change overnight. It will take seven, maybe eight weeks for that fat to turn over, so that’s why I like to see it come out of the last two diets — 40-50 days before slaughter,” he reinforces.

For those tempted to feed higher levels of DDGS (40-50%), Prusa thinks producers should look at other tradeoffs — does it affect pig appetite, growth, feed conversion or pork or fat quality?

Quality Measures

“Pork quality programs have evolved from focusing on pH and color to what we have been doing the last 3-5 years — focusing on fat quality. We don’t want to lose sight of color, pH and marbling. Our biggest goal is to help producers raise the highest quality pork they can — whether it’s loin, bellies, fat, lean tissue, whatever,” Prusa declares.

“Many of these byproducts have become staples in swine diets. We will probably be feeding more, different ingredients in the future, so we want to take an active role in monitoring the levels being fed and their impact on the quality of lean and fat.”

Some studies have shown feed ingredients high in sugar not only affects fat quality in bellies, but also the pH and color of loins. “We know that if we feed large quantities of sugar right before slaughter, there will be a rapid pH decline. We’re not yet sure whether that equates to a quality shift.” 

The biggest challenge the pork industry faces is finding a technology that would allow fat sampling of every pig, or every other pig, without slowing the packer’s line speeds. Prusa describes that as “the gold standard” that would gather a lot more information needed to help guide producers’ nutrition programs, slaughter weights and leanness levels.

How soon will that happen?

“I liken it to when we moved from producing fat pigs to producing lean pigs,” Prusa says. “There were incentives for leanness. Nobody thought we could measure or estimate the percent lean in a carcass at a rate of 1,000 pigs/hour. Now we have the Fat-O-Meater, the Autofom. Technologies have evolved.

“I think that’s where we’re at with IV. We’re in the infant stages of looking for that technology. It will probably be near-infrared technology in some type of a gun shape that will be able to measure IV on every carcass or every other carcass on the line. It’s becoming that important,” he notes.