Pet Food Ingredients de-mystified: Sweet potato still a questionable novel (?) protein source

The sweet potato has become an increasingly popular ingredient in specialty petfoods. However, according to Dr. Greg Aldrich, PhD and President of Pet Food & Ingredient Technology Inc., which facilitates innovations in foods and ingredients for pet, it’s use in pet foods has not been ewll researched yet. In an article for the PetFoodIndustry.com, an on-line magazine for pet food professionals, he writes:

“In the ever widening search for novel pet food ingredients, one candidate was literally right under our feet. That is, until the last couple of years, when sweet potatoes became the "darling carb" of new products and increasingly popular in specialty pet foods. For example, sweet potatoes can now be found in:
Elimination diets for the treatment of food hypersensitivities/allergies (e.g., salmon & sweet potato);
As an option for pet owners wanting to provide variety (e.g., sweet potato vs. corn or rice);
As an ingredient in the new no-grain and raw formulas; and
As a novelty in boutique and ultrapremium petfoods.

However, almost nothing has been published regarding sweet potato nutrition or usage in companion animal diets. Given we are starting from scratch, what information is available?

Not even a potato?
Sweet potatoes (Ipomoea batatas) originated in the equatorial forests of the Americas (Peru and Ecuador) and have been cultivated for 5,000 years. They made their way to Europe following the explorations of Columbus and then on to Asia in the 16th century. Today, global sweet potato production is ranked fifth by weight, with more than 95% produced by developing countries. This is in part because sweet potatoes are well suited for cultivation on small free holding farms in tropical climates.

Partially due to agronomic and visual similarities between sweet potatoes and yams, there's confusion about the names, but the two are not actually related. The yam (Dioscorea spp) is a monocot tuber that originates from Africa and Asia, whereas the sweet potato is a dicot storage root from the Convolvulaceae (morning glory) family.

In fact, despite its name, the sweet potato is not even related to the potato (Solanum tuberosum). The "Irish" potato is a member of the Solanaceae (nightshade) family, which includes tomatoes, red peppers and eggplants and originates in the Andean mountain chain of South America. While the yam, potato and sweet potato are all root crops, the yam and potato taste bland and starchy and contain insignificant amounts of carotenoids. The sweet potato, as the name implies, is sweet to the taste, and the orange varieties are high in beta carotene.

That sweet taste
According to the US Department of Agriculture (USDA) National Nutrient Database for Standard Reference, Release 21, raw sweet potato (edible portion only) is mostly water (>77%) and carbohydrate (20%). On a moisture-free basis, the carbohydrate fraction is mostly starch (55%) and dietary fiber (13%), with the remainder soluble sugars. The protein level is comparable to other tubers and grains (~7%), fat is insignificant (< 0.5%) and ash (~4.4%) is a tad higher than other starch sources. The sweet potato represents a decent source of potassium (~1.5%) and is enriched with beta carotene (375 ppm) to a level nearly 60% that of carrots.
The sweet taste of sweet potatoes is developed by the enzymatic production of maltose from starch. Maltose is a disaccharide-reducing sugar composed of two glucose units in a 1-4 linkage and has a mildly sweet taste. Maltose content in the fresh, raw sweet potato is negligible; however, during storage and, more importantly, cooking, carbohydrate hydrolysis occurs. As cooking temperatures exceed the gelatinization temperature (~75˚C), starch is hydrolyzed to amylose by β-amylase; then β-amylase converts the amylose to maltose.

These enzymes are stable long enough during normal cooking processes for the hydrolysis to occur before inactivation at around 95˚C. The variety of sweet potato, storage conditions and cooking practices all influence the production of maltose. Baking is more effective at increasing the maltose concentration than is boiling (canning). The degree to which maltose would evolve during extrusion was not found in the literature. However, one could surmise that the brief residence time in conditioning and extrusion during kibble production would limit maltose development.

Sweet potatoes in pet food are well liked by dogs and are neutral on palatability for cats. Although the sweet potato, like any other plant ingredient, contains some anti nutritional components (e.g., trypsin inhibitors), these are not identified as an issue. Further, there are no case studies available on dogs or cats in which toxicity, intolerances or sensitivities are an issue.

Processing and sourcing
Unlike other root crops that store well, sweet potatoes have a limited shelf life. For this reason, their cultivation is timed so they can be sold fresh, or they are harvested in a "campaign" and frozen, canned or dried to chips or flour.

In canned pet foods, raw sweet potatoes can be chopped to incorporate into the batter. Upon canning, sweet potatoes will retain most of their shape but become soft in texture. In dry pet food applications, fresh sweet potatoes can be a bit more of a handling issue as they must be ground and managed with the slurry, similar to other fresh ingredients like meats, fruits and vegetables.

The other option is sourcing dry sweet potato as a chip, flake or flour. At present, sourcing chips or flakes seems more common, although sweet potato flour is reportedly used as a wheat flour extender during times of scarcity.

Sweet potato as an ingredient is not currently defined by the Association of American Feed Control Officials, so the name likely follows the nomenclature of the common name or USDA standards. Cost is a substantial challenge: Sweet potatoes can run five to 10 times the cost of common grains.

Like other novel ingredients used in pet foods, sweet potato will likely have a long term fit. But this will be limited by the availability of animal specific nutritional information and a more pet food friendly supply infrastructure.

Names and stories
In North America, the sweet potato is often (mistakenly) called a yam. There are several stories as to why. In one, slaves of the US's antebellum South mistook sweet potatoes for the yams of their native Africa and called them nyami. Another story goes that over eager marketers in the 1930s, attempting to differentiate their sweet potatoes from another, incorrectly labeled theirs as yams and the name stuck.

Enough confusion still exists that today the USDA requires products sold as yams (which are actually sweet potatoes) to carry the extra "sweet potato" identification. But, by whichever name they are labeled, sweet potatoes, not yams, are the more commonly available ingredient for pet food applications.”