Sweet potato flour in extruded kibble: what the research says about expansion, texture and palatability

Why the starch source changes the kibble

Dry pet food is made by cooking a meal under heat, moisture and pressure in an extruder and forcing it through a die. The starch in the recipe gelatinises during this process and sets the structure of the kibble as it expands and dries. Change the starch source and you change how much energy the extruder needs, how the kibble expands, how dense and hard it is, and how it feels in the mouth of the animal.

The most useful public evidence on sweet potato flour comes from a university animal-nutrition research group that tested seven starch sources — corn, brown rice, sorghum, potato starch, sweet potato flour, chickpea and pea — as the sole starch source in dog diets formulated to about 300 g/kg starch, produced on a single-screw extruder at roughly 1,070 kg/h [1]. A companion paper from the same trial reports digestibility, fermentation and glycaemic results [2].

What happens in the extruder

In that trial, the sweet potato and potato diets needed more water in the preconditioner and showed higher specific mechanical energy in the extruder barrel than the corn diet [1]. The authors did not test these process variables statistically, because there were only two observations per treatment, so they should be read as a tendency rather than a proven effect.

The practical point for a plant is that a recipe switched from cereal to sweet potato flour may not run on the same settings, and the trial gives the parameters to watch: water addition, screw speed, die configuration and drying. In the trial, the sweet potato kibble came out of the dryer at 48.9 g/kg moisture, compared with 36.7 g/kg for the corn kibble, on that line and with those settings [1]. Whether the same happens on another line is something to measure, not to assume.

Work on sweet potato flour alone, outside pet food, points the same way. A study on a single-screw extruder found that expansion of sweet potato extrudates rose with barrel temperature when flour moisture was low, and that moisture and temperature were the variables that mattered most [6].

Kibble expansion, density and hardness

The clearest effect of sweet potato flour is on structure. Compared with cereal- and pulse-based kibble, the two tuber-based diets gave:

• higher expansion — expansion index 2.33 for sweet potato and 2.52 for potato starch, against roughly 1.6 for corn;
• lower density;
• more pores and larger pore area;
• hardness similar to the corn kibble for sweet potato, while the potato starch kibble was the hardest of the seven [1].

The authors attribute this to the starch itself: tuber starches have a higher amylopectin share, which gelatinises more easily, whereas pulse starches come with more fibre, which hinders cell formation and expansion. The pea diet, at the other extreme, gave the least expanded and densest kibble [1].

A separate dose–response study by the same group replaced corn with 0, 100, 200 or 300 g/kg potato starch and found that expansion, hardness and pore count rose and density fell as the tuber starch went up [4]. The pattern is consistent with the seven-source trial.

For the formulator, a more expanded, lower-density kibble has consequences downstream: bag fill and bulk density change, the kibble takes up fat and palatant coating differently, and breakage during handling needs to be checked.

Degree of gelatinisation

Gelatinisation is what makes starch digestible and what binds the kibble. In a cat study that compared potato, cassava, sweet potato, rice and wheat as the main carbohydrate source, the degree of starch gelatinisation measured 90.2% for the sweet potato diet, 87.3% for cassava, 80.8% for potato, 80.3% for wheat and 72.4% for rice [3]; the paper reports these as measured values for each diet without a separate significance test. Apparent starch digestibility was above 97.9% for all five diets and did not differ significantly between them [3].

A survey of 20 commercial dry dog foods found that formulas built on non-traditional ingredients such as cassava, potato, chickpeas and lentils contained less total starch than traditional cereal-based formulas (about 20–22% versus 35–36%) but a higher proportion of gelatinised starch (around 95–97% versus about 86%) [5]. These were different products made under different conditions, so the survey describes the market rather than proving that the starch source caused the difference; it is consistent with, but not evidence for, the tuber-starch results from the controlled trials.

Palatability: the result that formulators should note

In the seven-source trial, 16 adult Beagles took part in paired preference tests of each diet against the corn-based diet over two consecutive days. Dogs preferred every diet over corn (p < 0.001) except the sweet potato diet, whose intake did not differ significantly from the corn control [1]. Only the chickpea diet showed a significant first-choice preference.

The authors discuss two possible explanations. One is moisture: the sweet potato kibble was the wettest after drying, and dogs are known to distinguish small differences in kibble moisture. But because dogs in other studies preferred potato, pea and chickpea diets over corn even at similar moisture, they conclude that kibble texture and, through it, the coating probably play a part as well [1].

Two points follow for anyone using sweet potato flour:

1. In this trial, intake of the sweet potato diet did not differ significantly from the corn control. That is not the same as proving the two are equivalent — the test was not designed to show equivalence — and it is one formula, one coating and one group of dogs. It does not show that sweet potato flour is free of palatability issues in every formula, nor that it offers a palatability advantage.
2. Drying and coating deserve attention. The more porous, slightly wetter kibble made with sweet potato flour may behave differently when fat and palatant are applied, and this is where a plant can recover or lose preference.

Digestibility and gut response: the trade-off

The companion paper from the same trial (14 Beagles in the digestibility work) reports that the sweet potato flour diet had the lowest coefficients of total tract apparent digestibility of the seven starch sources, while brown rice and pea were the most digestible [2]. This is the result a formulator should be upfront about. It does not mean every sweet potato formula is less digestible; it means that the digestible energy of a sweet potato formula should be calculated and confirmed in a trial rather than assumed to match a cereal formula.

Against that, the same diet gave higher faecal short-chain fatty acid concentrations than the potato and brown rice diets, a higher abundance of Faecalibacterium prausnitzii and Blautia in the faeces, and a smaller postprandial glucose peak than corn and potato starch [2]. The authors conclude that sweet potato and pulses “improve indicators of gastrointestinal functionality”.

Whether these gut effects matter for a given product is a question for the brand’s nutritionist. The formulation and any claim on the label are the responsibility of the pet food manufacturer; none of the studies cited here tested a particular commercial ingredient.

Treats and baked products

The studies above all concern extruded kibble. Public, peer-reviewed data on sweet potato flour in baked or cold-formed treats is thinner, and we do not quote figures we have not seen. What the extrusion work does establish — easy gelatinisation and a strongly expanded, porous structure — is relevant to any process that relies on cooked starch to hold a piece together, but the exact behaviour in a treat line has to be confirmed on that line.

Practical checklist for a formulation trial

• Specify the ingredient properly. Ask for the moisture, starch and fibre content of the sweet potato flour, the particle size and whether the raw material was peeled. These affect gelatinisation and expansion, and they are also what determine whether the published results apply to the material you are buying.
• Check and re-tune the extruder. In the published trial the tuber diets took more water in the preconditioner, more mechanical energy and came out wetter [1]; treat these as parameters to measure on your line, not as settings to copy.
• Measure kibble density and durability on the first run, and adjust die size if the expansion is too high for your bag or feeder.
• Run a palatability test against your current formula after coating, not before. The coating step is where the texture difference shows up [1].
• Confirm energy. Calculate digestible energy for the actual formula and check it in a trial; the published trial found lower apparent digestibility for its sweet potato flour diet in dogs [2].