Functional fiber with color
Tomato pomace has the potential to provide additional nutrition and health benefits
According to the US Department of Agriculture, tomatoes are the second most popular vegetable crop behind potatoes, with an annual average per capita consumption of 71 pounds going into juice, sauce and paste. The backstory is that 10-30% of this is seeds, skin and pulp, with no ready market in the human food aisle. This translates into an estimated 750,000 metric tons of dried tomato pomace potentially available to pet and livestock feed markets.
Given that the US ranks about fifth in the world acreage of tomatoes grown, tomato pomace could be a readily available ingredient for petfood. To that end, we are seeing a growing number of petfoods for which tomato pomace appears on the ingredients list.
Is this growing popularity a function of cost and availability, or does tomato pomace impart some nutritional benefit to petfood? Yes and yes.
From vine to powder
Tomato pomace is commonly traded on an air-dry basis (approx. 5-10% moisture) at a composition of around 20% protein, 13-15% fat, 3-5% ash and 25-35% crude fiber. The Association of American Feed Control Officials defines it as "the dried mixture of tomato skins, pulp and crushed seeds."
Tomato pomace starts with the processing of whole tomatoes into juice, sauce or paste. The tomatoes are pressed to expel the juice, then separated from the seeds, skins and most of the pulp. The resulting residue contains enough moisture (approx. 60-70%) that it must be dried to keep from spoiling.
Once dried, the residue is ground or pulverized into a powder. Besides producing a consistent particle size, the grinding also liberates the oils found in the seeds. These tomato seed oils are substantial (about 35% of the seed) and predominately unsaturated (approximately 82%; Giannelos et al ., 2005) which makes them susceptible to oxidation. So after grinding, an antioxidant preservative is often added for long-term storage. The final product is an orange to pink, finely ground, almost dusty, freely flowable tomato-smelling powder.
Functional fiber option
One of the first research papers published evaluating tomato pomace in pet diets reported that when dogs were fed a corn and soybean meal diet supplemented with tomato pomace, digestibility was comparable to those diets supplemented with beet pulp or grape pomace (Allen et al ., 1981). A few years later, Fahey et al. (1990) reported that the total dietary fiber (a measure of nutritionally functional fiber) found in tomato pomace was comparable to beet pulp and wheat bran. When tomato pomace was included in dog diets at a similar level to other fiber sources, the digestibility, elimination frequency and fecal volume were similar.
More recently, Swanson et al. (2001) reported that tomato pomace had a total dietary fiber content of approximately 57% and a majority of this fiber was insoluble (53% vs. 4% soluble). Following 24 hours of bench-top ( in vitro ) fermentation, 35% of the tomato pomace was degraded. Short-chain fatty acid production was moderate relative to other fiber sources and resulted in high proportions of the beneficial short-chain fatty acid butyrate. A major concern with fiber sources is whether they lead to flatulence. These researchers reported that gas production during the 24-hour period was significantly less than from most other fiber sources.
Tomato pomace is commonly incorporated in dry formulas at 3-7% of the ingredient mix and has little to no impact on food processing.
Color is meaningful
Considering only this fiber contribution may miss a big part of the storythe tomato's positive connection to human health. Tomatoes and their content of the antioxidant carotenoid lycopene have been linked to preventing numerous human diseases, including some forms of cancer and coronary artery disease, via a reduction in cholesterol.
Lycopene is the pigment responsible for tomatoes' red color, and unlike many other vegetable carotenoids, it persists through processing. For example, the content of lycopene in tomato pomace has been reported at 281 mg/kg (Botsoglou et al ., 2004).
While a link between tomato/lycopene and cancer prevention or heart-health benefits has not been demonstrated for dogs and cats, experiments with dogs have shown that following an oral dose, lycopene is readily absorbed and distributed throughout body tissues (Korytoko et al ., 2003). Inclusion of tomato pomace at 1% of the diet, along with other antioxidant fruits and vegetables, aided cognitive function retention in older dogs (Milgram et al ., 2005).
Tomato pomace has also been reported to contain relatively large concentrations of vitamin E (224 ppm) and total mixed tocopherols (2,059 ppm), along with other phytosterols such as campesterol, stigmasterol and B-sitosterol (King and Zeidler, 2004).
Fit for the formulator
While we know a fair amount about tomato pomace, we still lack reports evaluating it in cat diets, and studies on the differences among supply channels are conspicuously absent.
Some critics may suggest tomatoes are unsafe due to their membership in the nightshade family or that tomato pomace is a bio-accumulator of pesticides and herbicides. These suggestions have no validity. Rather, tomato pomace appears to be safe and effective, with a demonstrated record as a functional fiber and viable source of bioactive antioxidant vitamins and carotenoids.
As an ingredient in the formulator's toolbox, tomato pomace is a cost-competitive, readily available ingredient that reads well on the label and fits nicely in a petfood formula.