In the world of petfood manufacturing, some ingredients are used that are not nutritional, by intent. Rather, their purpose is to enhance the nutritional value, flavor or stability of the food. Sodium bisulfate is just such a functional food additive that one might find in petfoods, especially cat diets. However, this ingredient isn’t well known to consumers and has created some confusion regarding its purpose.
Specifically, some people have confused sodium bisulfate with menadione sodium bisulfite complex, the vitamin K3 source. Further, it should not be confused with sodium bisulfite, a chemical preservative used in fruits and wine. So, maybe some background on this ingredient, how it behaves in a food, what it does for the animal and whether it is safe for use in petfoods is in order.
In short, the primary function of sodium bisulfate is acidification. Just like the phosphoric acid in your cola provides that tangy acid flavor you couldn’t or wouldn’t go without, so too does sodium bisulfate give an extra punch to the flavor of foods—especially if you are a cat.
For reasons not completely explained, cats have a taste or desire for foods with a pinch of acidity. For example, a leading supplier reports that in a standard split-plate palatability test, cats preferred a diet containing sodium bisulfate to one with a comparable level of phosphoric acid. Whether this result is sustained when not offering a choice is somewhat unresolved as chronic consumption of diets containing either sodium bisulfate or phosphoric did not differ in a study published by Spears et al. (2003).
Regardless of the equivocal palatability results, in the work by Spears, sodium bisulfate performed equal to phosphoric acid for the purpose of acidifying feline urine: The urine pH following meals was not different and did not improve as the proportion of the two acids were included incrementally from 0.2% to 0.6% of the diet.
This may seem insignificant to the casual observer; however, parity in performance in this case is important. There are often times when it is desirable to lower phosphorus (phosphoric acid can be a significant source) and concurrently calcium and ash in the diet for health and organ-specific pathologies. In these cases, having an ingredient tool to replace phosphoric acid as a urine acidifier—that also helps maintain mineral balance—can be a significant benefit.
Further, like phosphoric acid, sodium bisulfate has been used to help control bacterial growth on such disparate compounds as soft drinks in the food industry and chicken litter in agriculture. It has been hypothesized in today’s era of food safety that sodium bisulfate may also provide some protection against microbial pathogen contamination in extruded foods (Knueven, 2012). It most likely performs this activity as a function of dissociation into its ionic form at a pKa (pH at which half the molecule is dissociated) of 1.99, a value very comparable to phosphoric acid (pKa = 2.1), and by creating an unfavorable acidic environment for target organisms. If validated, this could provide a method for manufacturers to use against microbial pathogens.
Sodium bisulfate, sodium hydrogen sulfate or sodium pyrosulfate as it may be called in various disciplines, is a white, odorless, fine granule. The technical grades of the mineral have several industrial applications in metals, cement manufacture and bleaching of such things as wool or leather. The more purified forms may be found in cleaning compounds and, of course, as an additive in foods.
Sodium bisulfate is produced from a reaction between salt (sodium chloride), sulfuric acid and a small amount of water at temperatures exceeding 650ºF. The sulfuric acid originates from “roasting” sulfur containing ores and the salt from mines or dried sea beds. Because of these humble origins, some have made the case that sodium bisulfate conforms to the definition of “natural.”
The reaction of salt and sulfuric acid results in the production of sodium bisulfate (NaHSO4) and hydrochloric acid (as a gas). The sodium bisulfate is a molten liquid that is sprayed from a tower down into a rising column of hot air, where it crystallizes into fine granules as it falls. The composition of the material resulting from this reaction is approximately 20% sodium, along with other trace amounts of minerals such as chloride, potassium, calcium and iron.
The use of sodium bisulfate in petfoods is relatively straightforward. It is a dry, freely flowable powder most commonly purchased in plastic bags or totes or in bulk for large industrial applications. It is very hygroscopic, so warehousing in dry conditions is important.
Even though the product is commonly associated with palatability enhancement and would lead one to consider it for topical application, it is most commonly included with the dry components of the diet prior to processing. This might give one pause considering that acids are often corrosive to metal equipment; however, sodium bisulfate is not typically considered to be an issue by most equipment manufacturers. The only formulation concern is accounting for the quantity of sodium, but this typically is not a factor given the inclusion rates are fairly low—between 0.5% and 1.5%.
Sodium bisulfate is considered a special purpose product and classified as a general purpose feed additive with no limitations or restrictions identified (AAFCO, 2012). It is used today in soft drinks, fruity beverages and various food applications (including petfood) without incident. Incorporation into cat diets provides a benefit to palatability, urine acidification and potentially safety of the food.
Opportunities to expand use of this ingredient to other animal foods (e.g., dog foods) are likely considering safety concerns regarding microbial control continue to be a driving force for the production of foods compliant with the Food Safety Modernization Act. Sodium bisulfate may be a significant part of a successful strategy.
Find more columns by Dr. Aldrich at
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