Phosphoric acid is a strong, inorganic acid often used in colas and other dark-colored, carbonated sodas. Its strength and neutral flavor make it a desirable choice for low-pH foods and beverages.

When choosing a sweetener for an acidulated food or beverage, product developers face a host of choices. Besides imparting a sweet taste, sweeteners also influence other important product attributes, including viscosity, flavor intensity, aftertaste and the perception of other tastes in a product. For example, as sensory studies have confirmed, perceived sourness is reduced with increasing levels of sweetener. In sensory science, this general phenomenon is commonly known as mixture suppression, which can be quite useful in situations where acids need to be elevated to improve microbial stability. However, at high levels, the product can taste too sour. Increasing the formula’s sweetener level will help suppress some of the unwanted sourness.

Sensory adaptation is another important consideration, when formulating a sweetened food or beverage. Adaptation is defined as a reduction in the perceived intensity of a taste following multiple exposures to the tastant, the chemical that stimulates the sensory cells in a taste bud. As an individual continues to consume a sweetened food or beverage, the perceived sweetness tends to decline. Research in this area suggests that non-nutritive, high-intensity sweeteners are more likely to result in sensory adaptation, when compared to nutritive sweeteners. In sweet-sour mixtures, sensory studies have shown sweetness intensity declined over time, while sourness intensity remained constant. These findings emphasize the importance of evaluating food products under realistic eating conditions. A single sip of a beverage or a bite of food will not provide an accurate picture of a consumer’s sensory experience.

From a caloric perspective, non-nutritive, high-intensity sweeteners are a great alternative to nutritive sweeteners, such as high fructose corn syrup (HFCS) and table sugar (sucrose). Unfortunately, most of these high-intensity sweeteners also impart some bitterness and a lingering aftertaste to products. One potential solution is to create a blend of non-nutritive sweeteners that complement each other. For example, a sweetener with a faster onset of sweetness could be blended with another sweetener with delayed sweetness. In the case of acesulfame-K and aspartame blends, aspartame offsets the bitter taste associated with acesulfame-K. In addition to reducing bitterness, this blend is also synergistic: the perceived sweetness of the blend is greater than the sum of its individual sweetness ratings.

Sweetener Choices

The decision to use either nutritive or non-nutritive sweeteners will have a major impact on the acids used in product formulation. Nutritive sweeteners like HFCS and sucrose provide the fewest headaches from a taste perspective.

Despite the negative press surrounding HFCS in the past few years, HFCS 55% (a blend of 55% fructose, 42% glucose and 3% polysaccharides) is still the sweetener of choice for most soft drinks. HFCS 55% is designed to mimic the taste profile of sucrose. Sucrose, the disaccharide of glucose and fructose, is the default sweetener for most foods and beverages. The clean, sweet taste of sucrose is considered the gold standard of sweetness. In fact, the properties of sucrose are used as a benchmark to evaluate all other sweeteners. Nutritive sweeteners not only add sweetness, but also body and mouthfeel to a product. This property is particularly important in beverage applications where the mouth-coating characteristics of a beverage can influence the perception of sweet and sour.

High-intensity, non-nutritive sweeteners pose additional challenges for food scientists. Since these sweeteners are added at such low levels to foods and beverages, they do not have the bulking effects of nutritive sweeteners. The lingering sweetness of some high-intensity sweeteners--as well as the bitter aftertaste of others--can be difficult to control. When selecting a combination of a high-intensity sweetener and an acid, it is important to match the taste profile of the sweetener with that of the acid. For instance, malic acid is known to have a lingering sour taste, and this property can be exploited, when combined with the persistent sweetness of high-intensity sweeteners.

Among common high-intensity sweeteners, aspartame and sucralose are the most similar to sugar in terms of their sweetness and lack of bitterness. For aspartame, the duration and intensity of sweetness increases at higher concentrations. Acesulfame-K and saccharin, on the other hand, are known to be two of the most bitter high-intensity sweeteners. To counteract the bitterness, saccharin and acesulfame-K are usually blended with another, less bitter sweetener to create a blend such as saccharin-aspartame, acesulfame-K-aspartame or sucralose-acesulfame-K.

Acids: The Basics

Acids are known to enhance the flavor intensity of many foods and beverages, especially fruit flavors. Interestingly, this flavor enhancement has not been attributed to an increase in flavor release from the food or beverage; the exact mechanism of enhancement is likely a combination of several sensory factors. In the culinary world, acids are often added to dishes, especially slow-cooked foods, for freshness and brightness. Although high levels of acids can be quite objectionable, more moderate levels are welcome additions to foods and beverages, especially sweetened products. As mentioned previously, both acids and sweeteners suppress each other in a mixture. Some sensory studies have shown that acids suppress sweetness more than sweeteners suppress sourness.

Food acids elicit two responses in the mouth: sourness and astringency. Astringency is often associated with a drying sensation in the mouth, which is believed to be caused by an interaction between acids and saliva. In most cases, inorganic acids are perceived as more astringent than organic acids. However, there is little consensus in scientific literature on the relative sourness of different food acids.

Although scientists initially theorized that sour taste was solely a function of the number of hydrogen ions in a food or beverage, this simplified view of sourness is no longer considered accurate. Instead, scientists now believe sour taste is a function of pH (-log [H+]), as well as several factors associated with the acids used in a formula, such as concentration and quantity of undissociated acid. To illustrate this point, sensory scientists performed studies on the perceived sourness of acetic acid and citric acid, as compared to hydrochloric acid. Although all of these solutions were maintained at the same pH, sourness ratings for acetic acid and citric acid were higher than those for hydrochloric acid.

Although scientists do not know pH’s exact role in the perception of sourness, changes in the concentration of free hydrogen ions in solution influence sour taste and intensity. To compare the relative sourness of different acids in a food or beverage, all products must be evaluated at the same pH. Without this control, one acid perceived as less sour at a higher pH may actually be more sour at a lower target pH.

Common Acid Choices

Citric acid is probably the most common organic acid used in foods and beverages. Containing three ionizable hydrogens per molecule, this triprotic acid is known for a bright, tart flavor that dissipates quickly. This acid pairs well with fruit flavors, especially citrus. Acetic acid, a monoprotic organic acid, is another popular acidulant imparting both tart and sour tastes. Lactic acid, another monoprotic organic acid, is a fairly weak acid with an acrid flavor that works well in dairy products.

Malic acid, a less common organic acid known for its smooth tartness and lingering sourness, works well with high-intensity sweeteners, as its lingering sourness balances prolonged sweet tastes. Both tartaric acid and fumaric acid are organic acids known for strong tart tastes and for the ability to enhance fruit flavors. Both acids are more astringent than other organic acids. Fumaric acid is one of the few food acidulants that does not readily dissolve in water.

Phosphoric acid is a strong, inorganic acid often used in colas and other dark-colored, carbonated sodas. This acid is one of the strongest food acids on the same weight basis and same pH as other food-grade acids. Its strength and neutral flavor make it a desirable choice for low-pH foods and beverages; however, this corrosive acid has regulatory and safety issues associated with it.

Buffering It Up

The primary goal of a buffer is to control variations in pH, preventing batch-to-batch inconsistencies in a food or beverage. A buffer is defined as a weak acid and its conjugate salt, or a weak base and its conjugate salt. Although a buffer can be either weakly acidic or weakly basic, most food buffers (like most foods and beverages) are acidic. In general, it is best to formulate a buffer system with a weak acid and one of its own salts (for example, combining citric acid and sodium citrate), than combining different acids and acid salts.

In a buffered system, anionic salts of acids react with dissociated hydrogen ions in a food or beverage to form the ionized version of the acid. By reacting and sequestering excess hydrogen ions in the system, a buffered solution will maintain a relatively consistent concentration of dissociated hydrogen ions. Since changes in pH are the direct result of changes in the concentration of free hydrogen ions, buffered solutions are quite effective at maintaining a constant pH.

Buffer Choices

Sodium and potassium are the most common salt forms of food-grade buffers. During formulation, it is important to consider the contribution of salts from buffers in the overall formula, as high levels of cations can impart undesirable tastes. In tart foods and beverages, citrates are preferred over phosphates, as citrates create a more rounded, smoother sour taste. High levels of phosphates may also create a soapy aftertaste in buffered products. Other common food buffers include acetates (salts of acetic acid) and gluconates (salts of gluconic acid).

Another essential element to consider when choosing a buffer is the pH range of the final food or beverage. For example, the buffer combination of sodium acetate and acetic acid has an effective buffering range of pH 3.6-5.6. The effective buffering range of sodium citrate and citric acid is lower, at a pH between 2.1-4.7. Effective buffering ranges for the three anions of orthophosphates and pyrophosphates are pHs of 2.0-3.0, 5.5-7.5 and 10-12. The ability of phosphates to ionize over a range of pH means that, in food products, phosphates carry a negative charge. Although scientists do not understand exactly how phosphates function in foods and beverages, the negative charge carried by this class of ingredients plays a major role in their function as chelators, buffers and emulsifiers.

Summing It Up

Balance in foods, like many things in life, is never easy to achieve. In complex food systems, juggling sweet and sour tastes, along with a host of other issues, like microbial stability and product convenience, can be a formidable challenge. However, a product’s taste is what ultimately wins over consumers. Taste is the most important attribute of any food or beverage, and because of its significance, it should be a priority. Focus on taste and flavor first and foremost, because without it, there is little hope for product success. pf


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