Sourness models used in the selection and use of acidulants

1st April 2013

It is possible to reduce the time required for selection and level adjustment of acidulants and buffer salts by using mathematical models of sourness. These models are useful during initial product design. Their use can reduce the amount of benchwork required for prototype development. Daniel R Sortwell reports.

Three factors determine sourness ­ the specific acid used, the acid concentration, and the pH. Sourness is affected by pH because the pH determines the extent of acid protonation. A fully protonated, or undissociated, acid only occurs at low pHs and is quite sour. At high pHs, acids lose protons, or are dissociated, and are much less sour. It is useful to look at the difference between the extent of protonation of an acid at the pH in question and its extent of protonation at pH6.8, the normal pH of saliva.

The extent of protonation of an acid vs pH can be calculated using its dissociation constant(s). The calculated values for seven acidulants are shown in Fig.1. As shown in this graph, there are large differences in the extent of protonation between acids at lower pHs. Acetic acid, which is much more sour than other food acids, has the highest extent of protonation by far. Phosphoric acid, which has very low sourness, has a very low extent of protonation. Among the other acids shown, the relative differences change with pH, as would be expected from their respective dissociation constants.

Mathematical models of sourness based in part on the extent of protonation were developed and fitted to two sets of sensory data. The sourness model curves at 0.5per cent w/v concentration vs. pH are graphed in Fig.2. As shown, the relative sourness between acids changes with pH. In other words, acid selection to achieve more or less sourness would vary depending on the pH.

Another important point is that anions of weak acid buffer salts contribute sourness, again, depending on the pH. For example, 64 per cent of sodium citrate dihydrate is citrate ion. The sourness contributed by this citrate ion would follow the same curve shown for citric acid in Fig.2. No matter whether the starting point is citric acid or citrate ion, the extent of protonation is the same at the same pH.

Various interactions affect the impact of an acidic ingredient on the flavour profile of a food or beverage product. The product composition is a critical factor ­ beverages that are mostly water are very sensitive to changes in the acidic ingredients and buffer salts. For example, the level of buffer salt that can be used in a beverage system may be limited by the taste impact of buffer salt cations such as sodium or potassium. Products that contain hydrocolloids and fats are less sensitive. Mutual masking between sweetness and sourness tends to dampen sourness.

Three examples of using these sourness models follow:

Example A: Maintain sourness while raising pH to improve aspartame stability in a prepared beverage. As shown in Fig. 2, all acids are less sour at higher pHs. In this case, replace the acid and/or buffer salt with another that has more sourness at the same pH. For example, add sodium or potassium citrate to raise the pH to 3.9 and replace part of the citric acid with malic acid to maintain sourness.

Example B: Increase sourness while maintaining pH in a gelled system where a narrow pH range is required. In this case, increase the levels of buffer salt and acidic ingredient while maintaining the same ratio between them.

The pH will remain close to the same but sourness will increase since there is now more protonated acid present. As an example, if the current formulation includes 0.60 per cent malic acid and 0.12 per cent sodium citrate at a pH of 3.2, by increasing the levels of malic acid and sodium citrate to 0.80 per cent and 0.16 per cent, respectively, the pH will remain close to the same but sourness would increase.

Using the sourness models described earlier, it is possible to estimate the change in acid and buffer salt levels necessary to achieve a required change in sourness. In the example above, the levels were increased by 33 per cent. This would result in a 24 per cent increase in sourness.

Example C: Lower pH to improve microbial stability with minimal added sourness. At pHs above 4.0, use Lactic Acid to lower the pH with minimal added sourness. At pHs below 4.0, phosphoric and citric acids are used. Phosphoric acid is less sour than citric acid but is also more difficult to handle. Glucono delta lactone (GDL) is also effective in lowering pH with minimal added sourness.

enquiry no 52

Daniel R Sortwell is senior food scientist with Bartek Ingredients Inc, Stoney Creek, Ontario, Canada.





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