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pH is a measure of the acidity or alkalinity of a solution and is of the utmost importance in determining chemical behaviour and ability to support (micro )biological activity.

If water is taken as neutral, being neither acidic nor basic, a scale of acidity and alkalinity can be constructed depending on the concentration of acidic H+ or alkaline OH- ions in the solution.

Water dissociates (very weakly) to give one H+ and one OH- ion.

H2O  H+ + OH-

The amount of dissociation can be expressed as [H+][OH-] / [H2O], known as the dissociation constant K.

As water dissociates so little, the value [H2O] can be taken as constant, such that [H+][OH-] = K[H2O]  = Kw known as the ionization product of water which at 24°C is 1 x 10 -14.

In other words, the concentration of H+ and OH- ions are the same.

[H+][OH-] = [H+]2 = [OH-]2 = 10 -14

[H+] = [OH-] = 10-7

It is not easy to visualise such very small numbers, so to make matters more comprehensible a logarithmic scale is used to turn the numbers into multiples of 10. To make the numbers positive, a negative logarithm is used. The resulting number is known as pH.

pH = The negative logarithm (base 10) of the hydrogen ion concentration

pH = -log [H+]

In the case of pure water, pH = -log 10-7 =  7.0

In practical terms, solutions with a pH of less than 7 are increasingly acidic and solutions with a pH above 7 are increasingly alkaline.

The pH scale extends from 0 to 14, examples being strong Hydrochloric Acid to strong Sodium Hydroxide.

Because of the logarithmic scale, every pH unit represents ten times more or less concentration of the acidic or alkaline ions.

(There are important ‘complications’ in terms of the buffering effect of the weak acids their salts found in fruit juices. So a highly acid juice with a large buffering capacity may have a higher pH than expected).

All fruit juices are acidic in nature, so have pH values below 7.0. In fact, most juices have pH values in the range 2.0 – 4.0, examples being lemon or lime juice at the lower end and high ratio orange juices at the upper end.

The ability of a foodstuff to support the growth of pathogenic micro-organisms is largely determined by its pH. Of such importance is that fact, that legislation exists to specify the minimum heat treatment that must be applied to processed low acid foodstuffs, defined as those having a pH of 4.50 or greater. In the juice industry, it is quite common to specify the addition of an acidulant to high pH products (such as banana, tomato, papaya) to reduce the pH. In our business we adopt a maximum pH value of 4.20 for our fruit juices, unless the customer specifically requires non-acidified product, when we will ensure that the appropriate heat treatment is applied.

The rate and route of chemical reactions are influenced strongly by pH as well as temperature and concentration. In the case of fruit juices, lower pH values tend to result in more rapid browning reactions and well as more aggressive attack on processing equipment. When employing enzymes in juice processing, the pH range in which they function is crucial to a successful outcome.