To measure the acid-base status of a environment in water, the chemists have created a unit of measurement known as pH, whose values are between 0 and 14. The pH of pure water equal to 7 (at 25 ° C), represents the boundary between acids and bases: the solutions the pH is below 7 are acidic and those with the pH value is higher to 7 are alkaline.
The pH scale was introduced by the Danish chemist Søren Sørensen in 1909, when he controlled the quality of beer brewing. This scale is now widely used in chemistry, geology, ecology, medicine, industry and agriculture.
In aqueous solution, the pH is defined as:
pH = - log [H+]
When acid is added to water, the hydrogen ion concentration [H+] increases and is higher than the same concentration of ions in the water. Thus, the solution is acidic (over the concentration of H+ ions is high), over the pH is low, and vice versa.
If the pH level can vary from 0 to 14, the pH of the food is usually in the area of 2 to 7. The pH of the products of plant origin is often lower than that of foods of animal origin , especially fruit.
|Beef||5.3 to 6.2|
|Chicken||5.8 to 6.4|
|Fish||6.5 to 6.8|
|Fresh milk||6.3 to 6.5|
|Butter||6.1 to 6.4|
|Carrots||5.2 to 6|
|Potatoes||5.4 to 6.2|
|Onions||5.3 to 5.8|
|Tomatoes||4.2 to 4.9|
|Apples||2.9 to 3.3|
|Oranges||3.6 to 4.3|
|Lemons||2.2 to 2.4|
Influence of pH on the preservation of foods
Weathering reactions, whether chemical, enzymatic or microbiological, are largely influenced by the pH of the medium (see table below).
|Weathering reactions||Minimum pH||Optimum pH||Maximum pH|
|Growth of mold||1.5 to 3.5||4.5 to 6.8||8 to 11|
|Growth of bacteria||4.5||6.5 to 7.5||9.0|
|Growth of yeasts||1.5 to 3.5||4.0 to 6.5||8.0 to 8.5|
|Enzymatic browning||5||6 to 6.5||7|
|Maillard Reaction||1||4 to 7||9|
The above table shows that most microorganisms grow best at a pH close to 7 (4 to 7.5). Some bacteria, including lactic and acetic bacteria, are able to grow at pH lower than 4.
Furthermore, since the pathogenic bacteria and the majority of bacteria of deterioration can not grow at pH <4.5, we divide the food into two categories: low-acid products and acidic products.
The low acid foods with pH> = 4.5 are considered less stable. Their stabilization by heat treatment requires a heat sterilization to remove all pathogens and corruption, including bacterial spores.
Food acids, with a pH <4.5 are considered relatively stable. Their stabilization by heat treatment requires a less severe, such as pasteurization, to eliminate yeast, mold and some acidophilic bacteria, none of which is sporulating species.
Also, it is important to note that certain species of fungi are able to metabolize acids, thus transforming an originally acid into a weak acid.
The reduction of pH of food products can be achieved by two methods. The first is a direct method is to acidification of the product by adding one or more acids, organic acids are most commonly used acetic acid (vinegar) and citric acid. The second is an indirect method based on the use of microorganisms in fermentation.
The acidification is the addition of an organic acid (as acetic acid) or an acidic ingredient (such as lemon) to a food that is initially slightly acidic. The acid or acidic ingredient must be added to the proportions determined for the pH of the finished product is below 4.5.
Fermentation can be defined as the controlled use of selected microorganisms in order to preserve food by producing acid or alcohol and to modify their organoleptic characteristics. Improving the keeping quality of fermented foods is based on the effect of pH or organic acid (for the lactic fermentation) or the influence of alcohol (for the alcoholic fermentation) in microorganisms.
This technique, used by man for millennia on empirical basis, is not without risk if not controlled. It is necessary that the microorganisms that cause the desired effects are not pathogenic and do not generate no alteration.
Mastering this technique is based on the choice of allowing the fermentation microorganisms (fermentation) and the control of parameters that support the development of these organisms (temperature, pH, aw, nutrients, etc.)..