Sequential measurement of salt & acidity from one sample

Analytical testing is a cornerstone of the food production process. Whether a hobbyist or food scientist, many are involved in food quality and safety using a variety of analytical techniques to create safe, stable and, most of all, great tasting foods.

Salt in food

Sodium is a mineral that is present in many foods and beverages, most commonly in the form of sodium chloride, or salt. Sodium-containing compounds are often added as an ingredient for the purpose of extending the shelf life of the final product by inhibiting microbial growth, improving texture and appearance by retaining moisture and increasing product stability, and improving flavour by promoting desired tastes while masking off-flavours. A small quantity of sodium is needed in one’s diet to aid in bodily functions, but consuming too much sodium poses significant health concerns, such as hypertension (high blood pressure) and osteoporosis. Due to its hypertensive effects, sodium is one of the minerals that appear on product labels for people who must watch their dietary intake.

Several instrumental methods are available for determining sodium concentration in foods and beverages. For binary solutions, sodium content can be inferred by measuring salt with a refractometer or an electrical conductivity (EC) meter. For more complex foods and beverages, sodium can be measured directly with a sodium ion selective electrode (ISE), indirectly with a chloride ISE or by potentiometric titration of the chloride ion to infer NaCl. Other methods that directly measure sodium, including inductively coupled plasma mass spectrometry (ICP-MS) and flame atomic absorption spectrophotometry (FAAS), provide highly accurate results at low concentrations, but the sophisticated technology is very expensive.

Acidity in foods

Acidity in food and beverages is an important factor for the taste and composition of many products. Different from pH, which simply indicates whether a substance is an acid or base, acidity indicates the concentration of the acid in the solution. The total acidity is typically expressed as the predominant acid. For example, the predominant acid in orange juice is citric acid. Other common organic acids include malic, lactic, tartaric and acetic acids. These organic acids play a primary role in the taste of the final product by promoting a tart or sharp flavour as well as ensuring consistency of product for the food manufacturer so as to maintain a flavour that is associated with the brand. Some foods naturally have a high acidity, such as citrus fruits or tomatoes. Acidity can also be produced through microbial fermentation, such as in yoghurt.

Acidity of a substance is determined by titration; a base of a known concentration is added to a sample until all acids in the sample have been reacted with. The AOAC Method 942.15 is commonly used to measure the titratable acidity of various fruit products to a pH endpoint of pH 8.1. The AOAC Method 947.05 is used to determine the acidity of milk, which is reported as % lactic acid. This method specifies using a colour indicator or a pH electrode and meter to determine a pH endpoint of pH 8.2 or pH 8.3. The advantages of utilising a pH electrode over a colour indicator are that colour changes are subjective, especially when working with an opaque sample such as milk. Utilising a pH meter or automatic titration system provides improved accuracy and repeatability for acidity determinations in food and beverages.

Why measuring salt & acidity matters

Australian regulatory bodies and agencies have established criteria for nutrient content claims regarding the sodium content of foods. In terms of titratable acidity, there are guidelines as to the type of acid expressed as well as the value allowed for each food type.

The quality of analytical data depends on proper sampling, appropriate method selection, and effective execution. Although there are multiple methods available for measuring salt and acidity, the easiest way to obtain accurate results for a sauce or semi-solid product is by automatic titration using Hanna Instruments’ HI932 automatic potentiometric titrator. Food manufacturers will often use automated titration as their testing tool due to the consistent and accurate results it produces, as well as the ability to optimise methods and increase efficiency.

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