Acid hydrolysis

Explanation: Some foods contain fats that are occluded and bound to proteins or carbohydrates. To extract these fats it is necessary to break these bonds prior to the solvent extraction. Acid hydrolysis, e.g. with HCl, is therefore commonly used.

Examples of foods in which a significant proportion of the fats are bound to proteins and carbohydrates are cereal products, meat products, and baked products.

Several procedures have been described. The procedures differ with respect to acid strength, extraction temperature and extraction time, and drying procedure of the fats.

Methods:

Weibull methods: These procedures involve hydrochloric (HCl) acid digestion of the sample, passing the digest through filter paper, drying of the filtered residue, and then extraction of the residue in a Soxhlet-type extractor. After evaporation of the solvent, total fat is measured by weighing. Although this method is useful for releasing bound fats from composite products, it has many manipulative steps and the drying procedure is critical and can introduce losses.

Schmid methods: These procedures also involves hydrochloric acid (HCl) digestion. However, the fat is extracted directly from the digest using Mojonnier-type flasks or similar. This method also may produce nonfat ether extractables.

Generally the Schmid methods give slightly higher values for total fat than Weibull methods because of oxidation of the residue during the drying procedure resulting in a less efficient extraction in the Weibull procedure.

Characteristics

• Applicable to most foods (baked cereal products) except certain dairy products.

• In general high yields, because the procedures may produce non fat ether extracts, such as low molecular weight carbohydrates and amino acids.

• Not preferred for characterisation of lipid classification or determination of fatty acid profile, because hydrolysis of fats and subsequent losses of fatty acids may occur. However, sometimes acid hydrolysis is used when the food matrix leaves no other choice (e.g. cereals).