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PROXIMATE ANALYSIS

1. Define proximate analysis

Proximate analysis refers to the quantitative analysis of macromolecules in food. A combination of different techniques, such as extraction, Kjeldahl, NIR are used to determine protein, fat, moisture, ash and carbohydrates levels.

1. Explain the importance of  proximate analysis

In the food industry, Proximate analysis” is of key commercial concern as food-manufacturing companies need to ensure that their products meet the appropriate laws and legal declaration requirements as well as the safety aspects of the end products when released to the end consumer

In the fuel or energy industry, proximate analysis of a fuel provides the percentage of the material that burns in a gaseous state (volatile matter), in the solid state (fixed carbon), and the percentage of inorganic waste material (ash), and is therefore of fundamental importance for biomass energy use and environmental pollution

• Give four components of biological materials used in proximate analysis

 Water (moisture), ash, crude fat (ether extract), crude protein , Carbohydrates and crude fibre.

4. Explain the following  terms
   1. Total carbohydrate content of foods has, for many years, been calculated by difference, rather than analysed directly. Under this approach, the other constituents in the food (protein, fat, water, alcohol, ash) are determined individually, summed and subtracted from the total weight of the food. This is referred to as total carbohydrate by difference and is calculated by the following formula:

100 – (weight in grams [protein + fat + water + ash + alcohol] in 100 g of food)

It should be clear that carbohydrate estimated in this fashion includes fibre, as well as some components that are not strictly speaking carbohydrate, e.g. organic acids.. Total carbohydrate can also be calculated from the sum of the weights of individual carbohydrates and fibre after each has been directly analysed.

1. Available carbohydraterepresents that fraction of carbohydrate that can be digested by human enzymes, is absorbed and enters into intermediary metabolism. (It does not include dietary fibre, which can be a source of energy only after fermentation – see the following subsections.) Available carbohydrate can be arrived at in two different ways: it can be estimated by difference, or analysed directly.

• To calculate available carbohydrate by difference, the amount of dietary fibre is analysed and subtracted from total carbohydrate, thus:

100 – (weight in grams [protein + fat + water + ash + alcohol + dietary fibre] in 100 g of food)

This yields the estimated weight of available carbohydrate, but gives no indication of the composition of the various saccharides comprising available carbohydrate. Alternatively, available carbohydrate can be derived by summing the analysed weights of individual available carbohydrates. In either case, available carbohydrate can be expressed as the weight of the carbohydrate or as monosaccharide equivalents.

1. Dietary fibreis a physiological and nutritional concept relating to those carbohydrate components of foods that are not digested in the small intestine. Dietary fibre passes undigested from the small intestine into the colon, where it may be fermented by bacteria (the microflora), the end result being variable quantities of short-chain fatty acids and several gases such as carbon dioxide, hydrogen and methane. Short-chain fatty acids are an important direct source of energy for the colonic mucosa; they are also absorbed and enter into intermediary metabolism 
2.   Describe the kjeldahl procedure for the determination of  protein content in a food sample

Kjeldahl Method is a widely accepted method for the determination of protein in food products. The method follows three steps:

Digestion – Decomposition of organic matter by heating in the presence of concentrated sulphuric acid, the end product is ammonium sulphate solution.

Distillation – Ammonium sulphate is converted into gaseous ammonia by addition of an excess base, followed by boiling and condensation of the ammonia in a receiving solution (acid).

Titration – Quantification of the unreacted acid in the collecting vessel. The rate of digestion and the completeness of the breakdown of nitrogenous compounds to ammonium sulphate mainly depend upon the heat input, amount of boiling point elevator of acid (alkali sulphate), addition of catalyst (mercury, copper sulphate, titanium dioxide), oxidant (hydrogen peroxide), reflux rate of sulphuric acid and length of digestion. Ammonia is liberated from the acid digestion mixture by distillation in the presence of alkali (50% NaOH). A total recovery of ammonia from the digest can be obtained within 5 to 20 min by direct distillation and about 10 min by steam distillation.

Kjeldahl Nitrogen distillation assembly