Determining Macronutrients in Milk Powder

• Stefan Dodunski & Mustafa

Intro:

Proximate analysis, or Weende analysis, is the investigation of different macronutrients in food based on the scheme devised in 1860 by Henneberg and Stohmann in Germany. This analysis was originally devised to duplicate the digestion of animals where the extraction of fat was placed in an acid solution to replicate the acid in the stomach and further by alkaline digestion to replicate the environment in the small intestine. Further to this nitrogen, crude lipid, crude fibre and ash are determined chemically. The determination of crude protein is based and approximated on the nitrogen content.

For this project proximate analysis was used to determine percentages of moisture, fat, protein and ash in food. Nitrogen-free extract (NFE), a term used to cover all other material present in the sample, is not measured by proximate analysis but calculated by subtracting from 100, the sum of the percentages of moisture, fat, protein, fibre and ash. NFE represents a rough estimate of soluble carbohydrate and all errors of various determinations are reflected in this calculation. In this experiment total carbohydrate is estimated by difference as many foods are low in fibre and the fraction is often disregarded (Food Chem. for Nutrition Laboratory Manual, 2015).

The food industry is highly regulated by government protocols and international standards and policies which ensure appropriate quality control and supply chain management of food products. In order to ensure that food products meet these requirements food analysis of the composition and characteristics of the foods are mandatory. The entire supply chain is monitored and controlled, from raw ingredients, through to production and within the marketplace (Nielson, 2010).

Proximate analysis is the general way of estimating food composition and is very adept at comparing foods for nutritive value, legal aspects and blending of various foods in industry. Though this is a general comparison it is not specific to determining a specific substance or compound within a food but only a general estimation of moisture, ash and macronutrients (except alcohol). The determination of a specific substance is called “ultimate” analysis and is not used in the food industry for general comparisons. Methods of proximate analysis are empirical and are more easily performed and if procedure is done with good precision, can be reproducible (Food Chem. for Nutrition Lab Manual, 2015).

Moisture is normally considered to be the material lost by a foodstuff on heating at a temperature around that of boiling water or by allowing the sample to stand over a dehydrating agent or by some similar form of measurement. It is generally considered as water but is actually the total volatile matter lost under these particular conditions. The remaining residue is termed the total solid (Food Chem. for Nutrition Lab Manual, 2015). The moisture content of the samples were determined by using both air-oven method and vacuum-oven methods. The use of a variety of ovens in the food industry are a popular method with the opportunity for various amounts of samples that can be analysed concurrently (Nielson, 2010). However with less precision yields less accurate results as different types of ovens will produce different volumes of moisture. Vacuum ovens are less accurate than air fan forced oven as they have the least amount of variance of heat distributed throughout the oven <1%. Vacuum ovens have a larger variance as the glass door acts as a heat sink (Neilson, 2010).

The protein content of foods is often determined on the basis of total nitrogen content and the Kjeldahl method has been almost universally applied to determine the nitrogen content (Kavanagh, 1981). Nitrogen content is then multiplied by a factor to arrive at protein content. This approach is based on two assumptions: 1. the dietary carbs and fats do not contain nitrogen and 2. nearly all nitrogen in foods is present as amino acids. The factor is based on the ratio of nitrogen to protein on a specific food being analysed. The nitrogen is then used through a conversion factor to determine the amount of protein. In this experiment the conversion factor was 6.38 as milk products have a nitrogen content of 15.67%. During digestion H2SO4 (sulphuric acid), at high temperatures, is used to catalyse the reaction by oxidising the organic matter and combining any ammonium formed. After digestion NaOH is added to make the sample alkaline and distilled and the resulting NH3 in dissolved in boric acid. The solution is then titrated with HCL which neutralises the ammonium borate and a colour change from green untill red can be seen.

The Mojonnier method was used to determine crude fat. The method depends on the partition of the fat between an organic solvent and the aqueous phase. (Food Chem. And Nutrition Lab Manual, 2105). The principle behind this method is that fat is extracted by ethanol and diethyl ether. Petroleum decreases the solubility of the water during the ether phase and diethyl ether serves as a lipid solvent. Ammonium hydroxide and ethanol are also used. Ammonium hydroxide decreased the viscosity of the sample by neutralising the acidity and broke ionic and covalent bonds. Ethanol aids in separation of the ether water phase and prevents the milk form forming a gel. Milk powder and other dairy products need to undergo this process during fat removal due to their tightly bound lipids to proteins, and carbohydrates, making use of simple non-polar solvents inadequate (Neilson, 2010).

The ash of a food is the inorganic residue remaining after the organic matter has been burnt away. The ash is representative of all the minerals contained within the powder sample although the inorganic constitutes changes drastically. Metal ions are oxidised to oxides or combined with negatively charged elements. The ash obtained is not necessarily the exact same composition as the mineral matter present in the original food as there may be losses due to volatilisation or some interactions between constitutes. Therefore ash is regarded as the general measure of quality (Food Chem. For Nutrition, 2015).

Aim: The aim of this experiment is to determine the total moisture content, nitrogen and crude protein, crude fat and inorganic matter from a specific milk powder.

Materials:

Proximate 1:

• Pocket Refractometer Pal-1 by Atago made in Japan
• Air-Oven Catalogue 240 by Contherm Scientific Company made in New Zealand
• Burn off Furnace BWF 1100 by Carbolite made in the UK
• Analytical Balance Machine AE200 by Mettler made in America
• Electronic Moisture Analyser MA 150 by Sartorius made in Germany
• Vacuum Oven VOC-300SD by Eleya made in Japan

Proximate 2:

• 2100 Kjeltec Distiller by Foss Analytical made in Denmark
• Tecator Digestion System by Foss Analytical made in Denmark
• SuperVario-N Centrifuge by Funke-Gerber made in Germany
• Analytical Balance Machine AE200 by Mettler made in America
• Model 330 Boiling Bath by Contherm Scientific Company made in New Zealand

Results:

References:

Nielson, S. S. (2010). Food Analysis. (4^th ed.). New York, USA: Springer Retrieved from http://books.google.co.nz/books?id=JM– R91MDsiEC&pg=PA87&dq=moisture+content+of+food&hl=en&sa=X&ei=qeCFUYyTG4aFiAfX 94GYBg&sqi=2&ved=0CC0Q6AEwAA#v=onepage&q=moisture%20content%20of%20food&f =false

Food Chemistry for Nutrition Laboratory Manual. (2015) 151.231 Institute of Food, Nutrition and Human Health, Massey University: Palmerston North

Kavanagh, F. (1981), Official methods of analysis of the AOAC, 13th ed. Edited by WILLIAM HORWITZ. The Association of Official Analytical Chemists, 1111 N. 19th St., Arlington, VA 22209. Retrieved from

http://onlinelibrary.wiley.com/doi/10.1002/jps.2600700437/abstract

http://en.wikipedia.org/wiki/Powdered_milk