Appendix 3
Methods of preparation of foods for analysis

Documentation of sample preparation is as important as other aspects of the analytical protocols. Care should be taken to separate edible portions and inedible portions (refuse, waste) carefully, and record descriptions and weights of all parts. Sample preparation is also the appropriate time to record common measures or portion sizes with the description (e.g. slice), linear dimensions and weight. Finally, if a volume measure is possible (e.g. all liquids, powders, granular substances), the food's density should be measured and recorded.

Homogeneous foods

• Solids: 

-Friable: crumble and mix. 

-Sticky: freeze and crush at low temperature. 

-Hygroscopic: take portions rapidly into pre-weighed sealable containers for weighing.

• Emulsions. Take by weight rather than volume; warm and mix.
• Liquids with suspended solids. Homogenize, or sample during gentle mixing.

Reduction by quartering

Large items, if symmetrical, can be reduced in size by this technique. The principle is that the quarter should be representative of the whole. Any symmetrical food should be cut into quarters, and one-quarter of each batch taken for processing for analysis. Oval or elongated foods (e.g. potato or cucumber) should be cut into eighths, and two-eighths taken for a quarter, because each end may represent different parts of the plant (e.g. stalk and flower).

• Large item foods. Foods consisting of fairly large, separate, but similar portions, such as loaves of bread or joints of a meat, should be quartered and sampled then processed for analysis.
• Food lots of small items (flour, rice, legumes, small fruits, chopped mixed units). These foods are quartered as follows: the bulk is tipped into a uniform pile on a clean, inert surface and turned over several times with a polythene or glass spatula. The pile is levelled, and then divided into four equal segments. Two opposing segments are taken and the other two discarded. The remaining segments are mixed and further reduced in the same way.
• Segmented foods (the purchased item consists of a number of individual units). When sampling packets of biscuits, cartons of eggs, batches of bread rolls, etc., it is usual to take every fourth item to form a composite sample. For sliced loaves, it is adequate to take every fourth slice and one end slice, which then must be thoroughly crumbed before further reduction. The principle is to keep the crust–crumb ratio the same as in the original loaf (see below).  

Preparation of analytical samples for particular food types

• Cereals:

-Flours and grains. The units are mixed thoroughly on a clean, dry, inert surface with a polythene or glass spatula. The combined mass may be quartered (see above). Large analytical samples for inorganic analysis (ashing or wet digestion) should be taken at this point. Large grains (e.g. maize) may then need to be reduced in a hammer or ball mill. No reductions of fine flours should be necessary.

-Unsliced breads. Individual loaves are quartered, one quarter of each being taken, weighed, sliced, dried at ambient temperatures and weighed again. The air-dried quarters are ground with a pestle and mortar, then mixed well in a bowl with a spatula.

-Cakes, pastries, pies, cooked cereals, cereal-based puddings. Large items must be quartered. Quarters or small items should be chopped and mixed thoroughly in a bowl with a spatula. A large analytical portion should be taken for inorganic analysis and the remainder should be homogenized mechanically. If vitamin C is to be analysed (in fruit pies, for example) an unhomogenized analytical portion should be taken into metaphosphoric acid within a few seconds, but the remaining mixture can be thoroughly homogenized. Items resistant to homogenization can be frozen and crushed inside a polythene bag with a mallet (Osborne and Voogt, 1978).

-Biscuits. Every fourth item should be taken from the packet or batch, crushed with a pestle and mortar, and mixed together; then a large analytical portion should be taken for inorganic analysis. A grinder may be necessary for further reduction if nuts and/or dried fruit are present.

- Breakfast cereals. These can usually be quartered then crushed with a pestle and mortar; analytical portions can then be taken for inorganic analysis. High-fat, high-sugar cereals may require freezing and crushing in a polythene bag.

• Meats and fish (raw, cooked and processed). The fat and muscle of some meats are more conveniently analysed separately and the results combined to produce the final values. The edible portion of each unit is chopped coarsely with a sharp knife (fish is flaked with a fork) and mixed thoroughly in a bowl with a spatula. A portion is removed, frozen and crushed in a polythene bag, and used for inorganic analyses. The remainder of the analytical sample is minced and mixed thoroughly again; portions are taken for further analyses. Care must be taken to avoid fat separation during mixing.
• Vegetables: 

-Dry seed legumes. These can be treated as grains, with a large analytical portion taken for inorganic analysis before grinding. Loose seed coats must be carefully mixed into the mass of the food sample.

- Leafy vegetables and vegetable inflorescences. Small leafy vegetables such as Brussels sprouts should be mixed together in a bowl, chopped coarsely and mixed again briefly. A large portion should be taken for inorganic analysis and another portion into metaphosphoric acid for vitamin C analysis. Large tight-leaved vegetables (e.g. cabbage, iceberg lettuce) must be quartered. All large leafy vegetables must be chopped coarsely and mixed, and this must be done very quickly. After the mixing, analytical portions should be taken for analyses of vitamin C, vitamin A, carotenes, vitamin E and inorganic nutrients; the remainder can be chopped further. Stalks are often difficult to reduce and may have to be chopped separately and reintegrated into the food sample.

 -Roots and tubers. Large items may be quartered; quarters should be diced in a mechanical chopper for about 20 seconds and mixed rapidly. Portions can then be removed for all analyses.

- Others. Some vegetables, such as cucumber and tomato, must be treated as fruit.

• Fruit. Large fruits (e.g. pineapples or watermelons) and medium-sized ones (e.g. apples) must be quartered. Small fruits (e.g. cherries) should be quartered by the method used for particulate foods. Quarters should be coarsely chopped and combined, and unhomogenized analytical portions should be taken for immediate vitamin C and inorganic analyses. The remaining mixture can then be homogenized to produce an analytical sample for other analyses. Unripe bananas, and possibly some other fruits, should not be mechanically homogenized vigorously, because starch may break down to sugars. Dried fruit may be difficult to homogenize mechanically and may require manual chopping.
• Milk and milk products: 

-Liquid and evaporated milk. The contents of units should be swirled together gently in a stoppered glass or polythene container. 

-Dried milk. This should be treated as flour. 

-Cheese. The texture of the cheese will determine its treatment. Units of friable cheese can be crumbled and mixed; soft cheese should be mashed and mixed; hard or rubbery cheeses should be grated on a polythene grater.

-Yoghurt, cream, ice cream, condensed milk, very soft cheese. Units should be blended in a bowl with a spatula. Those containing fruit and/or nuts should be mechanically homogenized after a large analytical portion has been taken for inorganic analysis.

- Butter. See Fats below.

• Eggs: 

-Fresh. Fresh eggs should be shelled and mixed briskly with a fork; after analytical portions are taken for inorganic analyses, the remainder is homogenized mechanically.
-Dried. Dried eggs should be handled as flour.

• Fats and oils: 

-Oils. Units should be gently warmed, if necessary, then swirled at 30 °C. 

-Fats. Units of butter, margarine, lard or drippings should be softened over a warm water bath and then blended together gently. Units of suet can be crumbled and mixed with a fork. In homogenizing low-fat spreads, care must be taken to prevent breakdown of the fat/water emulsion.

• Nuts. Batches of nuts should be ground separately with a pestle and mortar, then mixed together thoroughly in a bowl. An analytical portion should be taken for inorganic analyses and the remaining mixture should be homogenized mechanically for further analyses.
• Sugars, syrups and confectionery: 

-Sugars. Refined sugars should be treated as flour. 

-Syrups. Syrups should be taken by weight rather than volume. Sticky syrups should be warmed and gently but thoroughly blended.

- Confectionery. Confectionery samples should be frozen and crushed on a chilled surface or blended in liquid nitrogen, which is then allowed to evaporate in a cold room. Any mixing of crushed units must also be done in a cold room.

•       Sauces: 

-Viscous sauces. Units should be gently warmed and blended thoroughly together. 

-Fluid sauces. These should be swirled together. 

-Biphasic sauces (e.g., salad dressings). These items must be thoroughly homogenized and mixed. Test portions should be taken for inorganic analyses and then the mixture should be rehomogenized for further analyses.

• Beverages. Carbonated beverages can be degassed by application of reduced pressure or by being poured from one vessel to another. Specific gravity should be measured by weight of measured volume; units should be mixed by swirling.
• Prepared composite foods and dishes. This is the form in which most foods are consumed. Items should be briefly homogenized, carefully mixed, then rehomogenized. It can be assumed that laboratory homogenization will not introduce any contamination greater than that arising during domestic or commercial food preparation. Care is required to blend in the individual pieces of muscle, fat, vegetables, etc., which may be found in mixed prepared foods. Portions for vitamin C assay are best taken from the mixed homogenate before it is rehomogenized. If the prepared foods are hot, speed is essential to prevent moisture loss. Total meals or diets can be handled in the same way.

Some practical equipment requirements for handling and preparation of laboratory and analytical samples

• General: 

Trays (for carrying foods) Bowls (0.5 litre to 4 litre capacity) Spatulas Chopping boards (polythene, wood) Kitchen knives, knife-sharpener Can-opener Spoons (various sizes) Plastic sieves, colanders Oven thermometer, meat thermometer Electric heat-sealer (for freezer bags) Large sheets of strong plastic (to cover benches, mix particulate foods) Kitchen cutlery and tableware

•  Homogenizers:

-Common domestic equipment: 
Domestic food processor (can be equipped with titanium or other special blades)
Coffee-bean grinder
Food blender
Bamix (hand-held homogenizer)
Mincer (hand, electric)
Graters, especially with non-metal cutting edges

- Laboratory equipment:
Sorval Omnimix
Turrax
Waring blender
Ato-Mix
Automatic pestle and mortar
Knife mill
Ball mill
Hammer mill
Robot Coupe blender (available in sizes appropriate for food service)