Food QC Sample Preparation: The Step Before the Measurement

Food QC results travel badly. A method that works in one laboratory produces a different number in another, and the instrument is rarely the reason. Homogenisation, portion size and the time between preparation and measurement account for most of the difference.

The sample is the method

A food sample is not homogeneous. A grain lot, a sauce, a baked product and a beverage each present a different sampling problem, and the portion actually measured is a few grams out of kilograms. The preparation step is what makes that portion representative, and it is the step most often described in one sentence.

Homogenisation

Grinding, milling or blending changes particle size, temperature and moisture loss. All three affect the result. The procedure should name the equipment, the duration and the endpoint — a defined particle size, a number of passes, or a visual criterion — rather than “homogenise until uniform”.

Portion size and sub-sampling

Portion size is a compromise between representativeness and what the instrument needs. A moisture analyser wants enough material to fill the pan evenly; a titration wants a portion that lands inside the burette’s useful range. Both need the portion taken from a mixed sample, never from the surface.

Preparation decisions that change the number

Holding time

Moisture migrates, fat separates, acidity changes with microbial activity, and colour darkens. The procedure has to state the maximum time between preparation and measurement, the storage condition in between, and what happens when the limit is exceeded — repeat the preparation, or record a deviation.

Different products, different preparation

Four product types cover most of the practical difficulty. Dry powders and grains need size reduction and a dividing step, because a few grams must represent kilograms. High-fat products need the fat distributed evenly, which usually means grinding carefully enough not to melt it. High-moisture products need mixing rather than grinding, and they lose moisture quickly while being handled. Beverages need degassing and often a dilution before any photometric measurement.

Writing the product type into the procedure is what stops a method borrowed from a similar product from failing quietly on this one.

Where preparation error hides

Three places: the sample left in the mill while the previous one is measured, the portion weighed once and used for two determinations with different requirements, and the container that was not dry. Each produces a result that looks normal and shifts the batch.

Writing it into the procedure

Describe preparation in the same detail as the measurement: equipment, duration, portion mass, container, holding time and temperature. The measurement is the visible part; the preparation is what makes it repeatable.