Chapter 5. Determination of moisture

Section 1. General

Moisture determiantion is one of important and most widely used measurements in the processing and testing of foods.

Dry matter

Stability and quality of foods

Chemical reactions

1. Water forms

Free water

Absorbed water

Bound water

2. Methods of moisture determination

Direct determinantion: remove the moisture in sample by the methods of drying, distillation and extration.

Indirect detremiantion: using sample’s density, refraction, conductivity or capacitance to measure the moisture content.

Direct determination: good accuracy and repetition, but spending long time.

Indirect determination: fast and automatic, but slightly week on accuracy .

Section 2. Determination of moisture

1. Drying methods

The procedures for determination the moisture content specified in food standards generally involve thermal drying methods.

The material is heated under carefully specified conditions and the loss of weight is taken as a measure of the moiture content of the sample.

Drying methods are:

Simple

Relatively rapid

Simultaneous analyses of large numbers of samples

Preconditions of drying methods

Water is the only volatile material.

Water can be removed drastically.

During heating, weight loss caused by chemical reactions in other components can be ignored.

1.1 Directly drying method

Principle: In certain tempatature (95-105℃) and pressure, samples are heated in oven and then water in samples is vaporized. The loss of weight is moisture content of samples.

Applying range: There is no other volatile materials besides water. furthermore, the sample is stable when heating.

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