First, the drying method (including atmospheric pressure, decompression, etc.) -- also called weight method.
1, drying oven (oven) direct drying method:
Principle: The direct drying method for determining food moisture is based on the fact that when the moisture in food is heated, the steam pressure generated is higher than the partial pressure of air in the electric drying box, so that the moisture in food evaporates. At the same time, the purpose of drying is achieved by continuously heating and discharging steam.
Application: suitable for foods with no volatile substances at 95 -105 C. Such as cereals, aquatic products, beans, milk, meat, and stewed products.
Determination method: Accurately weigh 2-10g sample, put it in a sealed weighing bottle which has been dried, cooled and weighed to a constant weight, move it into an atmospheric oven at 95-105 C, open the lid and bake for 2-4 hours, add it to the dryer and cool it for 0.5 hours, then weigh it. Bake 1H again, then cool 0.5h and weigh it. Repeat operation until constant weight. (two times before and after the mass difference does not exceed 2mg)
Formula: m1--- sample and bottle quality before drying.
Moisture (%) = M2 - dry weighing, m3--- weighing bottle quality.
2, vacuum drying oven (vacuum drying box) decompression drying method:
Principle: Using the principle of lowering the boiling point of water at low pressure, the weighing dish after sampling is placed in a vacuum oven and dried to a constant weight at selected vacuum and heating temperature. The quality of dried samples is water content.
Application: suitable for foods that contain sugar, monosodium glutamate, etc.
Sugar, especially fructose, is very unstable to heat, and when the temperature is higher than 70 degrees Celsius, it produces moisture and other volatile substances. Generally, the drying temperature is 70 C.
Two. Distillation method
Distillation: When a liquid is heated to a boiling point, it produces steam, which is collected, condensed and then returned to a liquid. This process is called distillation.
Principle: Based on the fact that the boiling point of two immiscible liquid binary systems is lower than the boiling point of each component, the water in food is azeotropically evaporated with toluene or xylene or benzene, condensed and collected distillate. Because of the different density, the distillate is divided into two layers in the receiver tube. According to the volume of the distillate, the sample can be calculated. Moisture content.
Application: suitable for foods containing more volatile substances, such as oils and spices.
Three. Carle Fischer method
Carle Fischer method is a chemical analysis method for determining water content by titration.
Principle: Based on the oxidation-reduction reaction of iodine and sulfur dioxide in the presence of water.
Suitable for the determination of trace and trace moisture (candy, chocolate, fat, lactose and dehydrated fruits and vegetables in food samples).