Etiket Arşivleri: moisture
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.
Stability and quality of foods
1. Water forms
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:
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.
Name of Experiment : DETERMINATION OF MOISTURE CONTENT
Number of Experiment : 1
Submitted by : Kenan ÖZ
Define the moisture content.
The determination of moisture is one of the most important and widely used analytical measurements in the processing and testing of food products.
The moisture content is frequently an index of stability and quality, and is also a measure of yield and quantity of food solids. It is closely concerned with the economics and legal aspects of food processing.
The moisture content of foods varies considerably: for fresh fruits, from 65% in ripe avocados to 95% in rhubarb; for fresh vegetables, from 66% in green beans to 96% in cucumbers; for fresh meat and fish, from 50 – 75%. The moisture content of processed foods varies even more from about 7 – 12% for dried vegetables to 27 -35% for jams and jellies.
What is the relationship between water activity and moisture content?
Water activity is an important means of predicting and controlling the shelf life of food products. Shelf life is the time during which a product will remain safe, maintain desired sensory, chemical, physical and microbiological properties, and comply with nutritional labeling.
The relationship between moisture content and water activity is complex. An increase in aw is almost always accompanied by an increase in the moisture content, but in a nonlinear fashion. This relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. These curves are determined experimentally. Moisture sorption isotherms are sigmoidal in shape for most foods, although foods that contain large amounts of sugar or small soluble molecules have a J-type isotherm curve shape. A moisture sorption isotherm prepared by adsorption (starting from the dry state) will not necessarily be the same as an isotherm prepared by desorption (starting from the wet state). This phenomenon of different aw vs. moisture values by the two methods is called moisture sorption hysteresis and is exhibited by many foods. Many disciplines use water content calculations to regulate product quality, however, water content measurement can be inaccurate, time-consuming and require a precision balance.
Write the importance of monolayer value for the foodstuffs.
The importance of monolayer value for the foodstuffs is physically bound as monolayer the surface of the food constituents.
What are the advantages and disadvantages of each methods (drying, distillation, chemical and instrumental methods)?
Advantages: Different temperature of drying may result in a different amount of free water loss, so it is important to compare results obtained using the same condition.
Disadvantages: Weight lost, volatile oils may be lost.
Advantages: It has advantages of needing little attention and do not separate with the water and so are not measured.