Minerals

• Generalities (this section)
  • Role of minerals
  • Sample preparation
• Next sections:
  • Analysis of minerals
    • Gravimetric Analysis
    • Colorimetric analysis
    • Titration methods Atomic absorption spectroscopy
    • Ion-selective electrodes
    • Atomic spectroscopy

Role of minerals

Mineral elements are important in many aspects of fish and shrimp metabolism. They provide strength and rigidity to bones in fish and the exoskeleton of crustacea. In body fluids they are involved mainly with the maintenance of osmotic equilibrium with the aquatic environment and in the nervous and endocrine systems. They are components of enzymes, blood pigments and other organic compounds. They are essentially involved in the metabolic processes concerned with energy transport. At the present time 26 of the 90 naturally occurring elements are known to be essential for animal life.

Diets deficient in a specific mineral can cause deficiency symptoms in fish or shrimp. Deficiencies are more likely to occur in highly intensive culture in tanks or cages than in ponds, where minerals are available from natural foods. Moreover; fish and crustacea can absorb minerals by other routes than from the digestion of food - through the ingestion of seawater and through exchange from their aquatic environment across body tissues such as skin and the gill membranes. Minerals are therefore probably not so important a component of the diet of fish and shrimp as they are in that of other animals.

Sample preparation

Many of the analytical methods used to determine the specific mineral content of foods require that the minerals be dissolved in an aqueous solution. For this reason, it is often necessary to isolate the minerals from the organic matrix surrounding them prior to the analysis. This is usually carried out by ashing a sample using one of the methods described in the previous section. It is important that the ashing procedure does not alter the mineral concentration in the food due to volatilization.
Another potential source of error in mineral analysis is the presence of contaminants in the water, reagents or glassware. For this reason, ultrapure water or reagents should be used, and/or a blank should be run at the same time as the sample being analyzed. A blank uses the same glassware and reagents as the sample being analyzed and therefore should contain the same concentration of any contaminants. The concentration of minerals in the blank is then subtracted from the value determined for the sample. Some substances can interfere with analysis of certain minerals, and should therefore be eliminated prior to the analysis or accounted for in the data interpretation. The principles of a number of the most important traditional methods for analyzing minerals are described below. Many more traditional methods can be found in the AOAC Official Methods of Analysis.