31 Chemical and functional properties of minerals in food

Aparna Kuna

In addition to the organic components like proteins, carbohydrates, fats and vitamins etc. our food also contains small amounts of inorganic species called mineral elements. These minerals play key functional roles in health and nutrition of humans. In the context of food and nutrition the minerals may be defined as the elements other than carbon, hydrogen, oxygen and nitrogen that are present in food. These are present in relatively low concentrations in the food. About 25 of the elements occurring naturally on the earth’s crust are known to be essential to life and are present in living cells. Further, since our food is derived from living plants or animals, these elements are expected to be present in our food.

The mineral elements can be divided into two groups on the basis of their amounts in the body. The major minerals are found in bulk concentrations in the body while the microminerals or the trace elements are present in very small concentrations. In terms of their biological roles, the mineral elements are divided into essential elements having known biological roles nonessential elements with unknown functions, and toxic elements. Sodium, potassium, phosphorous, iron, calcium, zinc, selenium, magnesium, copper, tin, cobalt, manganese and fluorine etc. are the examples of essential elements. These function as electrolytes, as enzyme constituents and as building materials in bones and teeth. Boron, aluminium, mercury, lead and cadmium are non essential elements. Of these boron and aluminium are non-nutritive and non-toxic whereas the rest are and non- nutritive, toxic elements.

Chemical and Functional Properties of Minerals in Foods

Even though minerals are present in foods at relatively low concentrations, they often have profound effects on physical and chemical properties of foods because of interactions with other food components. Details of mineral-food interactions for the broad array of minerals found in foods and their interactions as well as their roles are summarized in Table 28.1.

bioavailability can be extremely poor, so diets based on cereals and legumes are often inadequate in iron.

While it is possible in some cases to enhance the nutritional quality of crops through agronomic practices and plant breeding, the movement of mineral nutrients from the soil to the plant and from the plant to the animal or human is an extremely complicated process. Soils differ considerably in their mineral composition. Moreover, the concentration of an element in the soil may not be a good indicator of the amount that can be taken up by plant roots, since the chemical form of the element and soil pH have marked effects on mineral bioavailability to plants. For example, increasing soil pH by adding lime will lower availability of iron, zinc, manganese, and nickel to plants and will increase availability of molybdenum and selenium. Also, plants generally possess physiological mechanisms for regulating amounts of nutrients taken up from the soil. Therefore, we might expect that attempts to alter the mineral composition of food crops would meet with mixed results. For example, application of fertilizer does not significantly increase iron, manganese, or calcium content of food crops. On the other hand, fertilization with zinc at levels in excess of the zinc requirement of the plant has been shown to increase the level of zinc in pea seeds.

ii) Factors Affecting the Mineral Composition of Animal Foods

Mineral concentrations in animal foods vary less than mineral concentrations in plant foods. In general, changes in dietary intake of the animal have only a small effect on mineral concentrations in meat, milk, and eggs. This is because homeostatic mechanisms operating in the animal regulate tissue concentrations of essential nutrients.

Adequacy of Animal Foods for Supplying the Nutrient Needs of Humans

The composition of animal tissues is similar to that of humans; thus we might expect animal foods to be good sources of nutrients. Meat, poultry, and fish are good sources of iron, zinc, phosphate, and cobalt (as vitamin B12). These products are not good sources of calcium unless bones are consumed, which is usually not the case. Also, the iodine content of animal foods may be low. Dairy products are excellent sources of calcium. Thus, consumption of a variety of animal foods along with a variety of plant foods is the best way to ensure adequate intakes of all essential minerals.

Effect of food Processing on Minerals

The freshness, appearance, and nutritive value of foods changes when they are stored for long time. People in food industry work for procedures which make the foods retain their nutritive value even after a long time. The conversion of raw food materials into the acceptable food product by a variety of means is referred to as food processing. The techniques followed include, dehydration, freezing, heating at high temperatures, exposure to radiation (i.e. irradiation), fermentation, chemical preservation etc.

Processing of food has advantages and disadvantages both which results into desirable changes like enhancement of flavours, improvement of texture, and increase in shelf life etc. However, it may lead to some undesirable changes too. These include changes in colour, flavour, nutritional properties and development of toxicity. This affects, to some extent, all the components of food.

Minerals are comparatively stable under food processing conditions such as heat, light, use of oxidizing agents and extremes in pH. Therefore processing does not usually reduce the mineral contents. However, these minerals can be removed from foods by leaching or by physical separation. Cooking in water would result in some losses of minerals since many minerals have significant solubility in water. In general, boiling the vegetables in water causes greater loss of minerals from them as against steaming them. Canned foods such as fruit juices may take up metals from the container-tin and iron from the tin plate and tin and lead from the soldering.

During cooking sodium may be lost but the other minerals are well retained. Many selenium compounds are volatile and can be lost by cooking or processing. Further, it has been found that milling of cereals cause considerable loss of minerals. Since minerals are mainly concentrated in the bran layers and in the germ, during milling after removal of bran and germ, only pure endosperm remains, which is poor in minerals. For example, when wheat is milled to obtain refined flour, the losses in mineral content are to the extent of 76% in case of iron, 78% in zinc, 86% in manganese, 68% for copper, and 16% for selenium. Similar losses occur during milling of rice and other cereals.

As mentioned above, the minerals are quite stable to heat and pH during processing. However change in temperature, pH and concentration or dehydration may lead to the change in the status in food system. For example in milk 1/3rd 1/4th of the calcium and phosphorous is associated with casein while 66 to 80% are present as dissolved calcium and phosphorous. On heating these minerals change from the dissolved to the colloidal state. On the other hand, cooling of milk shift the colloidal calcium and phosphorous to the dissolved state. Decrease in pH from the normal value towards isoelectric side (pH 4.6) will caused the solubilization of these minerals while an increase in pH will causes a shift of colloidal calcium, magnesium and phosphorus to the dissolved state.

The minerals in meat products are in the non-fatty portions, when liquid is lost from meat, the maximum loss is of sodium and calcium, phosphorus and potassium are lost to a lesser extent. During cooking also, sodium is lost but other minerals are well retained. In fact, cooking dissolves some calcium from bone and enriches the meat with this mineral.

FORTIFICATION: NEED AND TYPES

A number of interventions are in practice that makes up for the loss of nutrients in the food. These are in terms of supplementing the food for the lost components. Nutrient supplements are added in order to:

• Maintain the nutritional availability of foods
• Provide the adequate level of nutrients in the food
• Impart additional nutritional value to the food

Nutrient supplementation obviously means adding nutrients to the food. It is generally achieved in two ways.

1. Enrichment: refers to adding nutrients in the food so as to maintain the amount that was present in food before its processing i.e., equal to the loss incurred during processing or preservation.
2. Fortification: According to FAO/WHO, Food fortification has been defined as the addition of one or more essential nutrients to a food, whether or not it is normally contained in the food, for the purpose of preventing or correcting a demonstrated deficiency of one or more nutrients in the population or specific population groups. Vitamins and minerals are most commonly used for fortification. Some of the foods and the fortifying agent used are given in Table 28.3.

Food fortification is especially useful for the people who generally belong to low income groups and face malnutrition. Food fortification has been very successful in eradicating diseases like goitre, rickets, beriberi and pellagra. Many important aspects must be borne in mind while carrying fortification at a large scale. Some of these are as follows:

• The food selected for fortification should be the staple food of the target groups so as to make it affordable.
• The fortified nutrient should have favorable physico-chemical and bioavailability characteristics. The former takes care of the colour, taste, odour and appearance of the food while the later is important from the stability point of view. For example iron and fatty acids may react in the fortified food and produce free radicals which are not good for the quality of food.
• It should be economically viable i.e., it should not be an expensive process.
• A number of factors like the solubility and stability of the fortifying agent affect fortification. The stability depends upon pH, light and temperature besides presence of oxygen. These must be considered while attempting fortification. In addition, a proper monitoring is essential while carrying out food fortification for fruitful results.

Books & Web resources

• Belitz, H.-D., Grosch, W and Schieberle, P.(2009) Food Chemistry 4th edn, SpringerPublications.
• DeMan, J.M. (1999). Principles of FoodChemistry 3rd edn, Aspean Publication.
• Damodaran, S., Parkin, K.L. and Fennema, O.R.(2007) Fennema’s Food Chemistry 4th edn, CRC Press.