FOOD CHEMISTRY

Module 5. Minerals in food

Lesson 14
MAIN ELEMENTS AND TRACE ELEMENTS IN EGGS, CEREALS & CEREAL PRODUCTS, VEGETABLES AND FRUITS

14.1 Introduction

While there is no universally accepted definition of “mineral” as it applies to food and nutrition, the term usually refers to elements other than C, H, O, and N that are present in foods. These four non mineral elements are present primarily in organic molecules and water, and constitute about 99% of the total number of atoms in living systems. Minerals are the constituents which remain as ash after the combustion of plant and animal tissues. Although mineral elements are present in relatively low concentrations in foods, they play key functional roles in foods. Mineral supply depends not only on the intake in food but primarily on the bioavailability, which is essentially related to the composition of the food. A series of food constituents, e. g., proteins, peptides, amino acids, polysaccharides, sugars, lignin, phytin, and organic acids, bind minerals and enhance or inhibit their absorption. They contribute to food flavor and activate or inhibit enzyme-catalyzed and other reactions, and they affect the texture of food.

14.2 Classification

Minerals have been classified as either major or trace, depending on their concentrations in plants and animals. The term “trace” was used to indicate the presence of an element that could not be measured accurately. Major minerals are calcium, phosphorus, magnesium, sodium, potassium, and chloride whereas trace elements include iron, iodine, zinc, selenium, chromium, copper, fluorine, lead, and tin. Minerals are divided into three classes:

• Main elements: The main elements (Na, K, Ca, Mg, Cl, P, S) are essential for human beings in amounts >50 mg/day.
• Trace elements: Trace elements (Fe, I, F, Zn, Se, Cu, Mn, Cr, Mo, Co, Ni) are essential in concentrations of <50 mg/day.
• Ultra-trace elements: Ultra-trace elements (Al, As, Ba, Bi, B, Br, Cd, Cs, Ge, Hg, Li, Pb, Rb, Sb, Si, Sm, Sn, Sr,Tl, Ti, W) are also very vital elements.

14.3 Nutritional and Functional Roles of Minerals

The important nutritional and physiological functions of the various minerals are discussed here.

14.3.1 Main elements

Sodium: The sodium activates some enzymes, such as amylase. Sodium absorption is rapid. The excessive intake of sodium can lead to hypertension. A low intake of sodium can be achieved by a non-salty diet or by using diet salt (common salt substitutes).

Potassium: Potassium regulates the osmotic pressure within the cell, is involved in cell membrane transport. Potassium deficiency may be a result of undernourishment or predominant consumption of potassium-deficient foods, e. g., white bread, fat or oil. Potatoes and molasses are particularly abundant sources.

Magnesium: Magnesium is a constituent and activator of many enzymes, particularly those associated with the conversion of energy-rich phosphate compounds, and as a stabilizer of plasma membranes, intracellular membranes, and nucleic acids.

Calcium: It is abundant in the skeleton and in some body tissues. Calcium controls essential processes like muscle contraction (heartbeat), blood clotting, activity of brain cells and cell growth. The main source of calcium is milk and milk products, followed at a considerable distance by fruit and vegetables, cereal products, meat, fish and eggs.

Chloride: Chloride serves as a counter ion for sodium in extracellular fluid and for hydrogen ions in gastric juice. Chloride absorption is as rapid as its excretion in the urine.

Phosphorus: Phosphorus, in the form of phosphate, free or bound as an ester or present as an anhydride, plays an important role in metabolism and, as such, is an essential nutrient.

14.3.2 Trace elements

Iron: Most of it is present in the hemoglobin (blood) and myoglobin (muscle tissue) pigments. The metal is also present in a number of enzymes (peroxidase, catalase, hydroxylases and flavine enzymes), hence it is an essential ingredient of the daily diet. Two food processing problems arising from mineral fortification are the increased probability that oxidation will occur and, in the case of wheat flour, decreased baking quality. Generally, iron is an undesirable element in food processing; for example, iron catalyzes the oxidation of fat or oil, increases turbidity of wine and, as a constituent of drinking water, it supports the growth of iron-requiring bacteria.

Copper: Copper is a component of a number of oxidoreductase enzymes (cytochrome oxidase, superoxide dismutase, tyrosinase, uricase, amine oxidase). Copper is even less desirable than iron during food processing and storage since it catalyzes many unwanted reactions. Cu²⁺-Ions are taste bearing. The threshold value 2.4–3.8mg/l was determined with aqueous solutions of CuSO₄ or CuCl₂.

Zinc: Zinc is a component of a number of enzymes (e. g., alcohol dehydrogenase, lactate dehydrogenase, malate dehydrogenase, glutamate dehydrogenase, carboxypeptidases A and B, and carbonic anhydrase). Other enzymes, e. g., dipeptidases, alkaline phosphatase, lecithinase and enolase, are activated by zinc and by some other divalent metal ions. Zinc deficiency in animals causes serious disorders, while high zinc intake by humans is toxic.

Manganese: Manganese is the metal activator for pyruvate carboxylase and, like some other divalent metal ions, it activates various enzymes, such as arginase, amino peptidase, alkaline phosphatase, lecithinase or enolase. Manganese, even in higher amounts, is relatively nontoxic.

Cobalt: Since it was discovered that vitamin B₁₂ contains cobalt as its central atom, the nutritional importance of cobalt has been emphasized and it has been assigned the status of an essential element. Its requirement is met by normal nutrition.

Chromium: Chromium is important in the utilization of glucose. For instance, it activates the enzyme phosphoglucomutase and increases the activity of insulin; therefore, chromium deficiency causes a decrease in glucose tolerance.

Selenium: Depending on the region, it can vary greatly because of the varying content of selenium in the soil. Selenium is an antioxidant and can enhance tocopherol activity. The enzyme glutathione peroxidase contains selenium.

Molybdenum: It is a component of aldehyde oxidase and xanthine oxidase. The bacterial nitrate reductase involved in meat curing and pickling processes contains molybdenum.

Nickel: Nickel is an activator of a number of enzymes, e. g., alkaline phosphatase and oxalacetate decarboxylase, which can also be activated by other divalent metal ions. Nickel also enhances insulin activity.

Fluorine: The addition to drinking water of 0.5–1.5 ppm fluorine in the form of NaF or (NH₄)₂SiF₆ inhibits tooth decay. Its beneficial effect appears to be in retarding solubilization of tooth enamel and inhibiting the enzymes involved in development of caries. Toxic effects of fluorine appear at a level of 2 ppm. Therefore, the beneficial effects of fluoridating drinking water are disputed by some and it is a controversial topic of mineral nutrition.

Iodine: Iodine absorption from food occurs exclusively and rapidly as iodide and is utilized in the thyroid gland in the biosynthesis of the hormone thyroxine (tetraiodothyronine) and its less iodized form, triiodothyronine. Iodine deficiency results in enlargement of the thyroid gland (iodine-deficiency induced goiter). There is little iodine in most food. Good sources are milk, eggs and, above all, seafood. Drinking water contributes little to the body’s iodine supply. To avoid diseases caused by low iodine supply, iodization of common salt is done in which potassium iodate, with 100 μg iodine added to 1–10 g NaCl. Higher amounts of iodine are toxic.

14.3.3 Ultra-trace elements

Tin: The natural level of tin in food is very low, but it can be increased in the case of foods canned in tinplate cans. Very acidic foods can often dissolve substantial amounts of tin. Thus, the concentration of tin in pineapple and grapefruit juice transported in poorly tin plated cans was 2 g/l. The tin content of foods in tinplate cans is generally below 50mg/kg and should not exceed 250 mg/kg.

Aluminum: It is resorbed in only negligible amounts by the gastrointestinal tract. The largest portion is eliminated in feces. It is not secreted in milk.

Boron: Boron seems to be an essential nutrient, which promotes bone formation by interaction with calcium, magnesium and vitamin D. In addition, there are indications that boron is involved in the hydroxylation of steroids, e. g. in the synthesis testosterone. Important sources include wine and water.

Silicon: Silicon, as soluble silicic acid, is rapidly absorbed. The main source is cereal products. Silicon promotes growth and thus has a biological role. The toxicity of silicic acid is apparent only at concentrations ≥100 mg/kg.

Arsenic: The main source is fish. Its metabolic role is not yet understood. It appears to be involved in the metabolism of methionine. Choline can be replaced by arsenocholine in some of its functions.

Table 14.1 Mineral content of eggs, cereals & cereal products, fruits and vegetable

Food products	Na	K	Ca	Fe	P
Eggs
Chicken egg yolk	51	138	140	7.2	590
Chicken egg white	170	154	11	0.2	21
Cereals & cereal products
Wheat, whole kernel	7.8	181	33	3.3	341
Wheat germ	5	993	49	8.5	1100
Corn flakes	915	120	13	2.0	59
Corn White	6	294	8	1.5	213
Rice, unpolished	10	238	16	3.2	282
Fruits
Apple	1.2	122	5.8	0.25	12
Orange	1.4	165	42	0.19	23
Strawberry	1.4	161	21	0.64	29
Grapefruit	1.1	148	24	0.17	17
Plum	1.7	177	8.3	0.26	18
Vegetables
Peas, green	2	274	24	1.7	113
Mushrooms	8	390	11	1.26	123
Potatoes	3.2	418	6.4	0.43	50
Carrots	60	321	37	0.39	35
Tomatoes	3.3	242	9.4	0.3	22

Table 14.2 Nutritional and functional roles of minerals and mineral salts/complexes in foods

Mineral	Food sources	Function
Aluminum	Part of ant acids & leavening agents	Essential nutrient, Acts as leavening agent & Texture modifier
Bromine	Brominated flour	Improves baking quality of wheat flour.
Calcium	Dairy products, Green leafy vegetables	Deficiency leads to osteoporosis in later life. Texture modifier.
Copper	Meat, seafoods, nuts	Catalyst in lipid peroxidation, ascorbic acid oxidation.
Iodine	Iodized salt, seafood	Deficiency causes goiter. Improves baking quality of wheat flour.
Iron	Cereals, legumes, meat	Deficiency leads to anemia. Catalyze lipid peroxidation in foods.
Magnesium	Cereals, green leafy vegetables	Removal of Mg from chlorophyll changes color from green to brownish
Manganese	Grains, fruits, vegetables	Cofactor in enzymes like pyruvate carboxylase, superoxide dismutase.
Nickel	Plant foods	Widely used catalyst for hydrogenation of vegetable oils.
Phosphates	Animal products	Acidulent in soft drinks. Acts as a leavening acid. Helps in retention of moisture in meats. Phosphates are used for the emulsification of processed cheeses and meats.
Potassium	Fruits and Vegetables	KCl may be used as a salt substitute.
Selenium	Seafood, cereals	Cofactor of Glutathione peroxidase.
Sodium	NaCl, MSG, milk	Used as flavour modifier. Used as a preservative. Many sodium salts are used as leaving agents.
Sulfur	Widely available	Present in methionine and cystine - essential amino acids. Sulfur dioxide and sulfites inhibit both enzymatic and nonenzymatic browning. Prevents, controls microbial growth.
Zinc	Meats, cereals	ZnO is used in the lining of cans for proteinaceous foods to lessen formation of black FeS during heating. Zn can be added to green beans to help stabilize the color during canning.