Macronutrients

Food components are divided into macro and micro-components. Macro-components, also referred to as macronutrients include carbohydrates, proteins, lipids and water. Micronutrients include vitamins and minerals (micronutrients) as well as pigments, antioxidants, anti-nutrients and toxins.

Carbohydrates

Carbohydrates are so called because they are hydrates of carbon. They consist of carbon, hydrogen and oxygen (C_n(H2O)_n), and are derived from plants. Carbohydrates is one of the main sources of energy in the body (1 gram = 4kCal). They include sugars, starches and fiber. Carbohydrates are classified into monosaccharides, disaccharides, oligosaccharides and polysaccharides.

Monosaccharides have one sugar unit, and are often referred to a ‘simple sugars’. Key monosaccharide of interest in food processing such as glucose, fructose and galactose have 6-carbons. These six-carbon sugars are called hexoses. Hexoses may be aldoses or ketoses depending on what type of functional group they have. If a hexose has an aldehyde group, it is called an aldose. If it has a ketone group, it is called a ketose. Monosaccharides may exist in two forms. They may be straight chained, or in a ring form. The straight chain is called the Fischer projection while the ring form is called the Haworth projection. A very small amount (less than 1%) is actually found in the Fischer projection. The Haworth projection is formed when the functional group (aldose of ketose) reacts with the hydroxyl (OH) group on carbon number 5. This produces a pyranose (six-sided) ring in aldoses e.g. glucose, and a furanose (5-sided) ring in ketoses e.g. fructose.

The carbon that is attached to the oxygen in the ring (not the one attached to the CH₂OH) is called the anomeric carbon. When this carbon is free (unbound), the monosaccharide is capable of participating in chemical reactions. Sugars with a free anomeric carbon are called reducing sugars, while those with bound anomeric carbons are called non-reducing sugars. All monosaccharides are reducing sugars. Reducing sugars can take part in reactions such as Maillard browning which provides color and flavor in foods such as grilled meat and baked bread. The free anomeric carbon can either be an alpha (α) or beta (β) carbon, depending on the position of the attached OH group. If the OH group on the anomeric carbon is on the opposite side to the CH₂OH on carbon number six, then the carbon is an α-carbon. If the OH group on the anomeric carbon is on the same side as the CH₂OH on carbon number six, it is a β-carbon.

Disaccharides are formed when two monosaccharides are joined. For example sucrose is formed from the combination of a glucose and a fructose unit. Where they link is called a glycosidic link. In sucrose, glucose and fructose are connected at the (α1→2) glycosidic link. That is, glucose at carbon number one (an α-carbon) is connected to fructose at carbon number 2. Apart from sucrose, other disaccharides include lactose (glucose + galactose) and maltose (glucose + glucose). Since the body metabolizes simple sugars, it must breakdown disaccharides and other complex carbohydrates for them to be used. Some people are not able to digest lactose sugar because they do not have enough of the enzyme lactase to breakdown the lactose. This makes them lactose intolerant resulting in gas, abdominal pain, and nausea. Lactose is an example of a disaccharide that is a reducing sugar since it has a free anomeric carbon. Sucrose however is a non-reducing sugar since its anomeric carbons are bound.

Oligosaccharides are carbohydrates generally ranging between 3-10 monosaccharide units. One example is raffinose, a three unit carbohydrate (also called a trisaccharide) consisting of galactose, glucose and sucrose. Raffinose can be found in edible beans. Since they require raffinase enzyme to digest (an enzyme that humans do not produce), they pass to the large intestine where they are fermented by bacteria producing gas. Beans that are cooked properly will have less of this effect due to partial breakdown of raffinose by heat.

Polysaccharides are complex carbohydrates generally consisting of monosaccharide units exceeding 100. They make up dietary fiber, producing health benefits such as improved colon and heart health, and may reduce the risk of cancer. They include both soluble and insoluble fibers. Soluble fibers include starch, pectins, gums and some hemicelluloses. Insoluble fibers include some hemicelluloses, cellulose and lignin.

Starch is the second most abundant carbohydrate in nature (cellulose is number one). It exist in granule forms that vary in shape depending on the source. They consists of several thousand glucose units. Two main constituents of starch are amylose and amylopectin. Amylose are straight-chain polysaccharides with glucose units connected at the (α1→4) glycosidic link. Amylopectin consists of both straight chains with glucose units connected at the (α1→4) glycosidic link, and branch chains with glucose units connected at the (α1→6) glycosidic link.

Cellulose is the most abundant carbohydrate and are more complex than starch. They can be found in the cell walls of plants. Similar to amylose in starch, they consist of straight chains of glucose units, but instead of having an (α1→4) glycosidic link, they have a (β1→4) glycosidic link. As a result, they are unable to be broken down by amylase, the digestive enzyme for starch in the human body. Since humans do not have cellulase, we are unable to breakdown cellulose.

Hemicelluloses are also found in plant cell walls. They are branched hetero-polysaccharides since they consists of several different types of monosaccharides (compared to starch and cellulose that just have glucose as the only monosaccharide constituent). Monosaccharides in hemicelluloses include xylose, glucose, mannose, galactose, rhamnose and arabinose. Xylose is the most common sugar found in hemicelluloses. Some of the sugars may be present in their acidic form e.g. glucuronic acid and galacturonic acid. Examples of hemicelluloses include xylan, glucoronoxylan, arabinoxylan, glucomannan and xyloglucan.

Lignins are complex polymers of aromatic alcohols found in plant cell walls. They are insoluble in water, non-digestible, and form a large portion of dietary fiber.

Pectins are found in plant cell walls along with cellulose and hemicelluloses. They consists of straight chains of polygalacturonic acid units linked at the (α1→4) position. Pectin is extracted and added to jams and jellies to provide a gel structure.

Hydrocolloid gums, found in plant cells are extrudates secreted in repose to injury. They are structurally related to hemicelluloses. Gum are used in the food industry as thickening agents.

Proteins

Proteins also provide energy (1gram = 1Kcal), but there main function in the body is to build structure. They are also involved in metabolism in the form of enzymes, hormones and antibodies, and are storage and transportation organelles. Main food sources of proteins are meat, poultry, egg, dairy, and legumes.

The smallest unit of proteins are called amino acids. The body uses 20 amino acids to make all the protein it needs. Ten (10) of these amino acids are essential. That means, they cannot be made by the body and therefore must be eaten. The ten essential amino acids are phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, arginine, leucine and lysine. You can remember them using the acronym PVT TIM HALL.

The structure of an amino acid consists of an amino group, a carboxyl group, and a side (R) chain. The side chain varies, and determines the functional properties of the amino acid depending on the type attached. Amino acids join together to form four types of structure. 1) a primary structure consisting of a peptide chain of several amino acids connected to each other at a peptide bond; 2) a secondary structure caused by side bonding of peptide double strands creating beta sheets and α-helix shapes; 3) tertiary structures caused by overlapping of alpha and beta sheets into 3-dimentional structures; and 4) formation of a quaternary structure caused by interaction of more than one secondary structures. Proteins may lose their natural form if their bonds are broken. For example, adding heat, high shear, acids, alcohol, and salts to proteins can disrupt hydrogen bonds. This process is called denaturation. A prime example of denaturation can be seen in the coagulation of eggs during frying.

Lipids

Lipids are fatty acids and their derivatives and are soluble in organic solvents. They provide the highest amount of energy per gram (1gram = 9Kcal). They include fats, oils, waxes, phospholipids and cholesterol. Lipids that are solid at room temperature are called fats, while lipids that are liquid at room temperature are called oils. Lipids used in food processing are primarily triglycerides (also called triacylglycerides). The basic structure of triglycerides consists of one glycerol unit and three fatty acids. If only one or two fatty acids are attached to the glycerol unit, it is called a monoglyceride and diglyceride respectively. Mono and diglycerides are used in foods as emulsifiers (additives used to prevent separation of lipids and water). The fatty acids attached to the glycerol unit may be saturated or unsaturated. This is where the terms saturated and unsaturated fats come from. Take a look at a food label containing fat ingredients. Saturated fats are so called because all the carbon units on its fatty acid chain are filled up (saturated) with hydrogen. Unsaturated fats are so called because not all the carbon atoms on its fatty acid chain is filled up with hydrogen. Instead, they have one or more double bonds to keep them stable. Saturated fatty acids have been associated with heart disease by increasing LDSs (bad cholesterols) and should therefore be consumed in moderate amounts. Meat fats are mostly saturated. Unsaturated fatty acids have been associated with lower risk of heart problems by reducing LDLs. Trans fats is another term that you might have heard. Take a look at the nutritional label again. You may see that trans fats in the product is zero for the serving size. Trans fats have been found to increase risk of heart disease by both increasing LDLs and reducing HDLs (good cholesterols). Therefore they are worse that saturated fats. (See explanation of LDLs and HDLs under cholesterols below).

Trans fats are formed during hydrogenation of oils. Hydrogenation is the chemical process of adding hydrogen to oils to make them more saturated. Different degrees of sanitation results in different functional properties including increased shelf life and texture. Unfortunately however, during the hydrogenation process some of the double bonds will turn to a ‘trans form’ instead of the natural ‘cis form’.

Waxes are used in plants for lubrication, protection and energy storage. On food, they are added to prevent dehydration. Chemically, they are esters of a long chain fatty acid and a long chain alcohol. Waxes may come from animals (bees wax), plants (carnauba) and mineral (petroleum) type.

Phospholipids are a modification of triglycerides, where phosphate groups are found on the carbon-3 position of the glycerol unit. Lecithin is an example of a phospholipid. It is used as an emulsifier.

Cholesterols are sterols characterized by three six-carbon rings followed by a five-carbon ring attached to an eight carbon side chain. Cholesterol is important for many metabolic functions including production of bile salts that is used in digestion. In order for cholesterol to perform its many roles, the body packages them in protein ‘wrappers’, converting them to 1) low density lipoproteins (LDLs) and 2) high density lipo-proteins (HDLs). LDLs have a higher percentage of fat to protein. They are responsible for delivery of cholesterol to parts of the body that needs it. HDLs have a smaller ratio of fat to protein and are responsible for vacuuming up excess bits of cholesterol and transporting them to the liver where they can be broken down and removed. This helps to maintain healthy cholesterol levels. Excess amounts of LDLs may lead to too much cholesterol in the arteries, ultimately resulting in heart disease. Therefore it is important to moderate intake of cholesterol food sources such as meat.

Water

Water is essential for life, since all metabolic reactions in the body occur in the presence of water. Approximately 65% of our body is water. In food processing, water is used in formulation and mixing, transporting materials, heating and cooling, cleaning and sanitizing, chemically, water is made up of two hydrogen atoms and one oxygen. The hydrogens are positively changed while the oxygen is negatively charged, making it a polar molecule. This polarity allows water to react with and dissolve many substances. In fact, water is called the “universal solvent” because it can dissolve more substance than any other liquid. Water can exists in solid, liquid and gaseous states. As a solid, it is less dense than when it is a liquid (1 kg/m³ as liquid versus 0.92 kg/m³ as ice), which is very unusual. Other materials become denser when they solidify. This is because, as water turns into ice, it forms crystalline structures with bonds that are further apart from each other (this also make water expand in volume when it freezes). If ice was denser than water, then during the winter, water would freeze and fall to the bottom of lakes, rivers and oceans, killing aquatic life. Another unique feature of water is its high heat capacity (4.2 KJ/kgK). A high heat capacity allows it to absorb a lot of heat before it can boil. This capacity to absorb large amounts of heat, make water a good cooling agent. On the other hand, once heated up, water takes a long time to cool down, therefore it is also a good heating agent. Water boils at 100^oC (212 ^oF) and freezes at 0^oC (32^oF) at atmospheric pressure. Boiling point can be depresses by lowering the pressure. For example, to allow food materials to loose water at lower temperatures, they can be dried in a vacuum where the pressure it low. This is important for foods that may be damaged (e.g. color and flavor deterioration) if heated at higher temperatures. Melting point of water can also be depressed by adding substances such as sugars and salts. Depressing freezing point means that a lower temperature than 0^oC is required to freeze the material. An example of this is when salt is poured on snowy roads to melt the ice. By adding salt, this makes it difficult for the ice to freeze at its normal freezing temperature. In foods an example is addition of sugars in ice-cream. Sugars depresses the freezing point allowing ice cream to have a soft texture instead of becoming rocky and icy.