ELEMENTS OF FOOD CHEMISTRY

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Food is made up of chemicals that include primarily water, proteins, lipids, carbohydrates and minerals.

The major components that are altered by processing include proteins, lipids and carbohydrates.

The chemical nature of foods is important in two ways in respect to food processing:

Food chemicals are altered by processing and these changes results in changes in the characteristics of the food and consumer acceptance of the product

Because of the lability of some food chemicals, the parameters used in food processing, such as temperature and shear, are limited to achieve minimal changes in the characteristics of the food and to maximize consumer acceptance.

Minimal processing results in the least change in the chemicals of food, provide the highest quality and result in foods with a very short shelf-life. As processing is performed to extent shelf-life (drying, canning), the is more chemical change and loss of perceived quality.

Composition of typical food products (%)

The products listed in the table differ in gross composition and also differ in the specific chemical nature of their lipid, protein and carbohydrates that influence processing methods and the effects of processing on the characteristics of the food.

pH:

The pH of foods (negative log of the hydrogen ion concentration) is important both in respect to the flavor, texture and food safety.  The FDA generally regards foods with a pH of less than 4.6 to be of less concern from a microbiological view point.  Examples of foods with  ranges of different pH values are shown in the following table.  Values greater than 4.6 are marked in red.

For an alphabetical listing of a wider range of foods, see:   

        http://vm.cfsan.fda.gov/~comm/lacf-phs.html

Water:

Water is an important component of foods, which influences textural properties and the extent to which the food may be subjected to microbial spoilage. Removing water through concentration, drying or freezing reduces the "free" water and prevents microbial growth.

Water activity is a measure of free (unbound) water available for chemical and biological activity.

A_w = vapor pressure of food product at a specified temperature vapor pressure of pure water

 The relationship between water activity and factors relating to food quality (microbiological and chemical) are shown in the following figure.

Generally bacterial require a water activity of >0.9 to growth and most yeasts and molds are inhibited by a water activity of <0.7.  The FDA considers that a A_w <0.85 to control the outgrowth of pathogenic bacteria.

It is well to remember that moisture migrates from high to low water activity.

Materials that are water soluble are call hydrophilic and those that are not water soluble are called hydrophobic.

The three components of food that have the greatest effect on the characteristics of processed foods are lipids (fats and oils), carbohydrates (sugars, starches and gums) and proteins. In addition, minerals are important in that they modify the functional properties of proteins. Vitamins influence the process of foods because they may be lost through the action of heat, light or oxygen.

Lipids:

May be classified as fats (solid at room temperature) or oils (liquid) at room temperature and contain a mixture of water insoluble components – the primary one being a triglyceride. A triglyceride contains 3 fatty acids that are esterified to the 3 hydroxy alcohol – glycerol.

Fatty acids have the general formula:

R-COOH

with the R group containing carbon C and hydrogen H. If the carbon atoms are bond together with a single bond C-C, the compound is saturated. If the carbon atoms are bound together with a double bond C=C, the compound is unsaturated. Fatty acids that are unsaturated can react with oxygen to product undesirable off flavors.

Triglycerides that contain mostly unsaturated fatty acids are oils and triglycerides that contain mostly unsaturated fatty acids are fats. Generally oils, because of the higher level of unsaturated fatty acids will oxidize over time. Protection against oxidation can involved: a) use of antioxidants, b) free radical quenchers - oxygen scavengers and c) packing in light protective packages and/or oxygen free packaging.

Hydrogenation (addition of hydrogen and removal of double bonds) is used to covert vegetable oils into semi-solid or solid fats to be used as ingredients in baked goods. These partially hydrogenated products are less susceptible to oxidation than the original oils.

Some lipids, such as phospholipids and mono- and di glycerides are used as emulsifiers. Phospholipids are normally occurring and have a phosphate and amine base substituted for one of the fatty acids. Mono- and di-glycerides are derived from triglycerides by the hydrolysis of the ester bond for one (mono-) or two (di) of the fatty acids

diglyceride

monoglyceride

Carbohydrates:

The carbohydrates in foods are mixtures of carbon, hydrogen and oxygen and can be classified as: a) simple sugars and polysaccharides.

Simple carbohydrates

Simple carbohydrates are water soluble and contribute to the sweetness of foods. There are two general types of carbohydrates : a) reducing and b) non-reducing. Examples of these are glucose (reducing) and sucrose (non-reducing).

Glucose (monosaccharide)

Sucrose (disaccharide)

Reducing sugars contain a reactive aldehyde (CHO) group that is absence in non-reducing sugars. Thermal processing can cause reactions between reducing sugars and the amino-group of proteins, causing browning and altering flavors. This reaction is termed the Maillard reaction. Very high heat processing in a low water environment can cause carmelization (polymerization) of also results in a browning reaction.

Monosaccharides may have 6 carbons and are called hexoses or they may have 5 carbons and are called pentoses. Glucose (sometimes called dextrose), fructose and galactose are three common hexoses. Ribose and deoxyribose are two common pentoses. Two monosaccharides may be linked together to form a disaccharides.

Sucrose is the most common disaccharide and is made of one molecule each of glucose and fructose. Sucrose is commonly referred to as sugar. Lactose is the major sugar in milk and is made up of one molecule of glucose and one of galactose. Maltose is a disaccharide made from two molecules of glucose. This linkage is formed by the removal of water(dehydration) and is broken by adding water back (hydrolysis)

Complex carbohydrates

Cellulose is the most common polysaccharide and the major component of plant cell walls. Cellulose is a polymer of glucose molecules linked together by beta 1-4 linkages and cannot by digested by humans. Thus, cellulose is a major component of dietary fiber. Starch is also a polymer of glucose, but the glucose molecules are joined together by alpha 1-4 linkages that can be digested by humans. In plants, starch is an energy reserve. In animals, small amounts of energy are stored in liver and muscle as glycogen, a highly branched polymer of glucose. Pectin is a polymer of galcturonic acid and is not digested. In plants, pectin "cements" cells together. Polysaccharides may be added to foods for a variety of reasons. Nutritionally, they are generally added to increase the dietary fiber content. Functionally, polysaccharides are added to thicken, form gels, bind water, and stabilize proteins. Starch is the most common polysaccharide added to food products. For some uses, starch may be chemically modified to improve stability or to alter its functional properties. Cellulose and cellulose derivatives are also added to a number of food products. The term, gum, is used describe some of the naturally occurring polysaccharides added to food.

Polysaccharides commonly added to foods include: Agar, Gum tragacanth , Algin, Locust bean (carob) gum , Carrageenan, Starch, Cellulose, Pectin, Guar gum, Xanthan gum , Gum arabic. These gums vary in the solubility in cold water and care must be taken to ensure that they are fully hydrated during food processing.

Starches, commonly used as thickening agents in food, are plant storage polysaccharides the are either branched (amylopectin) or unbranched (amylose). The partial structure of amylopectin is shown below:

The proportion of the two starches varies from plant to plant and influences the processing of the foods in which starches are used, as well as the characteristics of the food. Starches with 100% amylopectin (waxy starch) create a pie filling like texture, are clear with a long texture and do not form films. Starches that have >20% amylose have a pudding like short texture, are cloudy and do form films.

Regular starches require heating to replace the hydrogen bonds between starch molecules with starch-water bonds, which causes "gelatinization" and creates the thickening effect. Starches that are pre-gelatinized and dried are cold water soluble and are termed "instant" starches.

Proteins:

Proteins are polymers of amino acids linked together through a peptide bond.. The shape and thus the function of a protein is determined by the sequence of its amino acids. Digestion of proteins produce amino acids, some of which are essential to the nutritional well being of the human.

Amino Acid - contain an amino group ( -NH2 ) and an acid group ( - COOH ).

There are twenty amino acids that are found in proteins.

Peptide Bond- A bond formed by the condensation of the amino group ( -NH2 ) of one amino acid with the acid group ( -COOH ) of another amino acid resulting in the loss of water.

Condensation Reactions - Removal of water ( H2O ) and formation of a bond. The reversal of this is hydrolysis which involves the addition of water.

Peptide bonds are not easily broken. Mild thermal processing does not normally result in the breaking of peptide bonds to yield amino acids from proteins.

The function of protein in foods include:

	Furnish essential amino acids
	Add to structure ( Texture )
	Appearance ( myoglobin, browning )
	Contribute to odor and taste
	Stabilize emulsions
	Stabilize foams
	Form gels

Frequently the conditions used in processing are adjusted to optimize the effects of the processing on the proteins and subsequent characteristics of the food. In bread, for example, the brown crust is related to the Maillard reaction and the final structure of the bread is caused by the thermal gelation of the protein - gluten.