Proteins are macromolecules formed from simpler compounds, a-amino acids. An a-amino acid is a carboxylic acid that has an amino group bonded to the carbon atom next to the carboxyl group. The designation a denotes the position of the amino group. The carbon atom adjacent to the carboxyl group is called the a-carbon atom. The general formula for an amino acid compound is shown in Figure 10.8 . In the protein structure, amino acids are linked together by peptide bonds forming the long chains. These bonds are easily broken at high temperatures resulting in the formation of amino acids. Animal manure and food processing waste are rich in protein contents. For example, proteins comprise about 25% of the total solids in swine manure.

The radical, R, in the formula shown in Figure 10.8 differentiates the types of amino acids. The R may be simple hydrocarbons, ring compounds, additional amino or carboxylic groups, or — SH or hydroxyl groups. Alanine, leucine, aspartic acid (aspartate), and glutamic acid (glutamate) account for half of the total amino acids in swine manure (Figure 10.9). Proteins and amino acids are the major sources of organic nitrogen present in swine manure. Organic sulfur is mainly from two particular amino acids, cysteine (R = — CH2-SH) and methionine (R = — CH2CH2- S-CH3).

O

Stearic acid (CH3(CH2)14COOH)

Figure 10.10. Structures of dominant fatty acids in swine manure.

Lipids

Lipids are substances that can be dissolved away from biological material by solvents that are nonpolar or slightly polar. Since the classification is based on solubility, not structure, a wide variety of compounds fall under lipids. Fatty acids are long straight-chain carboxylic acids some of which are saturated and some of which contain one or more double bonds. Almost all fatty acids isolated from natural sources contain an even number of carbon atoms. Among the fatty acids identified in swine manure, stearic acid and palmitic acid are most dominant (Figure 10.10).