Food analysis amino acids

While most columns are general-purpose, a number of columns are marketed for specific applications. Examples are columns for environmental analysis (carbamates, polynuclear aromatic hydrocarbons) or food testing (amino acids, organic acids, sugars). These columns are often shipped with chromatograms demonstrating the performance of the specific application. More examples of specific applications and GPC columns for polymer characterization are described in Chapter 7. 

Another important property of the protein amino acids is that they are all L-isomers, with the exception of glycine (glycine only has one configuration). The body cannot use the D-isomers of these amino acids, and, in fact, some o-isomers may be toxic. An exception is D-methionine, which the body can convert into L-methionine through transamination. Naturally produced amino acids are L-isomer however, manufactured amino acids can be a racemic mixture with half of the molecules being D-isomers. These can then be separated and only the L-isomer used to fortify foods. The amino acid analysis methods normally employed in the nutritional laboratory will not differentiate the d- and L-isomers. There are, however, chiral chromatography and enzymatic methods that can be used if the product is suspected of containing D-isomers. 

Methods have been developed for analysis or deterrnination of free amino acids in blood, food, and feedstocks (116). In proteins, the first step is hydrolysis, then separation if necessary, and finally, analysis of the amino acid mixture. 

In terms of amino acids bacterial protein is similar to fish protein. The yeast s protein is almost identical to soya protein fungal protein is lower than yeast protein. In addition, SCP is deficient in amino acids with a sulphur bridge, such as cystine, cysteine and methionine. SCP as a food may require supplements of cysteine and methionine whereas they have high levels of lysine vitamins and other amino acids. The vitamins of microorganisms are primarily of the B type. Vitamin B12 occurs mostly hi bacteria, whereas algae are usually rich in vitamin A. The most common vitamins in SCP are thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, choline, folic acid, inositol, biotin, B12 and P-aminobenzoic acid. Table 14.4 shows the essential amino acid analysis of SCP compared with several sources of protein. 

Davies, A. M. C. (1976). The application of amino acid analysis to the determination of the geographical origin of honey. /. Food Technol. 11, 515-523. 

The structural analysis of membrane-associated peptides comprises two steps (a) the elucidation of the three-dimensional fold of the peptide and (b) the determination of the membrane-peptide interface. We will use our results gained for the 36 amino acid residue neuropeptide Y (NPY) [83] to demonstrate the approaches that can be used. NPY regulates important pharmacological functions such as blood pressure, food intake or memory retention and hence has been subject of many investigations (for a review see Ref. [84]). It targets the so-called Y receptors that belong to the class of seven transmembrane receptors coupled to G-proteins (GPCRs). 

Various important LC methods for amino acid, peptide and protein analysis were reviewed and evaluated126,127. A review of HPLC methods for the analysis of selected biogenic amines in foods appeared, including methods for extraction and for elimination of interfering compounds128. 

A study comprising five laboratories was carried out on the accuracy and precision of protein amino acid analysis. An important conclusion reached was that it is necessary to examine both accuracy and precision to achieve maximum improvement in either129. A commercially available single-cell protein, Pruteen, was proposed as reference material for the determination of amino acids and other substances in food. This recommendation was the result of a five-year-long study on the stability of this particular protein130. 

A number of criteria could be apphed to organize this chapter, depending on the point of view by which foods are considered. In this chapter, application of HPLC to food analysis will be described considering homogeneous classes of food components lipids, carbohydrates and related substances, proteins, peptides, amino acids, biogenic amines, phenolics, vitamins, and some selected contaminants. 

Detection of peptides in HPLC can be achieved by measuring natural absorbance of peptide bonds at 200-220 nm. Unfortunately at these wavelengths a lot of food components and also the solvents used for analysis absorb, demanding an intensive sample pretreatment and clean-up [129]. Peptides with aromatic residues can be detected at 254 nm (phenylalanine, tyrosine, and tryptophan) or 280 nm (tyrosine and tryptophan). Taking advantage of the natural fluorescence shown by some amino acids (tyrosine and tryptophan), detection by fluorescence can also be used for peptides containing these amino acids [106]. 

On the other hand, free amino acid analysis represents a powerful tool to characterize different foods and beverages, monitor proteolysis, assess freshness, detect adulterations, and safeguard consumer health. The occurrence of some potentially toxic nonprotein amino acids (some of which are neuroexcitatory) in commercially available seedlings has been reported by different authors [197-199]. Due to the incapability of humans to utilize the o-isomers of amino acids (some of which are thought to be toxic), the enantiomeric separation of d- and L-form of amino acids is also an area of growing interest [196]. 

Recently, Peace and Gilani [229] reviewed chromatographic determination of amino acids. HPLC analysis of free amino acids can be a powerful tool in the control of food authenticity. Cotte et al. [230] demonstrated that HPLC analysis of free amino acids, followed by statistical processing of... 

HPLC Applications for Free Amino Acids Analysis in Foods... 

Recent approaches to the amino acid analysis of foods require maceration of the sample, hydrolysis of the proteins with HC1, and filtration. The resulting material may still include proteins (and fragmented peptides), carbohydrates, salts, urea, and lipids. The solution is then passed through a small column of cation exchanger (with a nominal cross-linking of at least 8%) in... 

Determining protein quality analysis is important in food science, particularly for developing foods with targeted nutritional value, and in animal feeding and husbandry. Protein is the key component in the diet of any farmed species, particularly in aquaculture and the pet food industry, making an accurate assessment of protein utilization critically important. Protein quality analysis provides an estimate of the content and bioavailability of indispensable or dietary essential amino acids. 

PER is a method to metabolize or determine the quality of protein in foods. Quality is measured by the amount of usable protein and the growth resulting from it through an animal assay. Formerly, this method was used as the standard method for all protein quality analysis. However, there is some question as to whether or not it is a valid measurement. This is because PER does not account for the differences in amino acid requirements between humans and rats (Seligson and Mackey, 1984), nor does PER account for the protein needed for cell maintenance. Therefore, PER results often overestimate the requirements for some amino acids and underestimate others. Specifically, PER tends to underestimate the protein quality of lysine-deficient proteins such as wheat gluten (Hackler, 1977). 

Howell, N.K., Arteaga, G.E., Nakai, S., and Li-Chan, E.C.Y. 1999. Raman spectral analysis in the C-H stretching region of proteins and amino acids for investigation of hydrophobic interactions. J. Agric. Food Chem. 47 924-933. 

How do chemical analyses of foods differ from analyses used in chemistry, biochemistry and biology The same methods and techniques are often used only the purpose of the analysis may differ. But foods are to be used by people. Therefore, methodology to determine safety (presence of dangerous microbes, pesticides, and toxicants), acceptability (flavor, odor, color, texture), and nutritional quality (essential vitamins, minerals, amino acids, and lipids) are essential analyses. Current Protocols in Food Analytical Chemistry is designed to meet all these requirements. 

By far, the predominant methods for determination of amino acids in foods are based on HPLC. However, alternative methods for amino acid analysis do exist. Many of the earliest determinations for certain amino acids were based on microbiological tests (and other bioassays), but these are no longer widely employed. Cost and analysis time are obvious factors in the demise of these type of methods. Also, these types of methods are very prone to biased results and high variance. 

Gas chromatography (GC) for amino acid analysis is the alternative to HPLC that has found the greatest acceptance. It requires the preseparation derivatization of the amino acids to render them volatile. For this purpose, amino acids are frequently converted into acylated esters. N-Trifluoroacetyl-n-butyl esters and /V-heptafluorobutyrylisobutyl esters are most commonly employed. There have been comparative studies (3,4) that document similar (if not equivalent) analytical results for GC and the classic ion-exchange chromatographic method applied to a variety of food samples. Comparison (5) of GC to the reversed-phase HPLC determination of amino acids (phenylisothiocyanate derivatized) also shows excellent agreement. 

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