E-mail: jmorales@quetzal.innsz.mx

Knowledge of the amino acid composition of foods is essential to calculate their chemical score, which is used to predict protein quality of foods and diets. Though amino acid composition of many foods is reasonably well established, better knowledge is needed on native foods consumed in different regions and countries. This paper presents the amino acid composition of different presentations of raw and processed foods produced and consumed in Mexico. The amino acid composition was determined using Beckman amino acid analyzers (models 116 and 6300). Tryptophan was determined using the Spies and Chambers method. Of the different foods analyzed, some comments are made on native or basic foods in Mexico: Spirulin, where lysine is the limiting amino acid, with a chemical score of 67%, is a good source of tryptophan (1.16g/16 gN); amaranth contains high levels of sulphur amino acids (4.09 to 5.34 g/16gN), with a protein content of 15 g/100g; and pulque, a Pre-Hispanic beverage that contains high levels of tryptophan (2.58 g/16 gN) and sulphur amino acids (2.72 g/16 gN). Finally, insects are good sources of sulphur amino acids and lysine.

Key words: Amino acid, Food composition, Mexican foods.

Food protein quality, ie the proportion of ingested α-amino nitrogen that is retained by the body, is a key nutritional issue since it varies widely from one food protein to another, affecting dietary nitrogen requirements and the interpretation of intake data.

Taking protein quality into consideration is particularly important in evaluating new protein sources, in providing nutritional education and dietary counseling to normal individuals under different physiological conditions (athletes, pregnant or lactating women, elderly persons), in designing and regulating low-cost protein mixtures and in the development of products for special needs such as infant formulas, weaning foods, tube feeding, and treating inborn metabolic errors and other specific diseases (1).

The main determinant of food protein quality is its content of essential amino acids. Thus, the quality of a protein can be predicted from its amino acid composition calculating the chemical score and related indexes. The chemical score is the percentage of the most limiting amino acid in comparison to the so called reference amino acid pattern, which – theoretically – is fully utilized by the body. Reference scoring patterns changes as new knowledge on amino acid requirements is acquired. Although the FAO/WHO/UNU 1985 pattern is widely used (2), Young has proposed a new amino acid scoring pattern that takes into account results obtained in studies using amino acids labeled with stable isotopes (3).

A wealth of information on food composition has been gathered worldwide over decades of analytical efforts and research. Nonetheless, there is no data available on native foods or on a variety of regular foods consumed in different regions and countries. In Mexico, Cravioto, et al. began analyzing amino acid content in Mexican foods in 1947 (4, 5), but very little has been published subsequently on the subject. The Instituto Nacional de Ciencias Médicas y Nutrición, Salvador Zubirán (INCMNSZ) continued analyzing amino acids contained in different local foods and in 1996, some of this information was published in the Composition of Mexican Food tables (6). This paper includes data resulting from analysis done at the INNSZ during 1980-2002. Since referring individually to more than 180 foods analyzed would not be practical, the discussion and comments presented include only native foods for which no references can be found in literature on the subject and therefore no comparisons could be made.

Some products were received directly from people interested in obtaining an amino acid profile of their samples. For perishables, we got 1 kg samples and for industrialized foods we got three small packages of each brand selected. The samples were transported to the laboratory in bags or plastic containers, at room temperature (21±2°C, 50% RH). The samples weighed at least one kilogram and were wrapped as they are regularly sold. Foods that required cleaning were cleaned manually to eliminate foreign bodies and damaged parts and then cut into small pieces, ground and homogenized in a blender (Osterizer). After this process was completed, the sample was ready for determining moisture and nitrogen content and for amino acid analysis. Determinations of moisture and nitrogen content were carried out twice and for amino acids. Analysis were done in at least two samples; the average is reported.

Between 0.5 to 1.0g of the homogenized sample was weighed and digested with concentrated sulphuric acid (H₂SO₄) for 30 to 60 minutes using a selenium and potassium sulfate tablet as catalyst. After digestion, the balloon flask with the sample was left to cool at room temperature. The sample was diluted with 75 mL of distilled water; 40% sodium hydroxide was added to this solution and released ammonia was received in a 1% solution of boric acid. Ammonium borate was titrated with a 0.1N solution of H₂SO₄ (7, 8, 9) (Analyzer Procedure with Kjeltec 1035 Sampler System); (Technique described in the handbook Kjeltec Auto 1030 TECATOR).

The four hydrolysates were filtered through Whatman filter paper No. 42 into 250 mL boiling flasks. The flasks were rinsed three times with distilled water and filter rinse, evaporated until dry in a vacuum rotatory evaporator at 60°C, and then washed with distilled water until there was no acid odor. The residues were recovered from each flask using 25 mL of Na-S high performance sample dilution buffer, (Beckman reagent) and Whatman paper No. 42 and received in a dry and clean jar (7,14).

An aliquot part of 1 mL was taken from the filtrates and filtered through a 0.22 mm pore size polysulfonate or cellulose membrane; the filtrate was received in a 2 mL Eppendorf tube. A 100 µL aliquot part was taken from the latter and gauged with a Na-S high performance sample dilution buffer for each 1 mL. The analyzer coils were filled with this solution (Operate amino acid analyzer, according to manufacturer’s instructions) (15).

Beckman 116 and 6300 (System Gold 6300, Beckman Coulter de México, S.A. de C.V.) amino acid analyzers were used and programmed according to manufacturer’s instructions. Calibration curves were run with three injections of Beckman CD (Column Diagnostic Reagent and Standard) and STD (Amino Acid Standard for Hydrolyzate Analysis) mixtures. Samples were then placed in the corresponding coil, including a reference standard in one out of every five samples. For sulphur amino acids, the calibrating curve was run with a mixture of Beckman CD standards and methionine sulphonates from Sigma. The duration of each analysis was 80 minutes and working conditions were as follows: High performance sodium column (silicagel, 7.5 microns particle size), 12 cm long, a buffer flow of 14 mL/h, ninhydrin flow of 7mL/h; sequential temperatures were 48°C for 11 minutes, 65°C for 34 minutes, and 77°C for 74 minutes (15 - 17).

The amino acid composition, the chemical score and the limiting amino acid of 222 Mexican Foods, some of which are native foods, are presented in tables 1-7; empty spaces mean that there is no value available. Chemical scores were calculated using the scoring pattern for adults suggested by Young and Borgonha (3) because of its solid experimental support. Comments include only some particular items, mainly native foods of nutritional interest, as analytical considerations and literature comparisons are beyond the scope of this report.

Interestingly, historical documents indicate that spirulina was harvested from the waters of Lake Texcoco and consumed by the Aztecs as a food ingredient recently it has gained reputation as a cholesterol lowering functional food and it is sold in capsules as a food supplement in naturist stores. Spirulina protein is limiting in lysine with a chemical score of 67 but it is a good source of tryptophan ( % of scoring value).

Due to the importance of amaranth seeds in native religious practices, after the Spanish Conquest growing these plants was discouraged and even prohibited in the New Spain by the Spanish domination (1521 to 1810) and its food use was limited to the leaves and to a candy made from popped seeds mixed with honey. Currently, amaranth cultivation and consumption are gaining popularity and, due to its sensorial qualities, its incorporation in a variety of products is expected to grow.

As it may be observed in table 1, the seeds of the Mexican varieties of amaranth are limiting in lysine and leucine (~65%) but they are good sources of sulphur amino acids and have protein contents of around 15 g/16gN.

Pulque This Pre Hispanic beverage (table 1) is still widely consumed in rural areas of central Mexico. It is obtained from the alcoholic fermentation of aguamiel, the sweet juice from the stalk of several species of maguey (Agave sp.). Although on a wet basis it provides only 0.3 mg/16mg of nitrogen, the volume consumed by some people may be high and its amino acid content is interesting since its protein is high in tryptophan (2.35 g/16gN) and sulphur amino acids(2.72 g/16gN) although it is limiting in lysine (39%).

The Mexican varieties analyzed provide 8 to 9g of protein per 100g (table 2); they are limiting in lysine (56%) and poor in tryptophan (63%), but are good source of sulphur amino acids.

Insects Enthomophagy was, and still is, a common practice in Mexico, especially in rural areas of Central and Southern Mexico, but many restaurants in urban areas offer dishes based on different insects. To date, over one hundred edible species have been registered in the country (20).

Eating insects is not a result of poverty or famine; in Mexico it is a highly appreci0ated tradition with a significant ritualistic meaning. Furthermore, insects are quite expensive and are considered a delicacy that is frequently offered in fancy restaurant menus. As expected, insects have a high moisture content and provides around 20% of protein. The species analyzed have different amino acid compositions, but as a group they are usually limiting in tryptophan, with chemical scores ranging from 10% (jumiles) to 80% (escamoles). In general, all insects are good sources of sulphur amino acids and, with the exception of jumiles, they are also good sources of lysine.

Nopal (Opuntia sp.) is a cactus that is widely distributed throughout Mexico and it is used in many traditional dishes. It is extremely popular as a remmedy for diabetes, probably because of its high content of soluble dietary fiber. Its protein content is only 1.4 g/100g but its chemical score is close to 90%, leucine and tryptophan as the limiting amino acids.

Quiote is the flower of maguey. It is a delicacy reserved for special occasions due to its relative scarcity. Its protein content is only ~1g/100g but its amino acid composition is very well balanced, with a chemical score close to 100%.

Acknowledgements

17. Spies JR, Chambers DC. Chemical Determination of Tryptophan in Proteins. Analytical Chem. 1949; 21: 1249-1259.         [ Links ]