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Lime-cooking in rural areas
A number of researchers have described how maize is cooked in rural areas of countries where tortillas are eaten. Illescas (1943) first described the process as carried out in Mexico. It involves the addition of one part whole maize to two parts of approximately I percent lime solution. The mixture is heated to 80°C for 20 to 45 minutes and then allowed to stand overnight. The following day the cooking liquor is decanted and the maize, now referred to as nixtamal, is washed two or three times with water to remove the seed-coats, the tip caps, excess lime and any impurities in the grain. The addition of lime at the cooking and steeping stages helps to remove the seed-coats.
The by-products are either thrown away or fed to pigs. Originally, the maize was converted into dough by grinding it a number of times with a flat stone until the coarse particles were fine enough. Today the initial grinding is done with a meat grinder or disc mills and the dough is then refined with the stone. A portion of about 50 g of the dough is patted flat and cooked on both sides on a hot iron or clay plate.
In Guatemala a similar process (described by Bressani, Paz y Paz and Scrimshaw, 1958) uses either white or yellow maize, but the lime concentration varies from 0.17 to 0.58 percent based on the weight of maize, with a grain-to-water ratio of 1:1.2, and the maize cooking time varies from 46 to 67 minutes at a temperature of 94°C. The rest of the process is essentially the same, except that the dough is prepared with a disc mill and is cooked for about 5 minutes at a temperature of about 170°C at the edges and 212°C in the centre.
Tamalitos, for which the dough is steamed, are softer and keep longer. For recently harvested maize less lime is used and cooking time is decreased; the procedure is modified conversely when the grain is old and dry. The dry matter losses are about 15 percent, but they can vary between 8.9 and 21.3 percent.
Industrial lime-cooking
Factors such as the migration of people from rural to urban areas created a demand for ready-cooked or pre-cooked tortillas. Equipment for processing raw maize into lime-treated maize and then into a dough and tortillas was developed and industrial production of tortilla flour began in Mexico and other countries. Mechanized production in Mexico became important soon after the Second World War. Two types of industry are found in urban areas. One is the small family-owned home tortilla industry, where the process is as described above but with larger and mechanical equipment used to supply a larger market. This development became possible through the introduction of rotary mills and the tortilla maker designed by Romero in 1908. This equipment was later replaced by a more efficient type in which the dough is passed through a rotating metal drum where it is cut into tortilla shapes. These fall onto a moving belt or continuous cooking griddle, dropping into a receptacle at the end of the belt. This small industry may use whole maize, in which case the dough is cooked in large receptacles, or it may start with industrial tortilla flour.
The second type of industry is the large industrial conversion of maize into an instant precooked tortilla flour. The process has been described by various workers (e.g. Deschamps, 1985). It is based for all practical purposes on the traditional method used in rural areas. More recently, the process of producing the flour has been expanded to produce tortillas.
Maize is bought after the buyer has inspected its quality and sampled it. Batches of maize with a high percentage of defective grains are rejected. Those that are accepted are paid for according to the defects found in the raw material. Maize is also selected according to its moisture content, since very high moisture will result in storage problems. During the cleaning stage, all impurities such as dirt, cobs and leaves are removed. The cleaned maize is sent to silos and warehouses for storage.
From there it is conveyed to treatment units for lime-cooking. There it is converted into nixtamal, using either a batch or a continuous process. After cooking and steeping, the lime-treated maize is washed with pressurized water or by spraying. It is ground into a dough (masa) which is then transferred to a dryer and made into a rough flour. This flour, consisting of particles of all sizes, is forced through a sifter where the coarse particles are separated from the fine ones. The coarse particles are returned to the mill for regrinding and the fine ones, which constitute the final product, are sent to the packing units. Here the flour is packed into lined paper bags.
One complete unit must have equipment for lime treatment, milling, drying and sifting and a daily production capacity of 30 to 80 tonnes of flour. These figures are the minimum and the maximum; to increase its production capacity, a commercial enterprise must install several parallel units. The use of such units seems to be more a tradition than a technical necessity, since it would be perfectly feasible to design plants with a capacity lower than 30 tonnes or higher than 80 tonnes per day. Plants that are very large or very small are apparently not considered viable.
The industrial yield of alkali-cooked maize flour fluctuates between 86 and 95 percent depending on the type of maize, the quality of the whole kernels and the lime-treatment conditions. Industrial yields have been reported to be higher than those at the rural and semi-industrial levels, possibly because of the quality of the grain processed.
Tortilla flour is a fine, dry, white or yellowish powder with the characteristic odour of maize dough. This flour when mixed with water gives a suitable dough for the preparation of tortillas, tamales, atoles (thick gruels) and other foods. All maize flours made in Mexico must conform to the specifications of the government's Department of Standards and Regulations.
When the flour has a moisture content of 10 to 12 percent it is stable against microbial contamination. If the moisture content is over 12 percent it is easily attacked by moulds and yeast. The problem of bacterial attack is almost nonexistent since the minimum of moisture required for bacterial growth is so high that flour with this moisture content would already be transformed into masa. Another matter related to the stability of flour is rancidity, which is normally not a problem unless the flour is packed at high temperatures. The minimum time required for the flour to spoil in Mexico is four to six months during the winter and three months during the summer. Nevertheless, it is usually sold to the consumer within 15 days of being sold to retailers and wholesalers, while its shelf-life is one month (Delvalle, 1972).
Tortillas made from lime-treated maize flour can be made at home or in factories. Such flour has been a great advantage for households and for factories both large and small, although its use in rural areas is not widespread.
In Guatemala, about 3 000 metric tons of maize are produced yearly for tortilla flour production. This amount is significantly lower than that in Mexico; the population is smaller and there are few small tortilla factories. About 90 percent of the production is sold in urban areas and 75 percent goes into tortilla making. Other countries where lime-treated maize flour is produced are Costa Rica and the United States. In Costa Rica tortilla consumption per person is about 25.6 kg per annum. Approximately 62 percent of the production is commercial, 30.6 percent is home-made from commercial flour and 7.4 percent is home-made from grain.
Modifications of lime-cooking
The traditional method of cooking maize with lime to make tortillas at the rural level is both time-consuming (about 14 to 15 hours) and hard work. The cooking and soaking operations take up 70 to 80 percent of the time, which in a sense may be acceptable to the rural housewife. Nevertheless, the availability of an instant tortilla flour offers many advantages such as convenience, less labour and lower use of energy, for a safe, stable and nutritious product. At the industrial or commercial level, grinding and dehydration are large factors in the cost. Lime-cooked maize contains about 56 percent moisture, which must be decreased to 10 to 12 percent in the flour. Therefore, any method that would decrease both time and cost and still yield acceptable tortillas would be advantageous.
Efforts in this respect have been made by a number of workers. Bressani, Castillo and Guzmán (1962) evaluated a process based on pressure cooking at 5 and 15 lb pressure per square inch (0.35 and 1.05 kg per cm²) under dry and moist conditions for 15, 30 and 60 minutes, without the use of lime. None of the treatments had any effect on chemical composition and true protein digestibility, but all reduced the solubility of the nitrogen. Pressure cooking at 15 lb per square inch (1.05 kg per cm²) under dry conditions reduced the nutritional quality of the product, particularly when carried out for 60 minutes. The pressure cooking method without lime did not reduce crude fibre content, which is one of the particular effects of lime, and the calcium content was significantly lower than in dry dough (masa) prepared by the traditional method.
Khan et al. (1982) conducted a comparative study of three lime-cooking methods: the traditional way, a commercial method and a laboratory pressurecooking procedure. For each process maize was undercooked, optimally cooked and overcooked to measure some of the physical and chemical changes that might occur. Although the traditional method caused the greatest loss of dry matter from the grain, it gave the best tortillas in terms of texture, colour and acceptability. The pressure-cooking procedure yielded a sticky dough and undesirable tortillas. The commercial method was the least desirable. This study allowed the authors to propose a method to evaluate the completeness of cooking.
Bedolla et al. (1983) tested various methods of cooking maize and sorghum as well as mixtures of the two grains. The methods tested included the traditional one, steam cooking as tested by Khan et al. (1982) and a method using a reflux (condensing) system. They found that the methods of cooking affected the total dry matter lost during processing into tortillas.
Variation of cooking conditions can result in lower processing times. For example, Norad et al. (1986) found that a 40 percent reduction in cooking time could be achieved by pre-soaking the grain before alkali cooking. In these studies dry matter losses, water uptake, calcium content and enzyme susceptible starch increased, whereas amylograph maximum viscosity decreased in both presoaked and raw maize upon cooking. The decrease in viscosity and increase in the other parameters was faster in the pre-soaked maize.
Dry-heat processes have also been studied. Johnson, Rooney and Khan (1980) tested the micronizing process to produce sorghum and maize flours. Micronizing is a dry-heat process using gas-fired infrared generators. Rapid internal heating takes place, cooking the product from the inside out. The authors used this process to produce tortilla flour, claiming that it would be quicker and more economical than the traditional method.
Molina, Letona and Bressani (1977) tested production of instant tortilla flour by drum drying at the pilot plant level. Maize flour was mixed with water at a ratio of 3: I with 0.3 percent lime added on the basis of maize weight. After mixing, the dough was passed through a double-drum dryer heated with steam at 15, 20 or 25 lb per square inch (1.05, 1.40 or 1.75 kg per cm2), 93, 99 and 104°C surface temperature and 2, 3 or 4 rpm. The process produced an instant tortilla flour with physico-chemical and organoleptic characteristics identical to those of the reference sample prepared by the traditional method but different from those of a commercial product.
Extrusion cooking has also been evaluated as an additional technology for producing tortilla flour. Bazua, Guerra and Sterner (1979), using a Wenger X-5 extruder, processed ground maize mixed with various lime concentrations (0.1 to 1.0 percent). The dough and tortillas made by extrusion were compared with those made by the traditional process for their organoleptic properties as well as lysine tryptophan and protein content. No appreciable differences were noted at comparable use levels of calcium hydroxide. Both the traditional process and the extrusion modification induced losses of tryptophan related to the amount of lime added. With a 0.2 percent addition 8 percent of the tryptophan was lost, while with 1 percent lime more than 25 percent was lost. Some lysine losses were also observed. The organoleptic results suggested that it is possible to make culturally acceptable tortillas using extrusion as an alternative to the lime-heat treatment.
Maize for tortillas
Grain quality is a concept now growing in importance in breeding programmes aimed at increasing acceptance of genetically improved seeds by farmers as well as by consumers and food processors. The grain quality characteristics include yield, technological properties and, when possible, nutritional elements as well. Technological properties include stability during storage, efficiency of conversion into products under given processing conditions and acceptability to the consumer. The technological aspect of maize quality for tortilla preparation is of little importance to small farmers in the least developed countries, who seldom use seed other than that kept from harvest to harvest. Furthermore, the rural housewife knows how to adjust cooking conditions to the type of maize she will process for consumption. But maize is now being converted into a tortilla flour using industrial processes, where the grain being used may be of different varieties from various producers and different environments. It may have a variety of structures or may have been poorly handled after harvest, factors which influence the yield and physico-chemical and organoleptic as well as culinary properties of the product. This would appear to be of growing importance in countries such as the United States where the maize tortilla is becoming a very popular food.
That physical characteristics of maize are important became clear some time ago, when Bressani, Paz y Paz and Scrimshaw (1958) showed that the yield of dry matter in the form of dried-maize dough or flour was affected by the maize cultivar. In their rural home studies dry matter losses from white maize averaged 17.2 percent with a variability of 9.5 to 21.3 percent. Dry matter losses from yellow maize averaged 14.1 percent, with a range from 8.9 to 16.7 percent.
Cortez and Wild-Altamirano (1972) conducted a series of measurements on 18 cultivars of maize produced in Mexico. These included kernel weight, colour and lime-cooking time using a standard cooking procedure with 1.5 percent lime at 80°C and a steeping time of 12 hours. Cooking efficiency and time were measured by the ease with which the seed coat could be removed. Evaluations of the cooked maize included measurement of the volume of I kg of maize, the yield of dough from I kg of grain and the moisture content of the dough. The dough was further evaluated by measuring its strength and water absorption. The dehydrated dough was then ground to 60-mesh size and evaluated for moisture, colour, specific volume and other physical characteristics using a mixograph. The tortillas made from the dough of each maize sample were further evaluated for extensibility, volume, plasticity, softness and roughness of the surface.
From this extensive study, the authors reached several conclusions. Maize varieties or cultivars of higher weight per volume, a harder endosperm, more moisture and a high protein content produced the best tortillas. Two cultivars of popcorn maize were among the best types for tortillas. The Swanson mixograph was useful in establishing differences in maize types. The time required to cook the samples ranged from 30 to 75 minutes, and dry matter losses ranged from 10 to 34 percent. Rooney and Serna-Saldivar (1987) found that maize with hard or corneous endosperm required a longer cooking time. Bedolla and Rooney (1984) stated that the texture of the dough was affected by the endosperm texture and type, drying, storage and soundness of the maize kernel. MartínezHerrera and Lachance (1979) established a relationship between kernel hardness and the time needed for cooking. They reported that within a maize variety, higher calcium hydroxide concentration slightly decreased cooking time. Furthermore, knowing the initial hardness of a variety made it possible to predict the time required to cook it. Khan et al. (1982) and Bedolla and Rooney (1982) measured a parameter termed nixtamal shear force (NSF), an indication of kernel hardness. The measurement was related to both cooking time and processing method. These authors showed that the NSF measurement could reveal small differences in maize with similar endosperm texture and could be used to predict optimum cooking time.
Dry matter losses resulting from lime-cooking constitute a good index of maize quality for tortilla preparation. Jackson et al. (1988) reported that greater losses resulted from stress-cracked and broken kernels than from sound kernels. Therefore they concluded that any system for assessing maize for alkaline cooking should include measures of broken kernels, the potential for breakage and ease of pericarp removal. Specific studies on the effects of drying and storage on quality of maize for tortilla making are not readily available. Bressani et al. (1982) reported on QPM storage as related to tortilla quality. The Nutricta QPM variety was stored under a number of field or rural conditions. Containers made of cloth not treated with insecticides allowed insect infestation and therefore higher dry-matter losses during cooking; but the protein quality was not affected.
Possibly the most interesting feature of the process of converting maize into tortillas is the use of an alkaline medium, and particularly calcium hydroxide. The most obvious effect of adding lime is the facilitation of seed coat removal during cooking and steeping. According to Trejo-González, Feria-Morales and Wild-Altamirano (1982), added lime maintains an alkaline pH, which is needed to hydrolyse the hemicelluloses of the pericarp. Lime uptake by the kernel follows that of water, but the rate is lower than that of water. Norad et al. (1986) showed that soaking the kernels before cooking led to a higher calcium content in the grain. Calcium content of masa was affected by lime levels and also by cooking-steeping temperatures. Several other authors (e.g. Pflugfelder, Rooney and Waniska, 1988a) have shown in one way or another that lime uptake during alkaline cooking is affected by physical and chemical characteristics of maize dough.
Martínez-Herrera and Lachance (1979) found that higher calcium hydroxide concentrations slightly decreased cooking time, but the differences were not statistically significant. These authors also reported an interaction between maize variety and calcium hydroxide concentration. However, the coefficient of variation was high (29.1 percent); this was attributed to inherent variability in the kernels of the different varieties.
Bedolla and Rooney (1982) reported that increases in cooking time, cooking temperature, lime concentration and steeping time produced lower viscoamylograph peak viscosities at both 95 and 50°C, which was interpreted to mean a greater degree of starch gelatinization. Trejo-Gonzalez, Feria-Morales and Wild-Altamirano (1982) showed that calcium was fixed or was bound in some way to the starch of the maize kernel. Other effects included greater solid losses with increasing amounts of lime; changes in colour, aroma and flavour; and a delay in the development of acidity, which extends shelf-life. If added in exceedingly large amounts, lime affects organoleptic properties of the food; this effect is often observed when maize has been stored for a long time.
Acid porridges prepared from cereals are eaten in many parts of the world, particularly in developing countries, where they may form part of the basic diet. Some examples of acid porridges include pozol in Mexico and Guatemala, ogi in Nigeria, uji in Kenya and kenkey in Ghana. These porridges are usually made from fermented raw or heat-treated maize, although sorghum and millet are often used.
Ogi manufacture
The traditional process of making ogi has a number of slight variations described by several authors. Ogi is traditionally prepared in batches on a small scale two or three times a week, depending on demand. The clean grain is steeped in water for one to three days to soften. Once soft, it is ground with a grinding stone, pounded in a mortar or ground with a power mill. The bran is sieved and washed away from the endosperm with plenty of water. Part of the germ is also separated in this operation. The filtrate is allowed to ferment for 24 to 72 hours to produce a slurry which when boiled gives the ogi porridge. Ogi is usually marketed as a wet cake wrapped in leaves, or it may be diluted to 8 to 10 percent solids in water and boiled into a pap or cooked to a stiff gel.
Akinrele (1970) reported that the souring of the maize took place spontaneously without the addition of inoculants or enzymes. He identified the organisms involved in this unaided fermentation and investigated their effects on the nutritive value of the food. He identified the moulds as Epholosporium, Fusarium, Aspergillus and Penicillium species and the aerobic bacteria as Corynebacterium and Aerobacter species, while the main lactic acid bacterium he found was Lactobacillus plantarum. There were also yeasts: Candida mycoderma, Saccharomyces cerevisiae and Rhodotorula sp.
Although ogi is supposed to have an improved B-vitamin content, the results observed are quite variable, at least for thiamine, riboflavin and niacin. Banigo and Muller (1972) identified the carboxylic acids of ogi fermentation. They found 11 acids, with lactic, acetic and butyric acids being the most important.
The ogi-making process is quite complex, and the porridge can also be prepared from sorghum, rice, millet and maize. Therefore, laboratory procedures have been developed to learn more about the process and introduce changes to convert the grains to food more efficiently. These have been described by Akingbala, Rooney and Faubion (1981) and Akingbala et al. (1987), whose studies have been useful also in evaluating varieties of cereal grains for their efficiency in making ogi. The authors also reported on the yields of ogi from whole maize kernels (79.1 percent) and dry milled flour (79.8 percent).
The commercial manufacture of ogi does not differ substantially from the traditional method. Modifications have been introduced, such as the dry milling of maize into a fine meal or flour and subsequent inoculation of the flour-water mixture with a culture of lactobacilli and yeast. In view of the importance of ogi in the Nigerian diet, large-scale production is indicated. The material could be dried and packaged in polythene bags for a good shelf life. There is some problem in achieving a controlled fermentation with pure cultures. Some modifications include spray-drying the slurry or drum drying.
Other fermented maize products
Ogi has a number of other names such as akamu or ekogbona, agidi and eko tutu. These, with the Kenyan uji and Ghanaian koko, are substantially the same preparation with changes in the grain used or some modification of the basic process. For the Mexican pozol, maize is processed with lime as for tortillas. The nixtamal, or cooked maize without the seed-coat, is ground to a coarse dough which is shaped into balls by hand. The balls are then wrapped in banana leaves to avoid drying and are allowed to ferment for two to three days, or more if necessary. The micro-organisms involved are many.
Another major food made from maize, used daily in Colombia and Venezuela, is arepa. Mosqueda Suarez (1954) and Cuevas, Figueroa and Racca (1985) described the traditional preparation method as practiced in Venezuela. De Buckle et al. (1972) defined the Colombian arepa as roasted maize bread without yeast, round in shape, prepared from maize that has been degermed. Whole maize is dehulled and degermed using a wooden bowl called a pilon and a double-headed wooden mallet. The moistened maize is pounded until the hulls and part of the germ are released from the endosperm. The hulls and germs are removed by adding water to the mixture containing the endosperm. The endosperm is cooked and then stone-milled to prepare a dough. Small portions of this dough are made into balls, then pressed into flat discs which are cooked rapidly on both sides.
The traditional method of preparing arepas has been substantially modified by the introduction of precooked maize flour, which reduces the time from 7 to 12 hours to 30 minutes (Cuevas, Figueroa and Racca, 1985). There are two stages in the industrial process. The first is the preparation of maize grits by cleaning, dehulling and degerming the maize; the second is the processing of the grits to produce precooked flour. Efforts have been made to modify the process even further by extrusion cooking.
In Latin America there are many maize-based foods besides tortillas and arepas. Some of these are drinks like colados, pinol and macho, basically suspensions of cooked maize flour. These three products have a very low protein quality. The production of humitas, a tamale-like food consumed in Bolivia and Chile, was described by Camacho, Bañados and Fernandez (1989). Made from immature common or opaque-2 maize to which is added a number of other ingredients, humitas is produced from precooked maize flour which resembles the lime-treated masa. Other products include mote, made from cooked maize and cheese, pupusas, made from lime-treated maize and cheese, and patasca, which is like a lime-treated maize kernel. From immature maize a sweet, tasty atole of high nutritive value is made; Khan and Bressani (1987) described the process, which consists of grinding the maize in water followed by filtration and cooking. Immature maize, either common or opaque2, and sweet maize are also extensively consumed. Chavez and Obregon (1986) reported on the incorporation of the opaque-2 gene into sweet maize to provide a food of high nutritional quality.
Maize has also been used as a substrate for fermented beverages called chicha. Cox et al. (1987) have reported on the microflora of these fermented products, which are made by basically the same process but using a variety of additives.
The maize kernel is transformed into valuable foods and industrial products by two processes, dry milling and wet milling. The first yields grits, meal and flours as primary products. The second yields starch and valuable derived products.
Dry milling
The dry milling of maize as practiced today has its origins in the technologies used by the native populations who domesticated the plant. The best example is the method used to make arepa flour or hominy grits. The old technology was soon replaced by a grinding stone or stone mill, followed by the grits mill and finally by sophisticated tempering-degerming methods. The products derived are numerous, with their variety depending to a large extent on particle size. They are classified into flaking grits, coarse grits, regular grits, corn meal, cones and corn flour by means of meshes ranging from 3.5 to 60. Their chemical composition has been well established and their uses are extensive, including brewing, manufacturing of snack foods and breakfast cereals and many others.
Wet milling
The largest volume of maize in developed countries such as the United States is processed by wet milling to yield starch and other valuable byproducts such as maize gluten meal and feed. The starch is used as a raw material for a wide range of food and non-food products. In this process clean maize is soaked in water under carefully controlled conditions to soften the kernels. This is followed by milling and separation of the components by screening, centrifugation and washing to produce starch from the endosperm, oil from the germ and food products from the residues. The starch has industrial applications as such and is also used to produce alcohol and food sweeteners by either acid or enzymatic hydrolysis. The latter is done with bacterial and fungal alpha-amylase, glucoamylase, beta-amylase and pullulanase. Saccharides of various molecular weights are liberated yielding sweeteners of different functional properties. These include liquid or crystalline dextrose, high-fructose maize syrups, regular maize syrups and maltodextrins, which have many applications in foods.