African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 8, Num. 4, 2000, pp. 419-428

African Crop Science Journal, Vol. 8. No. 4, pp. 419-428

Physical, Chemical, and Water Absorption Characteristics of Tropical Maize Hybrids

B.B. Maziya-Dixon, J.G. Kling and A.E. Okoruwa
International Institute of Tropical Agriculture, c/o L.W. Lambourn & Co., Carolyn House, 26 Dingwall Road, Croydon CR9 3EE, England

(Received 4 February, 2000; accepted 13 June, 2000)

Code Number: CS00044

INTRODUCTION

The International Institute of Tropical Agriculture (IITA) initiated a maize hybrid breeding programme in 1979 with support from the Nigerian government (IITA, 1992). Release of the first hybrids in the early 1980s enabled the establishment of several private seed companies in Nigeria. A strong private seed sector can ensure a reliable supply of high-quality seed to farmers and serve as a catalyst for agricultural development (Smith et al., 1997). Hybrids developed at IITA were selected for their high grain yield, disease resistance and standability, with relatively little emphasis on post-harvest quality. Despite the superior agronomic performance of these hybrids, farmers still prefer their traditional cultivars in some areas, because they possess desired storage and processing characteristics for local maize dishes (Kling, 1993).

An understanding of the factors contributing to maize quality for various end-uses is of importance in plant-breeding programmes aimed at increasing acceptance of genetically improved food crops by farmers, consumers and food processors (Stroshine et al., 1986; Peplinski et al., 1989). Hybrids that produce shelled maize which dries rapidly, high in storage mould resistance and dry milling quality would offer advantages to producers, merchants, and end-users. Maize dry millers seek to optimise yield of large grits and would benefit from the development or identification of hybrids, which yield a greater percentage of flaking grits. Intrinsic quality characteristics such as starch, oil, and protein content can be directly related to end-use value (Hurburgh, 1989), and the potential for improving these characteristics through genetic manipulation is quite high (OTA, 1989). Other maize characteristics that have been found to affect yield and quality of maize products include test weight, kernel density and size, kernel hardness, water absorptivity, and average kernel weight (Paulsen and Hill, 1985; Pomeranz et al., 1986b).

The purpose of this study was to determine the physical, chemical, and water absorption characte-ristics of tropical maize hybrids developed at IITA as indicators of their suitability for primary processing in West and Central Africa.

MATERIALS AND METHODS

Maize hybrids. Eleven maize hybrids that were included in IITA International Trials in 1989 were selected for this study to represent the range of available hybrids adapted to the lowland tropics of West and Central Africa. Hybrids were grown at the IITA research farm, Ibadan, Nigeria (7°26’N, 3°54’E) under uniform conditions in 1990 during the first rainy season (March to August) to standar-dise possible environmental effects on grain quality. Within each hybrid plot, controlled pollinations were made to avoid pollen contami-nation from other hybrids. At maturity, the maize was hand harvested and ears with poor seed sets were discarded. Ears were artificially dried at 40°C in a dryer. The final moisture content ranged from 11 to 12%. The ears were hand shelled to minimise physical damage and were stored at 10°C and brought up to 25°C before analyses. Kernel colour and texture of the hybrids studied are presented in Table 1. The hybrids are of intermediate maturity (approximately 110 days to maturity in Ibadan).

Physical analyses. 1000-kernel weight was determined by counting and weighing 100 maize kernels and expressing the result as g/1000 kernels (Hilliard and Daynard, 1974). Kernel size (dimensions) was measured by selecting 10 kernels at random and measuring the major axes (length, width, and depth) with a vernier calliper (Martinez-Herrera and LaChance, 1979). Percent kernel components parts expressed on dry matter basis (endosperm, germ, and pericarp) were determined according to the method of Earle et al. (1946).

For the floaters test, the method of kernel density separation as described by Wischer (1961) was followed with slight modification. A sodium nitrate solution with specific gravity of 1.250 at ambient temperature was used to measure the percentage of floating kernels (50 kernels in 500 ml of solu-tion). The specific gravity of the solution was checked with a hydrometer before, during, and after measurement.

Test weight (bulk density) was determined by weighing the amount of grain held in a full to level calibrated 50 ml beaker based on a modification of the method used by Kikuchi et al. (1982). An average of 10 measurements represented one determination. Test weight was expressed as weight per unit volume and values were converted to kg m^-3.

The Stenvert Hardness Test (SHT) as described by Pomeranz et al. (1985) was performed on 20 g maize samples. Samples were ground in a micro hammer-cutter mill Type V (Glen Mills, Maywood, NJ), using a 2.0 mm aperture screen and a hammer speed of 3,600 rpm. The time to grind 17 ml of meal, measured to the nearest 0.1 sec, was used as a measure of grain hardness. Kernel density was measured by placing 25 pre-weighed kernels in a graduated cylinder containing 20 ml of 50% ethanol (Arnold et al., 1977).

Water absorption capacity was determined by soaking 25 g in 100 ml of distilled water at room temperature. After 6 hrs, the surfaces of the kernels were blotted dry, and the weight increase due to water absorbed by the kernels was measured. The increase in weight expressed as percentage of initial weight was calculated as the absorption index (Andah, 1977). All physical measurements were done in triplicates. Results were averaged and reported on a dry weight basis.

Chemical analyses. Fat content was determined by the Soxhlet method (AOAC, 1984). Ash and moisture content of ground samples and whole grains, respectively, were determined by AACC (1983) methods. Crude protein (N x 6.25) was calculated based on Kjeldahl Nitrogen (N) determined by a standard procedure using the Tecator Kjeltec System 1 (Tecator, 1979). Crude fibre was determined by the trichloroacetic acid (TCA) method of Entwistle and Hunter (1949).

The method of McCready et al. (1950) was used for the extraction of soluble sugars. Sugar content was determined by the method of Dubois et al. (1956). The residue from sugar analysis was used for determination of starch content. For amylose determination, starch was extracted by the method of Watson (1964). Amylose content of starch was determined with an Autoanalyzer (Model AA II, Technicon Inst.) using the method of Juliano (1971). Amylopectin content was obtained by subtracting the amylose content from the starch content of the sample. The water binding capacity of starch was determined according to the method of Medcalf and Gilles (1965). All chemical analyses were done in duplicate, averaged, and reported on a dry weight basis.

Statistical procedures. Data were analysed using the Statistical Analysis System (SAS) (SAS, 1985) using one-way analysis of variance with the Fisher’s Protected Least Significant Difference (LSD) test for statistical significant differences among means (Ott, 1988). The pooled variance among samples within hybrids was used as the error term. Means were grouped by the LSD test at the 5% level.

RESULTS AND DISCUSSION

Physical properties. Test weight, 1000-kernel weight, kernel density, percent floaters, hardness index, and water absorption capacity of maize samples is presented in Table 2. Among the hybrids examined, 8644-31 and 8425-8 had highest test weights while 8644-27 and 8338-1 had the lowest. All other hybrids were intermediate. Maize with low test weight often has a lower percentage of hard endosperm, and, therefore, produces a lower percentage yield of prime, large grits when milled. Paulsen and Hill (1985) showed that yield of flaking grits was significantly increased by selecting maize with high test weight and low breakage susceptibility. Therefore, hybrids 8644-31 and 8425-8 would be more desirable for industrial dry milling based on our test weight results.

Thousand kernel weight varied significantly among hybrids. Hybrid 8705-6 had the highest kernel weight while 8425-8 had the lowest kernel weight (Table 2). Both density of kernels and packing in the container influence test weight measurements. According to Thompson and Isaacs (1967), maize has an average void volume (space between kernels) of 42%. Therefore, measure-ments that eliminate void spaces such as thousand kernel weight give a more accurate volume measurement for density calculations. This might explain why Hybrid 8425-8 was highest for test weight and lowest for thousand kernel weight.

Kernel density varied greatly among the hybrids and ranged from 1.18 to 1.30 g ml^-3 indicating a fairly large spread in endosperm types, from soft to hard. Although hybrid 8425-8 had higher test weight, it had the lowest kernel density, 1000 kernel weight and high percent floaters (Table 2). Kirleis and Stroshine (1990) found that maize density was the best single predictor of dry milling quality and that a prediction model combining test weight and kernel density improved the prediction of milling quality of three dent hybrids. According to Watson (1987), differences in chemical composition among individual kernels of dent maize are probably too small to account for the wide differences in density observed. However, kernels do differ in the amount of void space within them and in the ratios of corneous to floury endosperm. Corneous endosperm is very dense, whereas floury endosperm is full of microfissures or void spaces.

Significant differences were observed for percentage floaters (Table 2). Among the hybrids, 8321-21 had the highest percentage floaters while 8321-18 had no floaters. In general, the samples were classified as either dent, semi-flint, or flint. This could explain the low range of percentage floaters (0-16%) observed in this study. The floaters test indicates the amount of corneous (hard) endosperm in the kernel. Hybrids with low percentage floaters would be expected to be harder than those with high percentage floaters, and, therefore, will be suitable for dry milling and for production of rice-like products.

Based on the Stenvert Hardness Index, four Hybrids (8321-18, 8522-2, 8425-8, 8705-6) were significantly softer than others. Hybrids 8705-4 and 8644-31 exhibited a significantly greater hardness among the Hybrids tested (Table 2). Hardness is an intrinsic characteristics that can be altered by genetics and environments. It is also closely related to the degree of adhesion between starch and protein. Hardness and softness are milling characteristics relating to the way the endosperm breaks during milling. The ratio of dense corneous endosperm to floury endosperm causes variation in kernel hardness. Maize with a higher proportion of corneous endosperm is typically rated harder by mechanical measures of hardness. In a study conducted by Pomeranz and Czuchajowska (1987), high yields were obtained from maize with the highest test weight and hardness index, as determined by percent of coarse fraction from the hardness tester. Pomeranz et al. (1986a) evaluated ten different commercial maize hybrids for relative hardness using several different methods of hardness evaluation. Stenvert Hardness values ranged from 20.4 sec (hard) to 12.0 sec (soft). In the present study, values ranged from 38.2 sec to 47.9 sec. Selection for resistance to ear rot and ear borers at IITA may have favoured selection of flintier grain type in comparison to hybrids developed in temperate areas where disease and insect pressures are less severe.

For the water absorption index of grain, 8321-18 had significantly higher value than 8644-31and 8705-4 (Table 2). The water absorption index for grain may be a measure of steeping performance. Steeping is the first critical step to ensure a clean separation of germ, endosperm, and pericarp during milling. Hsu et al. (1983) found a negative correlation between absorption rate and kernel size. The structure of the starchy endosperm was also found to be very significant in affecting the rate of moisture penetration, with the subaleurone region appearing to be rate limiting. Stenvert and Kingswood (1977) observed that the more ordered the endosperm structure, the slower the rate of moisture movement. Protein content and distribution were also of significant importance because they generally contributed to a more ordered endosperm structure. The rate of water penetration appears to relate to either grain hardness or protein content.

There were significant differences in kernel size (dimensions) and kernel components (Table 3). Kernel length ranged from 10.1 to 12.8 mm, width 8.13 to 9.03 mm and thickness 3.47 to 4.10 mm. These values fall within a range typical of maize cultivars as reported by Kirleis and Stroshine (1990). Among the Hybrids, 8338-1 had the highest proportion of endosperm and 8705-4 had the lowest. Hybrid 8505-5, which had intermediate percent endosperm and pericarp, had a signifi-cantly higher percent germ than any other hybrid. Hybrid 8338-1 had the lowest percent germ and intermediate percent pericarp. Percent pericarp was highest for 8705-4 and lowest for 8522-2.

Chemical properties. Hybrids exhibited signi-ficant variation for protein, fat, ash, crude fibre, and total sugars (Table 4). Hybrid 8644-31 had higher percent protein, ash and crude fibre but was low in percent total sugars. Protein content ranged from 9.45 to 11.40, which is within the common range for maize. Hybrid 8705-6 had the highest fat content while 8338-1 had the lowest. Normal maize grain contains between 3.5 - 5% fat. In the present study, most of the hybrids could be considered high for fat content, except 8522-2 and 8338-1. Although the lipid content of maize is controlled to a large extent by genetic factors (Watson, 1987), we cannot determine whether the high fat content observed in this study was the result of genetic or environmental effects, because the maize was grown in a single environment.

When oil is not a desired end product, low fat content in maize grain is preferred for most industrial and food-uses. High fat content in flour or grits causes rancidity during storage and interferes with efficiency and performance during secondary processing such as in the brewing industry. Even with traditional processing, high fat content limits the shelf life of whole maize meal, flour, and other products. Nonetheless, maize oil may be desirable in some situations as a source of the essential fatty acid (linoleic acid) in the diet. Fats contribute approximately 2.25 times more metabolisable energy (ME) than starch or protein on an equal-weight basis. The fat in maize contributes about 10-12% of the total ME provided by the maize (Wright, 1987). Therefore, maize with a higher amount of fat would be beneficial to the animal feed industry.

Differences in ash and crude fibre content among the eleven hybrids are presented in Table 4. Hybrids 8644-31 and 8505-5 had the highest values for both ash and crude fibre content, while 8522-2 had the lowest values. Peplinski et al. (1989) reported values of ash between 1.3 and 1.5% and a fibre content of 2% for six maize hybrids. Our results show a range of 1.4-3.3% for ash and 1.40-3.75% for crude fibre content. Total sugars ranged from 3.04 to 5.69%. Martin et al. (1976) reported a value of 7.4% total sugars for dent maize. Hybrid 8321-18 had the highest content of total sugars while hybrid 8338-1 had the lowest.

There were significant differences among hybrids for percent starch, amylose, amylopectin content and water binding capacity of starch (Table 5). Hybrid 8338-1 was high in starch content while 8644-27 was low in starch. Starch is the most important carbohydrate consumed on a world-wide basis because it is continuously available in abundant quantities at low cost. Amylose, which makes up 25-30% of maize starch, is essentially a linear molecule of glucose units while amylopectin, constituting 70-75% of normal maize starch, is a branched molecule. Although amylose content was significantly higher in Hybrid 8321-18, it was still within the normal range (24-28%) reported for maize. Hybrid 8321-21 had the lowest amylose content. Hybrid 8321-21 had higher amylopectin content but was low for amylose content.

The relative proportions of amylose and amylopectin greatly influence the physico-chemical properties of starch, and, therefore, its technological and nutritional properties. Although it has been indicated that higher amylose content is associated with lower starch digestibility (Martinez and Lausanne, 1996), it is not likely that the observed differences in amylose content would have any effect on starch digestibility. The amylose content of flour is a key parameter in the food industry since it can strongly influence the physical and chemical properties of flour, such as viscosity, retrogradation, solubility or water absorption (Martinez and Lausanne, 1996), as well as the bioavailability of starch and its interaction with other food components. Sterling (1978) noted that gelatinisation temperature seems to depend on the relative proportions of amylose and amylopectin, such that higher amylose contents are associated with higher gelatinisation temperatures.

The water binding capacity of starch ranged from 145 to 177% (Table 5). Water binding capacity of Hybrid 8644-27 was relatively high and differed significantly from other hybrids. Inherent differences in the proportion of crystalline and amorphous areas in starch granules may contribute to differences in water binding capacity, with presumably greater water binding capacity in starch containing large proportions of amorphous area. Medcalf and Gilles (1965) reported that, in general, the higher amylose starches have higher water binding capacity.

CONCLUSIONS

A wide range of differences in physical, chemical, and water absorption capacity exist among the eleven maize hybrids investigated. None of the hybrids excelled in all quality criteria. For example, Hybrid 8644-31 was relatively high for test weight, thousand kernel weight, hardness index, percent protein, ash, and crude fibre content. However, it also exhibited low starch content, total sugars, and water absorption index of grain. Hybrids with low percentage floaters took longer to grind as indicated by the hardness index, which may reflect a greater proportion of corneous endosperm. Hybrid 8321-18 was an exception, having zero percent floaters, but relatively low hardness index.

The presence of genetic variation among the hybrids suggests that a potential exists for improvement of grain quality to suit specific processing and food use requirements through selection and breeding. Breeding programmes in developing countries should target hybrid development to meet the requirements of producers, processors, and consumers. Hybrids with suitable quality characteristics for preferred food preparations and industrial uses could provide markets for the private seed sector, thereby promoting both availability of high quality seed for farmers and increased demand for maize products.

ACKNOWLEDGEMENTS

Financial support for this work was provided solely by Guinness Breweries, Nigeria, Ltd. and is highly appreciated. We are grateful for the assistance of N.S. Ilo, A.F. Adedapo, and T. Olatunji in carrying out the grain quality analyses.

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