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Weaning foods with improved energy and nutrient density prepared from germinated cereals. 2. Nutritional evaluation of gruels based on barley

Birthe Pedersen, Marianne Hansen, Lars Munck, and Bjorn O. Eggum

Introduction

Although barley is used extensively as a food in some areas of the world, few data are available on its nutritional qualities and dietary bulk in weaning foods.

As the dietary bulk of traditional weaning foods is a major constraint in providing enough food to small children [1], in the previous paper [2] we examined the dietary-bulk properties of barley-based gruels. Our findings indicated that gruel made from ordinary barley flours, especially when highly refined, is inadequate as a source of energy for small children, but that, if the barley is germinated or if a small amount (1%) of barley malt is added to ungerminated flours, gruels with acceptable energy densities can be produced.

The capacity of a weaning diet to meet the protein and energy requirements of infants depends, of course, on its nutritional qualities as well as on its dietary bulk. This paper describes the nutritional quality of barley gruels. The effects of germination, refining, and the addition of malt were also examined by chemical analysis and by rat bioassay.

The nutritional quality of cereals is likely to vary considerably between different varieties, such as may be grown in developing countries. Ethiopia, for instance, is known for the diversity of its native barley types, and the first highlysine variety identified was an Ethiopian line [3]. Both a high-lysine barley and a standard variety were used in this study.

Materials and methods

As reported in the previous paper [2], barley of the high-lysine variety Ca 700202 was used. It was milled into three different flours: wholemeal, semi-refined, and refined. Flours were also produced from grain germinated for three, five, or seven days. For comparison, a refined flour was produced from a normal barley variety, Triumph. The procedures used to make the different flours are outlined in the previous paper. Gruels were prepared from the ungerminated flours, from the germinated flours, and from the ungerminated flours mixed with 1% (dry basis) of the flour made from barley germinated for seven days. The dry matter contents of the gruels were those found to result in acceptable viscosity, as described.

After being cooked in a pot, the gruels were cooled to 40 °C, frozen immediately in liquid nitrogen, and Iyophilized.

Analytical methods

The two barley varieties and the flours were analysed for moisture [4], ash [5], protein (N x 6.25) [6], and fat [7]. Starch content was determined as described by Bach Knudsen et al. [8]. Insoluble and soluble dietary fibre were determined by the procedure of Asp et al. [9]. Sugar was measured as described by Jacobsen [10]. Energy was determined by bomb calorimetry (IKA-O 400, Janke and Kunkel).

Amino acid analyses were conducted according to the procedure of Mason et al. [11], and tryptophan was determined by high-performance liquid chromatography and fluorescence spectrophotometry after hydrolysis with sodium hydroxide (S. Bech-Andersen, personal communication, 1988). Zinc, copper, iron, and calcium were determined by atomic absorption spectrophotometry [ 12] and phosphorus by the colorimetric method of Stuffins [13]. The content of physic acid was determined [14].

Animals and diets

Groups of five Wistar male rats weighing on average 70 g were assigned to the different dietary treatments. The mean weight of the groups at the beginning of the experiment differed by no more than 1.0 g. The rats were housed individually in plexiglass cages with stainless steel mesh bottoms in a controlled environment (temperature 25 °C, relative humidity 50%, light-dark periods of 12 hours). Each rat received 10 g of dietary dry matter and 150 mg of nitrogen daily. Water was provided ad libitum. The diets were fed for eight days. Faeces and urine were collected separately during the last five days; body weight and food consumption were recorded. The faeces were Iyophilized and ground in a mortar into a fine powder for analysis.

The diets were prepared from the Iyophilized gruels and formulated to contain 1.5% of dietary nitrogen. A nitrogenfree mixture, consisting of autoclaved potato starch 80.7%, sucrose 8.9%, cellulose powder 5.2%, and soya bean oil 5.2%, was used to adjust the dietary nitrogen content. The diets contained 3.7% of a mineral mixture and 1.5% of a vitamin mixture on a dry-matter basis [16].

True protein digestibility (TD), biological value (BV), and net protein utilization (NPU = TD x BV) were measured. Corrections were made for endogenous excretion of nitrogen in the faeces ( 1.7 mg nitrogen per gram of dry matter consumed per rat) and in urine (76 mg nitrogen per five days per rat) as described by Eggum [15]. Utilizable protein (UP = NPU x protein content) was calculated.

Estimation of digestible energy was based on the analysis of diets and faeces. Digestible energy (DE) of the products was estimated by calculation, and corrections were made for the digestibility of energy in the nitrogen-free mixture. In a preliminary experiment, the DE of the nitrogen-free mixture was found to be 90.0%.

The data from the animal experiment were subjected to one-way and two-way analyses of variance, and differences between groups were identified by Tukey's test [17]. The minimum level of statistical significance accepted was P < .05.

Results

Effects of milling on composition

The composition of the barley flours and the barleys from which they were prepared are shown in table 1. The refined flours contained more starch and less of other nutrients than the wholemeal. The protein content of the most refined flour was reduced to 83% of that in wholemeal, but the amino acid composition was not changed by the milling process (table 2). The contents of minerals and physic acid were reduced to about 60% of the corresponding levels in wholemeal (table 3). Calcium was the mineral most severely affected by the process. Milling caused a slight decrease in the energy content of the flours, because the fat-containing germ was removed. The content of insoluble but not of soluble dietary fibre was also reduced by refining

TABLE 1. Chemical composition of barley flours and grains

a. Normal barley (Triumph variety), ungerminated. The flour was refined.

TABLE 2. Amino acid content (grams per 16 g N) of barley flours and grains

a. WM = wholemeal; SR = semi-refined: R = refined.
b. Normal barley (Triumph), ungerminated. The flour was refined.

Effects of germination on composition

Germination increased the concentration of sugar and decreased the content of starch and dietary fibre. The effect increased with germination time. Otherwise there was no major change in the content of most nutrients, including minerals. The level of physic acid, however, was lowered significantly by germination; thus, the germinated flours contained only half as much physic acid as the ungerminated semi-refined flour, with a comparable degree of refining. The level of physic acid was similar whether the grains were germinated for three, five, or seven days. Germination caused only minor changes in amino acid composition; the content of glutamic acid decreased, whereas the concentration of proline increased.

Composition of high-lysine versus normal barley

The high-lysine flour differed in composition from the normal barley, having more protein and less starch. The improved barley flour also had higher concentrations of several amino acids, including Iysine and threonine, which were increased by 38% and 22% respectively. The contents of the non-essential amino acids glutamic acid and proline were lower. The high lysine flour had a higher content of dietary fibre, but mineral contents and the level of physic acid in the two refined flours were quite similar.

Protein and energy utilization

The two-way analysis of variance showed that true protein digestibility and biological value (or protein quality), as measured in rats, increased slightly but significantly with milling (table 4). The effect was quite small; both protein digestibility and biological value were approximately 3% higher in absolute value in the refined flour than in the wholemeal. Germination resulted in a significant increase of about 5% in protein digestibility and a corresponding reduction in biological value. Adding 1% of malt, on the other hand, had no effect on protein digestibility or quality. Refining, germination, and malt addition had no consistent effect on net protein utilization, which ranged from 74% to 82%, with highest values in the refined flours.

There was no difference in the NPU of gruels made of refined flour from the improved variety and the control barley, because, although the high-lysine line had better protein quality, its protein digestibility was less than that of the conventional variety.

The amount of utilizable protein, however, was very low in the control gruel because of the low protein level of the flour. The content of utilizable protein in the high-lysine gruels varied between 9.0% and 10.3%; it was unaffected by germination or the addition of malt.

The digestibility (%) of the energy increased as a result of refining as well as of germination (table 5). It was not affected by the addition of malt. The highest value, 93%, was found for gruel made of reinfed normal barley. The amount of digestible energy (calories per gram), determined on a dry-matter basis, varied little.

Discussion

Effects of refining

Milling decreased the concentration of nutrients in the flours with the exception of starch and sugar. The changes during preocessing of the high-lysine barley into flour resembled those occuurring during refining of normal barley and other cereals [18]. On one hand, refining removes dietary fibre and antinutritional factors such as phytic acid. On the other hand, it greatly reduces the levels of vitamins and minerals and lowers the protein content.

TABLE 3. Mineral and phytic-acid content (dry basis) of barley flours and grains

a. Normal barley (Triumph). ungemminated. The Four was refined.

TABLE 4. Protein utilization in rats fed barley gruels

a-e. Mean values with unlike superscapt letters in the same column are significantly different (P < .05). Pooled SDs are: TD, 2.2; BV, 2.6; NPU, 3.0; UP, 0.3.
f. Nommal barley (Triumph), ungerminated, refined.

TABLE 5. Digestible energy (dry basis in rats fed barley gruels)

a-c. Mean values with unlike superscript letters in the same column are significantly different (P < .05). Pooled SDs are: DE (%), 1.3; DE (kcal/g), 0.057. d. Normal barley (Triumph). ungerminated, refined.

Refining caused an increase in protein digestibility as well as in protein quality (biological value). This is probably because the protein digestibility and the availability of amino acids of the outer layers are low compared to the endosperm [18]. The effect was small, however, and there was no great difference in the amount of utilizable protein in the flours of different degrees of refining. The increase in energy digestibility with increasing degree of refining is caused by a decrease in the content of fibre of low digestibility and a simultaneous increase in the concentration of highly digestible starch and sugar. The effect on energy digestibility was very small, and there was no difference in the amount of digestible energy between flours of different extraction rates.

More important perhaps than the effects on protein and energy utilization are the effects of refining on vitamin levels and mineral utilization. A highly refined barley flour contains only 20%-30% of the B vitamins present in wholemeal [19]. The mineral concentrations are also significantly reduced by refining, but factors interfering strongly with the utilization of some minerals, such as zinc, are removed as well. Hence, in spite of a much lower content of zinc, refined barley flour is probably a better source of available zinc than wholemeal, which seems to be a poor zinc source |20|. Zinc is an important mineral to consider, because zinc deficiency has been reported to be common among malnourished children. Breast milk may contain inadequate quantities, because the concentration decreases with time, and zinc supplementation has recently been shown to improve the nutritional rehabilitation of malnourished children [21].

Effects of germination

Germination has been reported to increase the Iysine content and improve the amino acid composition of cereal grains [22; 23]. Its effect on protein quality (measured as biological value or protein efficiency ratio [PER]) and utilization, however, is unclear. Thus, in spite of an increase in Iysine content, the PERs of ungerminated and germinated millets were not significantly different [23]. Similarly, Nattress et al. [24] reported that germination did not improve the PER of seed mixtures. Brandtzaeg et al. [25] found a lower BV in a malted millet-pulse mixture than in an unmelted mixture, and attributed this reduction to the deterioration of some amino acids during the roasting process of the germinated seeds. The net protein utilization, however, was not affected by germination. Taal et al. [26] found a small increase (6%-9%) in the Iysine content of sorghum and millet during germination, improved BV in sorghum but not in millet, and slightly improved NPU values for both.

In our experiment the composition of amino acids was largely unchanged by germination. The concentration of glutamic acid decreased and that of proline increased, in accordance with the results of Robbins and Pomeranz [22]. The Iysine level apparently rose slightly during the germination process, but the BV of gruels made from germinated barley was lower than that of gruels prepared from ungerminated barley. The reduction in the BV of the germinated gruels is probably due to Maillard products formed during the cooking process. Germinated flours have high levels of reducing sugars, facilitating the formation of Maillard products. When Maillard products are formed, Iysine is lost and protein quality reduced if Iysine is a limiting amino acid [27]. In conclusion, because the protein digestibility was increased by germination and the protein quality reduced correspondingly, germination had no effect on NPU. When malt was added at a 1% level, neither protein quality nor utilization was affected, probably because Maillard compounds are less likely to be formed when malt is used only as an additive.

Germination improved the digestibility of energy (%). Since the effect was small, however, there was no difference in the amount of digestibile energy on a dry matter basis (calories per gram) of the gruels made from germinated and ungerminated flour. Adding malt had no effect on the content of digestible energy of the barley gruels.

Germination did not affect mineral levels but markedly reduced the content of physic acid. Because of the reduction in physic acid, known to interfere with mineral absorption, it is possible that the availability of minerals is improved by germination [24]. However, it appears that physic acid is not solely responsible for the low zinc availability of unrefined barley flours [20], and the effects of germination on mineral availability need to be examined further [24].

The levels of some vitamins increase during germination while those of others do not [23-25]. For barley there is no big difference in the vitamin contents of ungerminated grains and malt [28].

Amino acid scores and protein intake

Leucine was the first limiting amino acid in the highly sine flours, on the basis of the FAO/WHO/UNU [29] amino acid scoring pattern for infants, followed closely by Iysine. The scores for leucine varied between 67 and 71, with the lowest value for the flour prepared after seven days of germination and the highest value for the ungerminated wholemeal. The normal barley flour was lowest in Iysine (52), followed by leucine and threonine. Because of the enhanced levels of Iysine and threonine in the improved variety, the BV of this variety is exceptionally high compared with that of common cereal grains [18].

All the gruels based on the high-lysine barley provide a safe level of protein intake for infants and small children. The calculations are based on the FAO/ WHO/UNU [29] recommendations; the amino acid scores are used and corrections are made for digestibility. Thus, if 60% of a one-year-old child's energy intake is derived from barley gruels, the gruels made from wholemeal, semi-refined, or refined flour will provide 94%, 87%, or 84% respectively of the child's total protein requirement. If the barley is germinated for three or seven days, the corresponding figures are 94% and 84%. In contrast, using the same assumptions, normal barley provides only 45% of the total protein requirement and thus does not provide a safe level of protein intake for children under one year of age.

Conclusions

The capacity of weaning gruels to meet the protein and energy requirements of infants depends on their dietary bulk as well as their nutritional quality. In the first part of this study [2], we found that if infants are fed germinated barley gruel, or gruel made from ungerminated barley with a small amount of malt added, they should be able to consume sufficient quantities to fulfil their energy requirements, but that ungerminated barley flours seemed to be inadequate as a source of energy for small children, especially when highly refined.

If sufficient quantities can be consumed, the high-lysine barley, but not the normal barley, provides a safe level of protein intake. In addition, although there are differences in the energy and protein utilization of barley gruels made from ungerminated versus germinated flours, and from wholemeal versus refined flours, these differences are small and are greatly superseded by the marked effects of germination and refining on viscosity. The addition of malt seems to have no effect on the nutritional quality of the gruels.

The effects of germination and refining on the content and availability of vitamins and minerals also needs to be determined.

References

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