Effect of Dehulling and Hydrothermal Treatment on the Amino Acid Content of Soriz (Sorghum oryzoidum) ()
1. Introduction
Cereals, more than any other type of crop, are grown in large quantities worldwide, due to its economic importance and energy intake. Scientific studies suggest that eating whole grains can protect against diabetes, obesity, constipation, cardiovascular disease and other lifestyle disorders [1]. It is the basic element in ensuring the food security of the population through the significant intake of carbohydrates, proteins, dietary fiber, vitamins, minerals and phytochemicals [1] [2] [3] [4]. Cereal proteins make up about 10% of the dry weight of cereals and are essential components in food systems, and their exploration, as an economical and sustainable source for a healthy diet, is current [5] [6]. It is generally characterized by a low content of essential amino acids, in particular lysine, threonine and tryptophan [7] [8] [9] [10]. The processing of cereals leads to changes in their nutritional quality, which could reduce both nutrients and anti-nutrients. At the same time, processing treatments can lead to improved sensory characteristics, digestibility or nutrient availability [4] [11] [12].
Knowing the nutritional value and changes that occur as a result of technological and culinary treatments can help establish policies on assessing nutritional status, formulating therapeutic diets, and investigating the relationship between diet, health, and disease [1]. Gluten-free cereal varieties attract attention as raw materials to improve the nutritional quality of food and to diversify the range of gluten-free products/diets [13]. Sorghum is a cereal crop well adapted to low rainfall regions and has therefore become a staple crop for millions of people around the world. As with all cereals, the chemical composition and nutritional value of sorghum vary within and between cereal crops [14] [15].
Soriz (Sorghum oryzoidum) is a hybrid of sorghum that is characterized by glassy endosperm, similar to rice. It was obtained at the Institute for Scientific Research for Maize and Sorghum in the Republic of Moldova, by crossing Sudan grass (S. sudanense) and bicolor sorghum (S. bicolor) [16] [17]. The advantages of soriz are manifested in the production process, which does not require major investments: the plant is not pretentious to soil conditions, fertilizers and has tolerance to diseases and pests. It has high productivity per hectare: in favorable conditions, the productivity of soriz reaches 6 - 8 t/ha, and in some years, in the northern parts of the republic, the harvest reached 12 t/ha (Chirsanova, A., et al.). The chemical composition of whole grain grains attests a starch content—74.12%…82.0% d.m., protein—about 13.0% d.m., sugars—0.24% - 0.37% d.m., lipids—0.1%…0.5% d.m., ash—0.36% - 2.0% d.m. In the whole soriz, the dominant protein fractions belong to prolamins (56.0% of total protein), followed by glutelin (22.4%), globulins (7.3) and albumin (6.7) [18]. An important feature of soriz is the absence of gluten, which makes it an optimal alternative in the diversification of gluten-free products—a relevant and extremely current field of research, especially for the Republic of Moldova [19] [20]. The novelty of the research consists in the fact that soriz is a relatively new cereal for the Republic of Moldova, but which reaches new borders. Respectively, research on the impact of processing, hydrothermal treatments on the nutritional value of soriz is current and necessary.
2. Materials and Methods
Whole grains of soryz and hulled soryz “Alimentar 1”, purchased at the Institute of Plant Protection and Organic Agriculture of the Academy of Sciences of Moldova, were used as research material. Hydrothermal treatment of whole grains and hulled grains was done in distilled water (Table 1).
The methods used in the research are presented in Table 2.
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Table 1. Raw materials used for research and their protein content.
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Table 2. Methods used for determining the analyzed indices.
3. Results and Discussions
The quality of the protein describes its characteristics in relation to its property to achieve defined metabolic actions [27] and is evaluated according to the ability of food constituent amino acids to meet the biological needs of the consumer [28]. Of the total amino acids, it was accepted that ten are essential amino acids for adults [24]. These amino acids cannot be synthesized by the human body from available materials and must be part of a healthy and balanced diet. However, the list of essential amino acids has been criticized [29], and the Food and Agriculture Organization (FAO) recommends addressing dietary amino acids as individual nutrients [30]. From a nutritional point of view, it is quite clear that some amino acids are absolute dietary needs if you want to maintain a normal growth [29]. Knowing the amino acid content, especially the essential ones, are important indicators in assessing the nutritional quality of traditional cereal products and those obtained from new sources of raw materials [31].
In native soriz grains, the sum of non-essential amino acids is about 66% of their total protein (Table 3). Soriz grains are characterized by a high content of glutamic acid (302.0%), alanine (96.0%) and proline (90.01%), which is also characteristic for most cereals [4] [14] [31].
Dehulling is the mechanical method that involves separating the pericarp from germs, which often leads to the elimination or reduction of phytic acid, as well as some minerals and vitamins [32]. Dehulling of soriz grains led to an uneven change in amino acid content. As a result, the total amount of essential amino acids decreased by 19.8% compared to the native grain. Sulfur amino acid content values decreased by 2.7%, but aromatic amino acid values increased by 11.6%. Dehulling has also led to an increase in the share of glutamic acid and alanine amino acids that are rich in gluten proteins (especially prolamins), which are found in large quantities in soriz derivatives [18]. Dehulling does not change the content of these amino acids.
Hydrothermal treatment. Scientific studies have shown that hydrothermal treatment improves the bioavailability of micronutrients by reducing the content of phytic acid and phenolic substances [33] [34]. Boiling whole grains without prior hydration led to 34% loss of essential amino acids compared to native grains, and pre-hydration of the grains and subsequent boiling reduced essential amino acid losses by 8% compared to cooked grains without hydration. Whole grains and dehulled soriz grains are a considerable source of branched-chain amino acids (BCAAs) (leucine, isoleucine and valine). Leucine is not only a precursor for muscle protein synthesis, but can also play a role in regulating intracellular signaling pathways that are involved in the process of protein synthesis [35]. Branched amino acids are considered the most important amino acids in protein synthesis, which are attributed a detoxifying role in dealing with ammonia and in preventing muscle trophism. But, nevertheless, there are still many controversies and research is needed to elucidate this topic [36] [37]. The hydrothermal treatment had an insignificant impact on the content of these
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Table 3. The impact of dehulling and hydrothermal treatment on the amino acid content of soriz.
amino acids, although the ratio between them, for the researched samples, is far from ideal (2:1:1), namely: in whole grains it is 4:1:2.7.
Hydrothermal treatment beneficially changed the ratio of the amino acids tryptophan: lysine: methionine (optimal, being considered the ratio of 1:3:3) from a ratio of 1:2.1:0.2 (in whole grains) to 1:2.3:1.3 (in boiled grains without hydration), 1:2.2:1.7 (in hydrated grains then boiled) and 1:2.3:1.4 (in hulled and boiled grains).
In boiled whole grains the ratio of these amino acids proved to be more balanced compared to the rest of the samples, surpassing whole grains such as sorghum (1:1.01:1.36), millet (1:1.78:1.3) and wheat (1:0.91:0.55). In dehulled soriz, the ratio of the respective amino acids was closer to the rice groats (1:2.5:1.3), but more balanced compared to the wheat groats (1:1:1.1) [38].
Summary index of essential amino acids. One of the methods used to determine the biological value of proteins is to determine the summary index of essential amino acids. The summary index of essential amino acids for native grains was 69.4, for grains cooked without hydration-56.6, and for those cooked with pre-hydration-68, which indicates that boiling grains, after prior hydration, is a more efficient method in maintaining an optimal ISAE. The summary index of essential amino acids (ISAE) for raw hulled grains was 58.3, and for cooked hulled grains-56.5. However, this method does not allow the determination of the limiting amino acid in relation to the standard protein, the amount of which depends on the complete assimilation of others.
Chemical score index (CSI). The most common method of determining biological value is to calculate the CSI. The index is expressed as a percentage and shows the ratio of the content of the essential amino acid to the same amino acid in the standard protein (Lopez and Mohiuddin). The modification of the amino acid content in the hydrothermal treatment of whole grains and dehulled grains of soriz also conditioned the change of their CSI (Table 4).
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Table 4. The impact of dehulling and hydrothermal treatment on the CSI of the soriz.
The quality of the protein when peeling the grains depends, to a large extent, on the yield of the peeled grains. Essential amino acids make up 34% of the total number of amino acids.
The amino acid score is the most limiting amount of amino acid in the food protein, expressed as a percentage of that amino acid present in a reference protein [39] [40] [41].
Dehulling unevenly modifies the CSI of soriz grains, namely: decreases the CSI values for lysine by 10%, methionine by 8%, threonine by 18%, tryptophan by 32%, but increases the CSI values for isoleucine by 7.0% and phenylalanine 25% for dehulling grains. The CSI values for leucine and valine practically do not change after dehulling. For both native grains and soriz derivatives, the limiting amino acid remains lysine, which is characteristic of most cereals except corn, in which the limiting amino acid is tryptophan [42].
The limiting amino acid remains lysine. The value of this amino acid determines the biological value and the degree of assimilation of the protein. According to the calculations obtained, pre-hydration of grains with subsequent boiling can be considered a more effective method, compared to grains cooked without hydration, in order to reduce the loss of amino acids.
Hydrothermal treatment of whole grains and dehulled soriz grains differently altered the CSI of proteins. However, these remain medium quality proteins, the limiting amino acid of which is lysine (CSI = 26-for hydrated and boiled grains and CSI = 13-for hulled and boiled grains) [43] [44] [45].
4. Conclusions
Soriz proteins contain all the essential amino acids, the limiting one being lysine.
Peeling and hydrothermal treatment led to an uneven change in the amino acid content and, respectively, the CSI value in the researched samples.
The obtained results attest that the method of boiling grains with pre-hydration had a positive effect on the CIS of the following amino acids: lysine, methionine and cysteine, leucine and tryptophan.
For threonine and branched-chain amino acids, this method of hydrothermal treatment has led to higher amino acid losses compared to cooked grains without hydration.
The obtained results confirm that the nutritional quality of the soriz grain proteins and derivatives obtained, evaluated according to the CSI, is low. Therefore, their association with other foods such as eggs, meat, fish, milk, whose protein is well balanced in essential amino acids is justified.
The obtained results will be able to be used in the capitalization of the mouse for the design of gluten-free products, for the rational formulation of therapeutic diets and in investigations on the relationships between diet, health and disease.
Funding
This research was funded by postdoctoral grant “Contribution regarding nutritional eradication of gluten consumption diseases” nr. 21.00208.5107.06/PD, running at Technical University of Moldova, https://utm.md/.