Moisture content is an important aspect of cereal grains that can significantly affect the storage period. Moisture content of about 13% and above exposes rice to a high risk of pest and disease attack; thereby affecting the storage period of the product [15, 16]. The moisture contents of the varieties were within the range (9.8– 13.0%) reported by Sahanob et al. [8] but vary from the values (9.25– 11.90%) reported by Chikwere et al. [17]. Faro 60 from Ebonyi State had the highest percentage moisture content while Faro 52 from Kano State had the lowest percentage moisture content. The higher percentage moisture content recorded from varieties from Ebonyi may affect the storage period of the varieties. The difference in moisture content recorded could be attributed to the inherent moisture content of the paddy or the effect of geographical area of study. Low moisture contents help to improve keeping quality of rice during storage. Hence Faro 52 from Kano State may have a longer shelf life compared to the other rice varieties due to the lower moisture content.
The fat content for the three varieties from the different agro ecological zones in Nigeria are lower than the values (1.00 to 1.26%) reported by Opara and Ntol [18] but vary from the range values (0.39 to 1.39%) obtained by Gabriel et al. [19]. Varying range was also observed from the findings of Verma and Srivastav [2] for different geographical areas. Despite the lower quantity of fat in rice, it has major influences in the eating quality [2]. The likely reason behind the difference in fat content among rice varieties from the different agro-ecological zones may be due to levels of removal of aleurone layer during milling operations as well as the milling practice and the degree of oxidation of unsaturated fatty acid [2]. According to Alaka et al. [20], with high content of fat, grains are exposed to spoilage during storage due to oxidation reaction. Therefore the rice varieties from the different agro ecological zones in Nigeria can be stored for a longer time without spoilage due to lower fat contents recorded in them.
The protein content for the three varieties from the different agro ecological zones in Nigeria were higher than the values (5.73–6.07%) and (7.33–9.97%) reported by Gabriel et al. [19] and Sahanob et al. [8] respectively. The differences in the protein content of the rice varieties may be due to agronomic management practices, genetic makeup of the varieties and environmental factors [8]. Proper balanced nutrient management, water management and sowing time are all agronomic management practices which cause variation in protein content [21]. The higher protein content obtained from the different varieties from the various agro ecological zones could be as a result of proper processing methods adopted. According to Oko and Ugwu [22] extended parboiling time lowers the protein content of rice. The nutritional value of rice is dependent on the total quality and quantity of the protein. On the basis of protein content, all the varieties contained sufficient amount which is above the reported values in this study.
All the tested varieties showed varying ash content which ranged from 0.66 to 2.04%. These values vary from the findings of Opara and Ntol [18] who reported 1.2–1.89% ash and also Chikwere et al. [17] who found 2.12–4.58% but within the range 0.5–2.00% reported by Oko and Ugwu [22] and 0.8–2.4% reported by Alaka et al. [20] except for faro 44 from Niger State which was slightly lower. The quantity of ash found in a given food sample plays a major role in determining the levels of essential minerals present in the food and may also contribute to the sensory quality of the rice [2]. The variations in the ash content could be as a result of the variations in mineral content of the soils and the water used for irrigation [2].
The fiber contents for the three varieties from the different agro ecological zones in Nigeria were high, with Faro 60 from Ebonyi State recording the highest value. These values were higher than the values (0.22–0.95%) and (0.35–0.74%) reported by Rathna et al. [4] and Sahanob et al. [8] but similar to the findings of Gabriel et al. [19] who found 0.90–6.85%. Dietary fiber is known as the edible parts of plant foods or carbohydrates which are resistant to digestion and absorption [23]. The differences in the fiber content of the varieties could be as a result of the milling effects. According to Oko and Ugwu [22] milling of rice generally reduces the fiber contents present in rice.
Carbohydrates, which constitute the bulk of our daily food, are majorly found in cereal plants and they play important roles in the daily lives of humans, [24]. The quantities of carbohydrates recorded in this study were similar to the values reported by Cherie and Dagnaw [25] for varieties of rice in Ethiopia and also similar to the values reported by Rathna et al. [4] from varieties of rice in south India. The levels of > 74% carbohydrates show that we can obtain the required energy needed by the human body for normal growth and development from local rice.
The correlation table indicates varying degrees of relationship among different proximate compositions, including moisture, protein, fat, ash content, fiber, and carbohydrate. The result showed that zone, variety and zone x variety had significant effects on the proximate compositions of the samples as (p < 0.05).
Minerals are very essential in the anabolism and catabolism processes in living organisms. Insufficient supply of these minerals as required by the body could lead to metabolic disorders. Rice is not among the food materials that contain adequate quantity of minerals. However, because it is a commonly consumed food within the people, it offers a certain quantities of minerals to the consumers [7, 26]. The calcium contents of the rice varieties were higher than the range (0.33–0.73 mg/100g) reported by Opara and Ntol [18], but lower than the values of (5.19-7.81mg/100g) obtained by Vunain et al. [27] for white rice varieties in Malawi, (3.47-4.67mg/100g) reported by Chikwere et al. [17], and (3.74–8.23 mg/100g) reported by Gabriel et al. [19]. Rice has shown to contain certain amount of calcium which when taken will help in blood clotting, nerve irritability in the blood and aid normal development and maintenance of bones and teeth [20].
Minerals, most especially iron and zinc are very important for normal growth and development as well as proper functioning of the human body [2]. The Zinc content of all the varieties varies from the values of (0.77 to 1.44 mg/100g) and (0.17%) reported by Ephraim et al. [28] and Sahanob et al. [8]. Metals like zinc are very important elements because they are needed in the body for proper biological activities. Zinc functioning as a metallo-enzyme helps in catalytic and regulatory functions and structural stability. Zinc also participates in DNA and RNA synthesis and as well as cell proliferation [29].
Magnesium is very important for all the biosynthetic processes in the body. These processes include cyclic AMP formation, glycolysis, genetic code transmission and transportation of energy dependent membrane. Magnesium is also important for electrical potentials of nerve and muscle maintenance and for signals transmission across neuromuscular junctions [30]. The magnesium content for all the varieties from the different agro ecological zones is lower than 0.19 to 0.26% reported by Oko and Ugwu [22]. According to Anitha et al. [31], when rice is consumed in large quantity compared to other foods, there will be adequate supply of minerals to the body from the diet. The differences in the mineral contents of the varieties could be as a result of rate of parboiling, fertilizer application and the amounts of soil nutrients which affect the minerals present in rice [22].
The correlation table indicates that zone, variety and zone x variety had significant effects on the mineral compositions of the samples as (p < 0.05).