**2. Technologies on the nutritional quality of maize crop**

This technological processing for increasing the nutritional quality of maize as a potential solution for nutritional deficiency can be classified into two main groups, preharvest technology and postharvest technology.

### **2.1 Preharvest technology**

Preharvest technology discussion will include crossbreeding and genetic manipulation, functional genomics and transgenic crop technology, biofortification, functional genomics and transgenic crop technology, and soil improvement.

### *2.1.1 Crossbreeding and genetic manipulation*

Crossbreeding method is used to transfer micronutrients density in unaccustomed sources into genetic with a high-yielding competitive background. For the farmers to accept and adopt the newly developed trait, end-use quality and agronomic attributes must be considered during crossbreeding [26]. An increase in protein content such as methionine, tryptophan, and lysine is evidence for advancement in diet's nutritional content through breeding that fundamentally focuses on nutritional quality.

Physical appearance, cooking and eating quality, milling trait, and nutritive value are those parameters used to determine grain's overall quality [18]. These properties are to a large extent, especially the eating and cooking qualities, influenced by the amylose content of the maize, and thus open to genetic manipulation. Amylose, a polysaccharide that is made up of 20–30% starch, is a significant form of resistant starch. However, the amylose biosynthesis is modulated by the enzyme granule-bound starch synthase 1 (GBSS1) and encoded by the Waxy gene. Gao et al. [27], and Wang et al. [28] disrupted the GBSS1 with clustered interspaced and short palindromic repeats-associated protein (CRISPR-Cas) to produce elect maize that contains low amylose content. Also, CRISPR-Cas to encode isoamylase-type debranching enzyme been edited with isoamylase 1 (ISA1) produced low amylose content [29]. CRISPR-Cas uses the system of knock-out and knock-in to improve the quality of crops.

Phytate, an antinutrient content in maize, also referred to as inositol 1,2,3,4,5,6-hexakisphosphate, forms insoluble complexes with minerals and protein and reduces their absorption when consumed. Zinc finger nucleases (ZFNs) blocked gene coding IPK1 for enzyme inositol-1,3,4,5,6-pentakisphosphate 2-kinase to reduce the phytate concentration in maize [30]. Also, Qi et al. [31] used CRISPR-Cas and RNA interference (RNAi) which targeted the gene ZmMADS47 encoding a MADS-box protein that interacts with *O2* to switch on zein gene promoter so that reduced zein protein content can be reduced. However, the decrease in zein content was 12.5% and 16.8% in the kernel of MADS/Cas9-21 and ZmMADS47 lines, respectively [31].

### *2.1.2 Functional genomics and transgenic crop technology*

Genome editing is the principle on which functional genomics is based. It is based on nuclease-based forms of engineering like transcription activator-like effector nucleases (TALENTS), clustered regularly interspaced short palindromic repeats (CRISPR) with the concerns of creation of mutations, precise incisions, and substitutions in eukaryotic and plant cells [32]. Transgenic crop technology directly inserts genes of interest into the plant genome. These are the only viable alternative for biofortifying crops with micronutrients that naturally do not exist in the crop [26]. Transgenic crop technology can be achieved at a low cost, short time, and without nutrition-based programs and ease the concurrent incorporation of the genetic system to reduce antinutrients, increase micronutrient concentration, and promote bioavailability.

A hybrid of QPM and provitamin A was developed by Zunjare et al. [33], which was speculated to help fight malnutrition among the populace where maize is used as a primary staple food. Based on QPM analysis, the required amount of tryptophan and lysine was achieved when switching conventional maize with a lesser amount of QPM [34]. Also, orange maize improved vitamin A in children's diet in Zambia [35].

Many kinds of cereal contain a high prevalence of phytic acid (PA), a significant zinc absorption inhibitor. Minerals like iron and zinc are bound by PA and prevent mineral absorption in the gastrointestinal tract. Some researchers like Brnić et al. [36] have reduced the PA in maize due to its nutritional consequences. Chemicals such as acetic acid and hydrochloric acid, microwave treatment or heat methods, and recombinant microbial phytate exogenously reduce PA in grains [37]. Transgenic corn expression phytate from *Aspergillus niger* was created to raise mineral availability by limiting PA through microbial phytate enzymes [38]. Eventually, low PA (lpa) phenotype cereal mutants have been developed in wheat, maize, and rice. The primary concern about this transgenic expression is its negative impact on agronomic performance and crop yield.

### *2.1.3 Biofortification*

Biofortification is a primary means of fighting micronutrient deficiency in the world. Biofortification and supplementation are the traditional means of adding minerals and vitamins to food crops. The three essential means of biofortifying crops are biotechnology, conventional plant breeding, and adding an inorganic or mineral compound to fertilizer [37]. Essential micronutrients and β-carotene have been used to biofortified maize in other to maintain healthy living. In the 1970s, researchers developed quality protein maize (QPM) to increase maize's tryptophan and lysine content. These newly developed biofortified crops have a tremendous amount of micronutrients in the edible parts of the crops. Conventional breeding in maize has also been used to upgrade its nutritional content [25].

Nutritionally improved crops, such as orange maize (enhanced with zinc and biofortified with provitamin A carotenoids), quality protein maize (QPM) (biofortified with amino acids), have been developed by plant breeders. The commercialization of maize with provitamin A carotenoid is gaining traction in Western and Southern Africa [38]. Sowa et al. [39] reported 85% retention of carotenoid provitamin A in biofortified flour in the preparation of muffin and porridge. Adoption of biofortified crops is mainly tested by consumer acceptability of rural households where it is used to produce the different menus and prepared in their local ways. Consumers need to accept and use biofortified crops to prepare local foods to make the most of them. However, QPM was preferred by rural mothers in Ethiopia to conventional maize in the preparation of complementary food for children and infants [40]. Li et al. [41] and Muzhingi et al. [42] proved that biofortified maize efficiently converted provitamin A carotenoid to vitamin A. Hence, the daily requirement of vitamin A may be met by consuming biofortified maize.

Although the stability of carotenoids during storage is a significant challenge in provitamin A maize, consumption without dehulling improved the storage stability of carotenoids. Taleon et al. [43] studied different processing methods and storage environments of hybrid maize in Zambia. They proposed that maize biofortified with provitamin A should be sold just before consumption as whole grain and milled as such. Carotenoid degrades over time; therefore, fortified maize variety should be consumed before the white-unfortified maize.

The milling process, mode of cooking (refined versus whole grain flour), and container used for cooking will determine the zinc retention in biofortified maize. Zinc absorbed by Zambia children through zinc-biofortified maize helped to meet their zinc requirement [44]. QPM boosts tryptophan; hence, niacin (vitamin B3) can be partially met [25] because tryptophan is free by changing the leucine isoleucine ratio for niacin biosynthesis.

### *2.1.4 Soil improvement*

Protein content and yield react positively to nitrogen fertilizer used to supplement the soil in which maize was planted. This is the advantage of adding nitrogen fertilizer to low-nitrogen soil. Fertilizers that are biofortified with micronutrients and applied to the soil are the simplest biofortification method [37]. This practice has been affected by the regular application of micronutrients to the soil, increasing labor and cost, accumulation and mobility of minerals among plants, and variations in soil composition at a specific location. It also increases the micronutrients temporarily as there is a need to apply the fertilizer constantly. These micronutrient fertilizers raise molybdenum, nickel, copper, selenium, iodine, and zinc in different levels in their edible parts [26].

People in low-income countries whose diet is based on cereal grains low in zinc (Zn) are affected by Zn deficiency. Zn deficiency can cause poor immunity, birth complications, impaired mental development, and stunted growth [37]. Zinc concentration in grain crops has been improved by nitrogen management improvement through the availability of Zn in the soil. Nitrogen availability in the soil represents a significant component in the biofortification of zinc in grains and, therefore, enhances residence's nutritional status in developing countries. Another micronutrient in low quantity in grain due to its insufficient amount in the soil is selenium (Se) [45]. The organic form of Se (selenocysteine and selenomethionine) are more significantly bioavailable than inorganic selenium (Se). According to Poblaciones et al. [45], Se-rich fertilizer increases Se's bioavailability in grain and boosts total yield. The author discovered that chickpea could store a high concentration of Se in the grain after applying fertilizer.

### **2.2 Postharvest technology**

Reducing the wastage and losses of food becomes crucial in ensuring adequate nutrition, food security, improvement in rural livelihood, poverty, and food availability among the populace. However, food wastage can result from poor grain storage affected by temperature, relative humidity and grain moisture. Microorganisms, insects, and rodents are maize biological deteriorating agents [25]. *Penicillium, Fusarium* and *Aspergillus* are the general mycotoxigenic fungi of importance in maize. The most prominent mycotoxins in foods, aflatoxin is produced as secondary metabolites by *Aspergillus flavus sp*. The contribution of aflatoxins to crops losses has a negative effect, either directly or indirectly, on general nutrition, health, food security, and the economy at large [46]. The prevalence of aflatoxin can be reduced by both post and pre-harvest intervention. The preharvest intervention could be in the form of developing insect and Aspergillus resistant, heat, and drought-tolerant varieties [47]. According to Suwarno et al. [48], provitamin A carotenoid enriched maize can reduce aflatoxin contamination. Postharvest interventions include suitable moisture at harvest, humidity and temperatures during storage, and suitable containers and space.
