Enrichment characteristics of nutritional components in different parts from corn kernels

Maize (also known as corn) is the third most important food grain after wheat and rice. Corn kernels are the fruits of corn as a vegetable in cooking. This study aims to clarify the distribution patterns and enrichment characteristics of nutritional components in the corn kernels after semi-wet layered milling. Four parts of the Zhengdan 958 corn kernel were collected, named corn bran 1 (CB1), corn bran 2 (CB2), corn germ (CG), and corn endosperm (CE). Standard experiments were used to analyze the chemical contents of the corn kernels. The basic nutritional compositions included the contents of starch, protein, fat, ash, and crude fiber, as well as the content and composition of minerals and dietary fiber. The targeting metabolomics was used to clarify the basic metabolites using the GC-MS detection technique, including free sugar, free and hydrolyzed aminol acids, and fatty acids of the four separated parts, compared with the whole corn kernel (WC). The results showed that the contents of the total starch, protein, fat, ash, and crude fiber were 72.60%, 9.51%, 4.96%, 1.44%, and 7.72%, respectively. Significant differences were observed among these contents in the different parts of the kernel (P<0.05). About 74% starch was found in the CE, while the CG was found rich in protein, fat, and ash, contributing 21.46% of protein, 53.16% of fat, and 57.84% of the ash to the WC. The CB1 and CB2 contained most of the fibers, with the amount of 39.35% and 28.90%, respectively. There were significant differences in the contents of total dietary fiber (TDF), insoluble dietary fiber (IDF), and soluble dietary fiber (SDF) in different parts of corn kernel (P<0.05). The contents of TDF, IDF, and SDF in the CB1 were 43.47%, 36.76%, and 7.54%, respectively, while the IDF content was 4.88 times higher than the SDF. While in the CB2, the contents of TDF, IDF, and SDF were 32.47%, 24.55%, and 8.83%, respectively, while the IDF content was 2.78 times higher than the SDF, indicating a relatively higher ratio of the SDF content in the CB2, compared with the CB1. A total of 10 types of free sugar were detected in the WC, as well as the four separated parts. There was the highest content of free sugars in the CB2, with an amount of 17.47 mg/g, followed by the CB1 with 12.80 mg/g. Glucose, galactose, and fucose were the most abundant free sugar in the corn bran samples. K, P, and Mg were the minerals with the highest contents in corn. Most of the essential trace elements were abundant in germ, whereas, Fe, Zn, Mn, and Cu were also distributed in corn bran. The refined processing of corn led to the loss of these mineral elements. Compared with other parts, the CG contained more abundant hydrolyzed amino acids (P<0.05), while the leucine and glutamic acid were the essential and non-essential amino acids with the highest concentration, respectively. The amino acids for the sweet flavor accounted for 24.49% of the total free amino acids in the CG, which was higher than those in the CB1, CB2, and CE. The total content of free fatty acids in CB2 was also higher than in CB1. The fatty acids constituting triglyceride were linoleic acid (about 40%), oleic acid (37%), and palmitic acid (14%), indicating the rich content of unsaturated fatty acids in corn. Consequently, the CB2 part rich in nutrients can be expected to compose mainly of the seed coat, aleurone layer, and part of the peripheral endosperm. The water-soluble dietary fiber content in this part was significantly higher than the rest (P<0.05), indicating that CB2 was a source of the extraction of SDF. The systematic qualitative and quantitative metabolomics research on the endogenous basic metabolites can also provide new ideas, and then greatly contribute to the improvement of the sensory properties, including the flavor and colors of corn products, as well as the bioprocessing technique and nutritious food development. This finding can provide strong reference data for the enrichment of nutrients and utilization of value-added byproducts during corm processing, particularly for the development of nutritious and healthy corn foods. In addition, it can also offer to optimize and then update the corn layered milling processing.