Corn kernels are not inert materials but biologically active organisms. During storage, three types of biological activities in the grain bulk jointly cause quality deterioration: kernel physiological metabolism, pest and mite damage, and microbial proliferation.
Corn kernel itself: Continuous respiration and enzymatic reactions consume internal nutrients;
Pests and mites: They feed on kernel endosperm and embryos, causing kernel breakage and weight loss;
Microorganisms (molds, bacteria, yeasts): They multiply rapidly under suitable temperature and humidity, decompose kernel nutrients and produce mycotoxins.
The synergistic effect of the above three biological factors leads to two major quality changes of stored corn: quantitative loss and qualitative deterioration.
1. Quantitative loss: Dry matter loss and weight reduction caused by kernel respiration, pest and rodent infestation, and microbial decomposition.
2. Qualitative deterioration: Including internal aging and external mildew. Internal aging is characterized by increased fatty acid value, protein denaturation and starch aging; external mildew is manifested by mold growth, toxin accumulation and deteriorated sensory quality.
Lipid is the most unstable nutrient in corn kernels and serves as the key early indicator of corn storage deterioration.
The dissociation degree of nutrients such as amino acids and minerals in the medium varies with pH changes, which directly affects the absorption and utilization efficiency of nutrients by microorganisms.
In mixed microbial culture systems, different strains have distinct nutrient utilization capabilities. pH fluctuations can alter the nutrient competition relationship among strains, thereby affecting the growth and reproduction of dominant microflora.
Corn lipid deterioration mainly occurs simultaneously through hydrolysis and oxidation pathways.
1. Hydrolysis: Endogenous lipase of corn and exogenous lipase secreted by molds jointly catalyze lipid hydrolysis, producing glycerol and free fatty acids (FFAs). FFAs are difficult to be utilized by microorganisms and accumulate continuously in kernels, directly increasing the fatty acid value.
2. Oxidation: Unsaturated fatty acids undergo lipid oxidation to form peroxides, which further decompose into aldehydes and ketones, causing corn rancidity, bitter taste and typical stale oily odor.
Key data of lipid oxidation:① The fatty acid value of fresh corn is about 20 mgKOH/100g;② The fatty acid value rises rapidly after warehousing, increases significantly after 1-year storage, and rises sharply after more than 2 years of storage;③ Under the conditions of 30℃ and 70% relative humidity, corn becomes severely unfit for storage after 100 days;④ The content of malondialdehyde (MDA, end product of lipid peroxidation) in corn stored for 4 years increases 20 times by 20 folds.
The crude protein content of corn shows a slight increasing trend during storage, which is a false increase caused by water loss and dry matter concentration; while the true protein content decreases significantly.
Core protein deterioration mechanisms: Proteins are hydrolyzed into peptides and free amino acids by proteases; protein spatial structure denatures with increased disulfide bond crosslinking and reduced solubility. Maillard reaction between reducing sugars and amino acids causes the loss of available lysine.
Storage significantly reduces the digestibility of corn protein: The in vitro protein digestibility decreases after 6 months of storage at both 25℃ and 37℃, with more severe deterioration under high temperature.
Starch content presents a trend of "first increase and then decrease": The starch content is the lowest at the initial harvesting stage and rises slightly after kernel ripening; during long-term storage, amylase continuously hydrolyzes starch into dextrin, maltose, monosaccharides and other small molecules, resulting in reduced total starch content.
Functional deterioration of starch:
1. Decreased gelatinization viscosity: Peak viscosity and final viscosity drop significantly; the final viscosity of corn with 14% moisture decreases by about 300 mPa•s after 60 days of storage;
2. Reduced resistant starch content: Corn with 12% moisture shows the maximum resistant starch reduction of approximately 29% after 1.5 months of storage;
3. Reduced digestibility: Free fatty acids combine with amylose to form complexes, inhibiting starch hydrolysis and reducing digestibility.
Corn test weight decreases continuously during storage. The test weight of corn with 14% moisture declines obviously after 60 days of storage, mainly caused by dry matter consumption via respiration and kernel shrinkage due to water loss.
Kernel color gradually darkens and loses fresh luster; the natural fresh flavor fades and stale flavor appears. In severe deterioration, lipid oxidation causes rancid odor, and mold growth causes musty odor. The sensory deterioration is more obvious in high-moisture stored corn.
For more corn processing information, please visit the official website of Beijing Meckey Engineering Co.