Browning of kernels occurred through three different pathways
(i.e., (i) microorganisms, (ii) Maillard reaction, and (iii) enzymatic
browning) based on the postharvest treatments used. Soon after
the nuts achieve maturity and falls on the ground, it is susceptible
to browning by microorganisms. Microorganism contamination
could occur through the open micropyle or faulty shell formation or
cracks. It is therefore recommended to harvest the nut as soon as it
falls on the ground and use good agricultural practices to minimise
unwanted microbial attack. In addition, the wet husk attached to
the NIS could be a source of microorganisms. Prompt removal of
the husk needs to be strictly followed. Sucrose is hydrolysed into
glucose and fructose. These reducing sugars together with a protein
component can lead to browning induced by the Maillard
reaction during drying. This was evidenced by microscopy work
in which high concentration of proteins were distributed in the
brown kernel sample. Thus kernels with high protein content are
more susceptible to browning. Regardless of sucrose concentration,
the amount of glucose and fructose hydrolysed during drying are
the key determinants in the degree of browning. The third possible
mechanism, enzymatic browning is related to phenol content as
the brown sections of the kernels had higher bound phenolic concentrations
than the white sections of the same kernel. The brown
sections result from esterification of the phenolic compounds with
associated protein substrates. The ‘Daddow’ variety had the highest
activity of polyphenol oxidase compared to other varieties studied
but it contains lowest phenolic content. Thus, that phenolic content
as well as polyphenol oxidase activity play a role in kernel
discoloration.