Insights into the Physiological, Biochemical and Molecular Basis of Postharvest Deterioration in Cassava (Manihot esculenta) Roots

Abstract

Due to its favorable agronomic traits, tolerance to abiotic stresses and adverse environments, cassava is the most important source of dietary carbohydrates for 750 million people around the world, and is produced mainly by subsistence farmers in marginally agricultural land. Physiological postharvest deterioration (PPD) of cassava roots is an endogenous and complex process that restricts their storage potential to only a few days after harvest. This physiological phenomenon is one of the main constraints in cassava agriculture with an enormous impact on the cassava market chain. It is estimated that losses due to PPD in cassava production in Latin America and the Caribbean and in Asia reach 10% and 8%, respectively, whereas in Africa they reach 29%. Several years of research have been accumulating evidence to consider PPD as a wounding stress deficient process involving changes in enzymatic activity and oxidative stress. The primary symptoms, the development of dark bluish or brownish radial veins or streaks near xylem vessels of the root pith tissue, appear within 2-3 days after harvest and spread to the neighboring parenchyma tissues producing a more general browning discoloration throughout the root. Secondary post-harvest deterioration, often appears when the roots suffer moderate to severe damage at harvest and is mediated by a wide range of pathogenic microorganisms, Several strategies have been proposed to overcome the problem, but each alternative has its limitations due to the variable results, lack of objective and systematic methodology for PPD evaluation, applications not conducive for use at farmer-level, limited genetic variability or absence of genetic and biochemical information. The present review examines the socioeconomic impact of PPD, the physiological, biochemical and molecular processes occurring in the root during PPD, as well as the current and future alternatives to overcome the problem.

Effect of Surface Coatings on the Shelf life and Quality of Cassava

“…This was affected by the storage duration and PPD development as was reported by Salcedo and Siritunga (2011). The decline was between 73.57% and 76.81% for the 1.5% guar gum treated root and the 2% guar gum coated root.…”

“…The 1% xanthan/ guar gum treated roots decreased by 52.58% while 1.5% and 2% xanthan/guar gum treated roots decreased by 74.01% and 69.73% at 20DAH. The treated root samples varied in their delay in phenol reduction and this may have been due to the different efficiencies of the coating solutions to delay PPD and oxygen depletion which caused enzyme inactivation (Salcedo & Siritunga, 2011). There was also notable delay in the browning of the coated cassava roots which may have been due to the inhibition of oxygen penetration by the coating solution as was reported by Baraiya et al (2015).…”

“…Two peaks were observed during cassava respiration as recorded by Salcedo and Siritunga (2011). These two peaks occurred generally at the second day after coating while the second peak depended on the efficiency of the coating solution ( Figure 6).…”

“…The crop undergoes PPD primarily due to the wounding which occurs during harvesting and subsequent microbial deterioration which occurs 5-7 days after harvest (Venturini, M. T., Santos, V. D. S., & Oliveira, 2015). The PPD is accompanied by unpleasant odor and flavor and occurs as blue-black streaks on the vascular tissues of the xylem (Salcedo & Siritunga, 2011). It is estimated that the postharvest losses due to PPD in cassava can be as high as 25%.…”

“…Oxidation of the phenolic content of the cassava root and cyanide production have also been associated with the decreased shelf life of the cassava due to their various activities. Phenol oxidation has been found to lead to the production of highly reactive quinones that form the blue/black colored complexes on the vascular bundles of the cassava root (Salcedo & Siritunga, 2011). The wounding that occurs during harvesting of the cassava roots leads to production of hydrogen cyanide.…”