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Effects of postharvest treatments on sweetpotato (Ipomoea batatas) storage quality : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, New Zealand
After harvest, sweetpotato (Ipomoea batatas) storage root quality is reduced due to
weight loss, sprouting and rots. There are also hidden quality losses relating to loss
of nutritional compounds. In order to maintain sweetpotato quality during storage,
sweetpotatoes need to be stored at 13 - 15 °C and 80 - 90% RH. However, controlled
temperature methods are difficult to achieve for subsistence farmers in less
developed countries who have limited access to electric power. This work was
undertaken to determine the potential postharvest techniques that would extend
sweetpotato storage life without compromising phytochemical concentration.
Postharvest treatments investigated in this work were hot water dipping (with or
without coating) and ethylene (with or without 1-MCP) treatments. The work was
undertaken using mainly ‘Owairaka Red’ and ‘Clone 1820’ sweetpotato cultivars.
Following treatments, these sweetpotatoes were stored at 25 °C and 80 - 90% RH for
4 to 12 weeks.
Hot water dipping (HWD) at 51 °C for 11 min delayed sprout growth by 2 weeks but
increased weight loss. Coating (carnauba wax 5%) significantly reduced weight loss,
but increased sprout growth in ‘Owairaka Red’. A combination of HWD and coating
was effective in reducing both sprout growth and weight loss. ß-carotene content
measured in ‘Clone 1820’ ranged from 17.3 to 25.6 mg/100 g dry weight. The
concentration was not affected by HWD or coating, but declined by about 30%
during 12 weeks storage. The calculated retinol activity equivalent (RAE) ranged
from 363 to 537 RAE, per 100 g of edible portion of sweetpotato. Based on the
recommended daily intake for vitamin A, a serve of 100 g would supply more than
25% of daily retinol requirement for all age groups, suggesting that even after
storage ‘Clone 1820’ is a good source of vitamin A. In addition, no treatment
adversely affected the phenolic acid and anthocyanin concentrations. Roots that were
HWD showed a subtle increase in total phenolic content, phenolic acids and
anthocyanin concentration when compared to control roots, but the effect was shortlived.
Previous studies have demonstrated that ethylene is a potential sprout inhibitor, but
causes darkening of flesh colour and the development of off-flavours after cooking.
Ethylene-induced responses may be inhibited by 1-methylcyclopropene (1-MCP). 1-
MCP and ethylene combined effects on sweetpotato physiological, flesh colour and
phytochemical variables were assessed during storage. Ethylene treatment with or
without 1-MCP inhibited sprout growth, increased root respiration rates by 2-fold,
and caused root stem-end split leading to high weight loss and rots. Ethylene
treatments also caused flesh darkening, and this was not prevented by a single 1-
MCP (1 µL L-1) pre-storage treatment. When roots stored in continuous ethylene
were subjected to multiple 1-MCP (1 µL L-1) treatments, the ethylene-induced root
splitting and flesh darkening were delayed/reduced whilst maintaining minimal
sprout growth. This implies that ethylene-induced negative responses in sweetpotato
can be mitigated with on-going 1-MCP treatment. The sensory results showed that
roots stored in air were highly preferred by consumers over roots stored in ethylene;
nevertheless, acceptance means scores of all treatments were above five, indicating
that ethylene-induced flesh darkening was not severe enough to cause consumer
Based on these findings, it is proposed that a combination of HWD and coatings can
be used to extend non-refrigerated storage life of sweetpotato with no major effect on
phytochemical content. The results on ethylene are consistent with previous findings
that ethylene suppresses sprout elongation. However, the associated negative effects
outweigh the benefits of using ethylene as a sprout control. Future research therefore
should focus on finding ways to get the benefit of ethylene for sprout reduction
without incurring risk of root splitting.