Investigating carotenoid loss after drying and storage of

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9. Discussion
2008). The side effect of this structure however is that light can be damaging for the
molecule. The degrading effect of UV and short visible wavelengths (blue light)
(between 410-450nm) on mayonnaise colour has been described by Lennersten and
Lingnert (2000). By comparing the use of different screening filters in solar drying (open
sun drying, UV blocking-Visqueen®-covered solar dryer and non UV- blocking
polyethylene covered solar dryer) it has been proved that UV-blocking polythene could
significantly better preserve carotenoids of cowpea leaves and mango compared to the
other coverage (Ndawula et al. 2004). Consequently the effect of light screening
coverage has been investigated in this study. Preliminary trials on sweet potato flour
under light-screening polythene and exposed to artificial sun light demonstrated that
colour of samples under the red polythene were significantly less affected than with the
other plastic sheeting (Chapter 2). These results were in accordance with Lennersten and
Lingnert (2000) and agreed with usual recommendations to work under red light during
carotenoid analysis (Rodriguez-Amaya and Kimura 2004). When the same red polythene
was tested in the field as solar dryer coverage it did not result in significant improvement
in carotenoid retention compared to UV-resistant and non-UV resistant polythene
coverage (Chapter 4). This was believed to result from slower drying due to the red
coverage that blocked a significant amount of the sun-light wavelengths (between 300-
600 nm) and consequently cooled the inside of the dryer (Chapter 4). Moreover there
was no significant difference between solar dried (under UV-resistant or non-UV-
resistant; or under red-plastic) and sun-dried samples (Chapter 4). Therefore the effect of
light in drying was not proved significant on carotenoid catabolism during the drying of
sweet potato in accordance with Bengsston et al. (2008).
The effect of light on carotenoid retention was also tested in storage. When storing sweet
potato chips in transparent or opaque packaging at ambient temperature in Uganda, there
was no difference in the total carotenoid content independently of packaging type after
four-months. These results were in agreement with Cinar (2004), working on sweet
potato extracted pigments exposed in the light or in the dark, where no significant
difference between total carotenoid contents exposed or non-exposed after 45 days at
25ºC was found.
Light is also known to be responsible for cis-isomerisation. Cis-isomerisation of !-
carotene can decrease the provitamin A activity of dried sweet potato since cis-isomers