Investigating carotenoid loss after drying and storage of

204
9. Discussion
(the main isomers formed are 9-cis and 13-cis) have half of the provitamin A activity of
the trans- !-carotene molecule (Rodriguez-Amaya and Kimura 2004). Isomerisation
under the effect of light has been reported in storage of carrot juice (Chen et al.1996)
and mango puree (Vasquez-Caicedo et al. 2007). The effect of light on isomerisation
during storage (in clear or opaque packaging) was not determined; however it was
measured during the pilot-scale study during drying. In the pilot-scale study (Chapter 3),
individual carotenoids (including cis and trans-!-carotene) were measured after drying in
solar (UV-resistant polythene), sun or hot air drying (non-exposed to sun-light). There
was no increase in cis-isomerisation (formation of 9-cis and 13-cis-!-carotenes) after
drying in these three dryers. A comparable result was found on a study comparing sun
and solar dried leafy-vegetables by Mulokosi and Svanberg (2003) and Kidmose et al.
(2007), who suggested that all stereo-isomers; trans-!-carotene, 9-cis and 13-cis were
likely to be oxidised following the same trend (Chapter 3).
9.5.2 Temperature
Cis-isomerisation could also be caused by temperature (Doering et al. 1995). Formation
of 9-cis and 13-cis isomers due to boiling, canning, cooking and drum drying treatments
of sweet potato has been described by Chandler and Schwartz (1988). Hiranvarachat et
al. (2008) showed that a minimum of 5 h at constant temperature of 60ºC was necessary
to induce formation of 13-cis-β-carotene in oven-dried diced carrot. Using dried-sweet
potato flour put in the oven (to simulate accelerated and extreme storage conditions)
(Chapter 2), it was demonstrated that at 40ºC there was a negligible isomerisation after
103h whilst at 100ºC there was a significant isomerisation after 3h of treatment.
Throughout storage at 100ºC 9-cis increased whilst 13-cis decreased. The same
observation was made for samples under light at ambient temperature however with
lower levels of cis-isomerisation (Chen et al.1996; Vasquez-Caicedo et al. 2007).
Cis-isomerisation is therefore highly temperature-dependent. The absence of
isomerisation in solar and sun drying could be explained since the average drying
temperature in all the devices was around 30-40ºC and never went beyond 55ºC
(Chapters 3, 4 and 5).