analysis of fresh tomatoes (Fig. 8). The results indicated that strategy 2 produced a less firm tomato (good dried structure) compared
to strategy 1. Fig. 8shows that the power intensity had a large
influence on the mechanical response of the skin and pulp. However in strategy 1 firmness significantly increased (P< 0.05) with
the specific power, whereas in strategy 2 firmness decreased with
the specific power. This could be due to the fact that MWHA drying
with power control made less firm the product structure (good
structure for dried tomato) compared to MWHA drying without
power control. So MWHA drying with microwave power control
improved tomato quality. To have weak mechanical changes of tomato after MWHA drying, an osmotic pretreatment will be necessary as was doneHeredia et al. (2007)on the tomatoes before the
dehydration.
3.3.3. Structure assessment
The effects of different drying methods and conditions on the
structure of dried tomatoes were observed under scanning electron
microscope (SEM).Fig. 9presents the SEM observations of the
structures of tomatoes dried by MWHA with (strategy 2) and without (strategy 1) power control and fresh tomato structures. In
strategy 1 dried samples (Fig. 9B) there was very few open structure and pores compared to strategy 2 dried samples (Fig. 9C), indicating severe tissue shrinkage and collapse during strategy 1
drying, which indicates a low quality structure (Giri and Prasad,
2007). The reduced shrinkage in strategy 2 dried samples may be
due to the temperature control at a low level. The more porous
structure was possibly linked to a massive and fast vaporization
during strategy 2 drying, which would be a good structure (Therdthai and Zhou, 2009). Thus, tomato structure obtained with strategy 2 is better quality because it facilitates rehydration of the
product compared to the more compact structure of strategy 1.
3.3.4. Lycopene assessment
The effects of different strategies of drying methods and conditions on residual lycopene rate is illustrated in Fig. 10. Results
showed that drying conditions had no significant (P> 0.05) difference on residual lycopene rate for strategy 1. However for the drying strategy 2, the residual lycopene significantly decreases with
increasing microwave power. This result was reported by Heredia
et al. (2010)in their study on the effect of osmotic pre-treatment
and microwave heating on lycopene degradation and isomerization in cherry tomato. The weak lycopene content of dried sample
of strategy 1 was caused by the high temperature of tomato sample
at the end of drying (Mercadante, 2008) due to isomerization and
oxidation ofall translycopene (Shi et al., 2008; Lee and Chen,
2002). In this strategy output power was constant toward the