Influence of a double-stage solar tunnel dryer on the preservation of quality characteristics and the Modelling of colour variations in red chili peppers

Abstract

In this work, the influence of two-stage solar tunnel drying on the preservation of quality attributes of both chili pepper varieties, Mareko Fana (MF) and Bako Local (BL), was studied. Both varieties were dried in different drying modalities, i.e. solar tunnel and open sun drying. The quality attributes were evaluated during the drying process using FTIR and spectrophotometry techniques. Different colour evolution models were implemented and a suitable model was selected. The results showed that after drying of the Mareko Fana chili variety, the lightness retention was found in the range of 81 %–89 %, the redness retention between 25 % and 42 % and the yellowness retention between 14 % and 38 %. For the Bako Local variety, the values were in the range of 78 %–82 %, 42 %–60 % and 36 %–55 %, respectively. The first order model gave a higher correlation coefficient (R2 > 0.96), which indicated the suitability of the model for predicting the color variation for both varieties. The volatile aromatic compounds in the chili peppers were greatly lost during open sun drying, while solar tunnel drying maintained these compounds for both varieties. However, during the solar tunnel drying process, a significant amount of alkyl halides or alkyl-chlorides were lost. The amount of dihydrocapsaicin and capsaicin from the chili peppers ranged from 10,172 μg/kg to 16,313 μg/kg and 16,676 μg/kg to 27,189 μg/kg for both dihydrocapsaicin and capsaicin, respectively. In tunnel, open-air, and uncontrolled open-air solar drying, the MF chili variety lost copsaicinoid content by 3.4 %, 14.8 %, and 38.3 %, respectively; in the BL variety, comparable losses were 1.8 %, 4.4 %, and 13.6 %. A minimum loss of ascorbic acids was recorded during solar tunnel drying. The results showed that well-designed double stage solar tunnel dryers are important for effective drying processes that preserve quality attributes of the chili pepper.