A comparative investigation of the impact of infrared drying coupled with cold storage on selected physicochemical attributes of two Moroccan date (Phoenix dactylifera L.) cultivars

After harvest, date fruit (Phoenix dactylifera L) is characterized by a moderate moisture content (MC) that might negatively affect its storage ability, especially for soft cultivars. Accordingly, drying is often applied at postharvest to reduce MC and maintain the quality attributes of dates during cold storage. In this investigation, the effect of the infrared drying method as an innovative drying technique was tested in association with controlled cold storage conditions to elucidate the changes in the main quality attributes of two soft Moroccan date cultivars with high commercial value. Thus, fresh ‘Mejhoul’ and ‘Boufeggous’ cultivars were subjected to infrared drying (IRD) at 60 °C/50 Hz using an industrial dryer (CONVECO Sp. z o.o., Glinianka, Poland) for 150 minutes. After drying, infrared dried date fruit samples were stored at +4 °C/55% and +4° C/65% of relative humidity (RH) for four months and assessed for water loss, color attributes, total soluble solids (TSS), acidity, and total sugars based on glucose and fructose. Therefore, the structure of the experimental design included four blocks:

• IRD ‘Mejhoul’ stored at +4 °C/55% (M-IRD55).
• IRD ‘Mejhoul’ stored at +4 °C/65% (M-IRD65).
• IRD ‘Boufeggous’ stored at +4 °C/55% (B-IRD55).
• IRD ‘Boufeggous’ stored at +4 °C/65% (B-IRD65).

The results showed that the initial water activity prior to cold storage was, respectively, 0.568 and 0.662 for ‘Mejhoul’ and ‘Boufeggous’, and it was a critical attribute, in addition to RH, in impacting the storage ability of each cultivar. Thus, the highest water loss was recorded for B-IRD55 with -3.97%, followed by B-IRD65 and M-IRD55 with a slight dehydration of -0.78% and -0.32%, respectively. However, a water gain of +1.67% was documented for M-IRD65. All color attributes (lightness, redness, yellowness, and chroma) were changed during storage and influenced the browning index that increased for all blocks after four months of storage, reaching, respectively, 43.4, 38.8, 34.4, and 29.1 for M-IRD65, M-IRD55, B-IRD65, and B-IRD55. Acidity showed an opposite and slight fluctuation for the two cold storage conditions according to the cultivar since this parameter increased for ‘Boufeggous’ and decreased for ‘Mejhoul’. For TSS, only B-IRD55 was highly impacted at the end of storage, while date cultivars stored at lower RH (55%) exhibited a slightly higher sugar content compared to those stored at 65%.

Acknowledgment. This research is part of the project No. 2022/45/P/NZ9/03904 co-funded by the National Science Centre and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 945339.