Optimization of Heat Pump–Assisted Intermittent Drying

Abstract

In the present study, heat pump–assisted drying of salak fruit was optimized by dividing the dehydration process into three distinct phases, namely, the initial, intermittent, and final stages. Drying variables considered for the optimization were the intermittent duration (X ₁), intermittent ratio (X ₂), and intermittent cycle (X ₃); the response variables studied were the total drying time (Y ₁), total heating time during intermittent drying (Y ₂), total heating time after intermittent drying (Y ₃), total color change (Y ₄), ascorbic acid content (Y ₅), and total phenolic content (Y ₆). Response surface methodology was used to determine the best combination of the drying variables that could provide the shortest drying period and premium product quality. Experimental results showed that all of the response variables were improved under the optimized intermittent drying conditions compared to the conventional method using constant drying conditions. The optimized heat pump–assisted intermittent drying reduced the drying time by 36% and improved phytochemicals retention with ascorbic acid and total phenolic content recorded at 18.4 ± 1.8 mg ascorbic acid/100 g dw and 43.3 ± 2.2 mg gallic acid equivalent (GAE)/g dw, respectively. The color change of the final product was minimum with a ΔE* value of 7.26 ± 2.03.

Keywords:

• Heat pump
• Intermittent drying
• Optimization
• Response surface methodology

ACKNOWLEDGMENTS

The authors are grateful to the Ministry of Agriculture for the financial support through e-science funding (05-02-12-SF0014) and support from the University of Nottingham, Malaysia campus.

Notes

X ₁ = Intermittent duration (min); X ₂ = Intermittent ratio, α; X ₃ = Intermittent cycle.

AA = mg AA/100 g dw; TPC = mg GAE/g dw.

Mean values within the same column marked by the same letter are not significantly different (p > 0.05).

^a Constant drying until equilibrium moisture content.

X ₁ = Intermittent duration (min); X ₂ = Intermittent ratio, α; X ₃ = Intermittent cycle; Y ₁ = Total drying time (min); Y ₂ = Total heating time during intermittent drying (min); Y ₃ = Total heating time after intermittent drying (min); Y ₄ = Total color change, ΔE*; Y ₅ = Ascorbic acid content (mg AA/100 g dw); Y ₆ = Total phenolic content (mg GAE/g dw).

^a Significant at 99.99% confidence level (p < 0.0001).

^b Significant at 95% confidence level (p < 0.05).

^c Not significant (p > 0.1).

^a Significant at 99.99% confidence level (p < 0.0001).

^b Not significant (p > 0.1).

Y ₁ = Total drying time; Y ₂ = Total heating time during intermittent drying; Y ₃ = Total heating time after intermittent drying; Y ₄ = Total color change; Y ₅ = Ascorbic acid content; Y ₆ = Total phenolic content.

NA = not applicable. Mean values within the same row marked by the same letter are not significantly different (p > 0.05).