Microwave and Blanching Pretreatments for Hot Air Drying of Orange-Fleshed Sweet Potato Slices (Ipomoea batatas)
Ernest Ekow Abano
Abstract
Microwave and steam blanching as pretreatments to hot air drying of orange-fleshed sweet potato (OFSP) were studied. The air-drying experiment was performed at constant temperature of 70°C and airflow of 1.0 m/s. The effective moisture diffusivity varied from <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" id="M1"> <a:mn>1.5</a:mn> <a:mo>×</a:mo> <a:msup> <a:mrow> <a:mn>10</a:mn> </a:mrow> <a:mrow> <a:mo>−</a:mo> <a:mn>9</a:mn> </a:mrow> </a:msup> </a:math> to <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" id="M2"> <c:mn>4.4</c:mn> <c:mo>×</c:mo> <c:msup> <c:mrow> <c:mn>10</c:mn> </c:mrow> <c:mrow> <c:mo>−</c:mo> <c:mn>9</c:mn> </c:mrow> </c:msup> </c:math> m2/s, and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" id="M3"> <e:mn>1.1</e:mn> <e:mo>×</e:mo> <e:msup> <e:mrow> <e:mn>10</e:mn> </e:mrow> <e:mrow> <e:mo>−</e:mo> <e:mn>10</e:mn> </e:mrow> </e:msup> </e:math> to 7.9 <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" id="M4"> <g:mo>×</g:mo> <g:msup> <g:mrow> <g:mn>10</g:mn> </g:mrow> <g:mrow> <g:mo>−</g:mo> <g:mn>10</g:mn> </g:mrow> </g:msup> </g:math> m2/s, for the microwave and blanched assisted hot air drying, respectively. The activation energy obtained for the various microwave-assisted hot air drying was 29.1 W/mm for 4 min, 68.1 W/mm for 3 min, and 79.7 W/mm for 2 min. Ascorbic acid degradation and formation of brown pigments in the OFSP slices were lower in microwave than in steam blanch-assisted drying. Microwave-assisted drying of OFSP is best governed by Page model, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" id="M5"> <i:mi>M</i:mi> <i:mi>R</i:mi> <i:mo>=</i:mo> <i:mi mathvariant="normal">exp</i:mi> <i:mfenced open="(" close=")"> <i:mrow> <i:mo>−</i:mo> <i:mtext>k</i:mtext> <i:msup> <i:mrow> <i:mi>t</i:mi> </i:mrow> <i:mrow> <i:mtext>n</i:mtext> </i:mrow> </i:msup> </i:mrow> </i:mfenced> </i:math> , while the blanch-assisted followed the logarithmic model, <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" id="M6"> <n:mi>M</n:mi> <n:mi>R</n:mi> <n:mo>=</n:mo> <n:mi>a</n:mi> <n:mtext> </n:mtext> <n:mi mathvariant="normal">exp</n:mi> <n:mfenced open="(" close=")"> <n:mrow> <n:mo>−</n:mo> <n:mtext>k</n:mtext> <n:mi>t</n:mi> </n:mrow> </n:mfenced> <n:mo>+</n:mo> <n:mtext>c</n:mtext> </n:math> . To produce better quality OFSP flour, it is recommended to cut the tubers into 3 mm slices, microwave at a power of 630 W for 2 min or blanch for 1 min, 43 seconds prior to hot air drying.