Effects of drying and extrusion on colour, chemical composition, antioxidant activities and mitogenic response of spleen lymphocytes of sweet potatoes

Antioxidative and physiological property changes of freeze-dried, hot air-dried and extruded products made from two different colours of sweet potatoes (yellow and orange) were investigated. The results showed that there were no significant differences of dietary fibre content amongst all the treatments, except that the extruded products of orange sweet potatoes had higher dietary fibre contents. However, the water solubility index (WSI) values and antioxidant content were significantly different between yellow and orange sweet potatoes, and significantly different amongst freeze-dried, hot air-dried and extruded samples. The WSI values of the extruded samples were significantly higher than those of the hot air- and freeze-dried samples. The freeze-dried samples of orange sweet potatoes had more total phenolic compounds, β-carotene, and anthocyanin and had better scavenging effect on DPPH radicals than had freeze-dried yellow sweet potatoes. However, the scavenging effect on DPPH radicals was appreciably greater for hot air-dried than freeze-dried yellow sweet potatoes, but this trend was opposite for orange sweet potatoes. The extrusion process significantly increased the WSI values, scavenging effect on DPPH radicals and total phenolic compounds but decreased the β-carotene and anthocyanin for both yellow and orange sweet potatoes. At lower concentration (25–50 μg/ml), the methanolic extract of freeze-dried yellow sweet potatoes possessed a greater capacity of increasing the mitogenic response than did freeze-dried orange sweet potatoes; at higher concentration (100–200 μg/ml), the proliferation of the lymphocytes increased when stimulated with mitogen Con A for all the samples. This suggests that mitogen Con A selectively promotes T-cell-dependent proliferative activity.