Microstructure and β-carotene bioaccessibility of processed and processed and stored orange-fleshed sweet potatoes (Ipomoea Batatas(L.)LAM)

Abstract

Orange fleshed sweet potatoes (OFSP) [(Ipomoea batatas (L.) Lam] are a good source of provitamin A carotenoids and their content of β-carotene is sufficient to meet vitamin A dietary requirements. They can therefore be useful in alleviating the problem of vitamin A deficiency (VAD). The amount of β-carotene available to humans is, however, influenced by retention during processing and preparation as well as by its bioaccessibility. Bioaccessibility is particularly relevant because there are many factors that limit the release of carotenoids from the OFSP matrix. This study investigated changes in β-carotene bioaccessibility associated with processing and storage of orange fleshed sweet potatoes and its relationship to the changes in tissue microstructure. The effect of fat and dietary fibre on the bioefficacy of β-carotene from OFSP was also studied.

Roots of two varieties of OFSP; Ejumula and NASPOT 10 O were each stored in a pit (17-21oC, RH 90-100%), saw dust (19-23oC, RH 86-100%), dark room (24.5-28oC, RH 68-100%) and ambient (24-27 oC, RH 68-100% ) conditions. Samples were drawn monthly from each of the storage treatments and analyzed for changes in β-carotene content using high performance liquid chromatrography (HPLC), in vitro bioaccessibility and tissue microstructure. In vitro bioaccessibility of β-carotene was determined using simulated in vitro digestion model followed by membrane filtration to separate the micellar fraction containing bioaccessible β-carotene. Microstructure was studied by obtaining OFSP roots sections and staining them with Periodic Acid Schiffs (PAS) reagent followed by visualisation in the light microscope. To study the bioefficacy of β-carotene from OFSP, 3% OFSP powder diets with varying amounts of fat (3, 6, and 12%) and soluble fibre (0.24%) were fed to vitamin A (VA)-depleted Mongolian gerbils (n = 85) for 3 wk (8 groups, n = 10/group, except control, n = 9). Before the feeding experiment started, there was a baseline kill (n = 6). Two additional 3% OFSP diets containing 6% fat and 3 or 9% white-fleshed sweet potato (WFSP) powder with soluble fibre content of 0.42% and 0.80%, respectively were fed to gerbils.

Results showed that the percent in vitro bioaccessibility in stored OFSP remained highest in Ejumula roots stored under pits (22.9±1.59 to 19.6± 1.34) followed by the roots stored under saw dust (23.4±0.51 to 15.9±1.78). Roots of OFSP stored under ambient and dark room conditions retained the least amount of β-carotene and percent in vitro bioaccessibility. The cell walls of roots stored under ambient and dark room conditions showed higher levels of lignification compared to those stored under saw dust and pits implying that conditions of storage affected the microstructure of the roots. Processed OFSP had significantly higher (P < 0.05) β-carotene bioaccessibility compared to the raw forms. The highest amount of bioaccessible β-carotene was found in Ejumula (150.79 ± 9.67 μg/g dm) and NASPOT 10 O (101.14 ± 9.04 μg/g dm) processed by deep frying. Bioaccessibility varied with processing treatments in the order; raw < baked < steamed/boiled < deep fried. Light microscopy showed that the microstructure of OFSP was disrupted by the processing methods employed. The cell walls of OFSP were sloughed by the heat processing methods applied.

All OFSP diets that were fed to gerbils maintained vitamin A status while 12% fat and WFSPadded diets improved VA status above baseline (P < 0.05). β-carotene bioefficacy in gerbils fed 12% fat (3.5 + 1.4 μg β-carotene:1 μg VA) was lower than in 3% fat and β-carotene groups (6.5 + 3.7 and 6.7 + 3.7 μg β-carotene:1 βg VA, respectively) (P < 0.05), but not statistically different from the WFSP-added groups or the 6% fat group with no added WFSP. A positive linear relationship between dietary fat and β-carotene bioefficacy was observed while soluble fibre did not reduce β-carotene bioefficacy. Processing improves bioaccessibility of β-carotene in OFSP while temperature influences β-carotene content and increases bioaccessibility in stored OFSP roots. The findings show that OFSP varieties studied are adequate sources of β-carotene and may be important in combating VAD in developing countries.