Abstract:

The maca residue left after juice extraction, though rich in dietary fiber including soluble and insoluble fibers,
is often regarded as a waste material. Soluble dietary fiber (SDF) is believed to be beneficial to human health. In the present
study, response surface methodology was employed to optimize the conditions for alkali extraction of SDF from maca
residue and some physiochemical properties of the resulting SDF were characterized such as monosaccharide composition
and average degree of polymerization. As a result, the optimum conditions are found to be to be 9%, 90 ℃, 79 min and
1:48 for alkali concentration, temperature, extraction time and solid-to-liquid ratio, respectively. The actual yield of SDF
under the optimized conditions was 52.6%, which is in close agreement with the predicted one (53.0%) with a deviation of
0.0076. The crystallinity of maca SDF was 69.05% as examined by X-ray diffraction. Some typical absorption peaks of the
SDF were analyzed by FTIR. The SDF was made up of six monosaccharides including arabinose, mannose, galactose, rhamnose,
xylose and glucose. Viscosity-average and number-average molecular weights were approximately 39471 g/mol and 48589 g/mol,
respectively. Its average degree of polymerization was 299. In addition, irregular appearance was also observed together
with large holes in its interior structure and its expansive force was 2 mL/g. Moreover, the oil-absorbing capacity was good,
absorbing 2.69 g of corn oil per g of maca SDF.

Key words: maca, soluble dietary fiber (SDF), response surface methodology (RSM), characterization