Mechanism of cassava tuber cell wall weakening by dilute sodium hydroxide steeping

- Dilute NaOH steeping solubilises mainly the cassava cell wall pectin fraction.
- NaOH steeping with fine milling reduced cell wall cellulose crystallinity slightly.
- NaOH steeping weakened the cassava cell walls through formation of micropores.
- Dilute NaOH steeping increased cell wall disintegration in ground cassava pieces.
- NaOH steeping effect was additive to milling attrition in increasing starch yield.

Steeping of cassava root pieces in 0.75% NaOH in combination with wet milling was investigated to determine whether and how dilute NaOH modifies cassava cell walls. Gas chromatography data of cell wall constituent sugar composition and Fourier transform infrared (FTIR) data showed that NaOH steeping reduced the level of pectin in cassava cell walls. FTIR and wide-angle X-ray scattering spectroscopy also indicated that NaOH steeping combined with fine milling slightly reduced cellulose crystallinity. Scanning electron microscopy showed that NaOH steeping produced micropores in the cell walls and light microscopy revealed that NaOH steeping increased disaggregation of parenchyma cells. Steeping of ground cassava in NaOH resulted in a 12% decrease in large residue particles and approx. 4% greater starch yield with wet milling. Therefore dilute NaOH steeping can improve the effectiveness of wet milling in disintegrating cell walls through solubilisation of pectin, thereby reduced cell wall strength.