Influence of pH and xanthan gum addition on freeze-thaw stability of tapioca starch pastes

Effects of pH (3, 7, and 9) and xanthan gum (XG) on freeze-thaw (FT) stability of tapioca starch (TS) pastes (6.0% or 24.0% w/w, total solids) were studied as a function of TS/XG mixing ratios (6.0/0.0, 5.7/0.3, and 5.4/0.6) and FT cycles (−20 °C, 22 h and 30 °C, 1 h up to five cycles). Syneresis results showed that xanthan was most effective in reducing the syneresis at pH 7 and this effect increased with increasing gum concentrations. At pH 3, however, xanthan was much less effective reflected by the highest syneresis produced within two FT cycles. DSC data demonstrated that neither transition temperatures nor enthalpies of the FT-TS/XG pastes were affected by pH and xanthan addition. Photographs of stained starch granules exhibited a phase separation between the added xanthan and starch polysaccharides at all pH values tested. SEM micrographs illustrated the largest ice crystals formation in the pH 3 pastes and in the 0.6% xanthan-added pastes of all pH values. Xanthan maintained textural properties of the tapioca starch pastes during FT treatments better at pH 7 and pH 9. Lower starch molecular weight (revealed by HPSEC) and a reduction in viscosity of xanthan solution caused by acid hydrolysis was probably the cause of the lower FT stability of starch pastes at pH 3.