5. Conclusion
The superficially similar soluble fibres carboxymethylcellulose
and guar gum can both impart a significant improvement in the
in vitro starch digestion behaviour of pasta, reducing the amount
of reducing sugars produced over 300 min by up to 18% (1.5%
CMC incorporation) and 24% (20% GG incorporation). While significant
negative effects on sensory and technological properties were
seen on incorporation of enough GG to reduce the rate of in vitro
digestion, equivalent reductions with CMC incorporation were
accompanied only by an increase in the yellowness parameter,
b⁄, which is a desirable consumer attribute. The dramatic differencedifference
in the amounts of soluble fibre required to bring about equivalent
reductions in digestion rate points to the possibility that
different mechanisms may be involved in the two cases. While
confocal laser scanning microscopy showed the apparent existence
of a ‘‘matty’’ coating of the starch granules at high levels of GG
incorporation, consistent with previous speculation by Brennan,
et al. (2008) and Brennan and Tudorica (2008), no such coating
could be observed in CSLM of the CMC-containing samples. A specific
competitive inhibitory or electrostatic effect of the interaction
of the negatively-charged CMC with a-amylase may have been
responsible for slower starch digestion rate seen with CMC-containing
pasta.