Biochemical and rheological properties of gelling pectic isolates from buttercup squash fruit

In order to examine the gelation properties of pectins isolated from ‘Supermarket’ buttercup squash (Cucurbita maxima × Cucurbita moshata), flesh tissue was extracted with 100 mM HCl, 200 mM citric acid or 200 mM HNO3, at 100 °C. All extracted material contained <40% pectin (galacturonic acid equivalents), along with starch and a significant proportion of co-extracted, unidentified carbohydrate material. Citric acid-extracted pectin was of much longer chain length than that extracted with HCl or HNO3. Pectin extracted with HCl or citric acid was 40–50% esterified, while HNO3 extraction reduced esterification to ∼32%. The isolates were used to gel high sugar/low pH solutions. Storage (G′) and loss (G″) moduli recorded during sample cooling and rewarming showed that, despite their low degree of esterification, pectins in isolates extracted with HCl or citric acid produced typical biopolymer gels with G′>G″ and little frequency dependence of both G′ and G″ at 20 °C. Pectins extracted with HNO3 formed only weak gels. Gels containing the citric acid-extracted pectin were already forming solid structure at 90 °C, similar to a commercial rapid-set pectin, while a sol–gel transition began for the HCl-extracted samples at <70 °C, similar to a commercial slow-set pectin. A longer extraction time coupled with removal of starch by enzyme digestion during preparation affected isolated pectin chain length and esterification, resulting in altered subsequent gel rheology, depending on the extraction acid.