Effect of hydroxypropylation on the structure and physicochemical properties of native, defatted and heat-moisture treated potato starches

Perera, Chandani (1998) Effect of hydroxypropylation on the structure and physicochemical properties of native, defatted and heat-moisture treated potato starches. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Native potato starch was physically modified by heat-moisture treatment (100°C, 16h, 30% moisture) and defatting (75%, n-propanol water, 7h). The changes in structure and physicochemical properties on heat-moisture treatment and defatting were monitored by scanning electron microscopy (SEM), X-ray diffraction, differential scanning calorimetry (DSC), Brabender viscosities, swelling factor (SF) and amylose leaching (AML). SEM showed that neither defatting nor heat-moisture treatment altered the size, shape or the surface appearance of the native starch granule. Heat-moisture treatment decreased X-ray diffraction intensities and altered the 'B' type X-ray diffraction pattern to 'A+B'. The decrease in X-ray intensities on heat-moisture treatment is indicative of crystallite disruption and/or rearrangement of double helices. The gelatinization enthalpy (∆H), Brabender viscosity (at 95°C), SF and AML decreased on heat-moisture treatment, whereas gelatinization transition temperatures (GTT), and thermal stability increased. Defatting increased the X-ray diffraction intensities and altered the X-ray pattern from 'B' to 'A+B'. The increased X-ray intensities on defatting is indicative of interactions between amylose - amylose (AM-AM), amylopectin - amylopectin (AMP-AMP) and amylose - amylopectin (AM-AMP) chains. These interactions in turn, increased GTT, ∆H and thermal stability. However, SF, AML and Brabender viscosity (at 95°C) decreased on defatting. -- The reagents (NaOH and Na₂SO₄) used during hydroxypropylation did not alter granule morphology and AML in native, defatted and heat-moisture treated starches. X- ray diffraction patterns of native and defatted starches changed on alkaline treatment, whereas that of heat-moisture treated starch remained unaltered. These changes reflected double helical disruption (within the amorphous regions), and altered crystallite orientation. In all three starches, alkaline conditions decreased ∆H and Brabender viscosity (at 95°C), and increased SF. The extent of the above changes followed the order : native > defatted > heat-moisture treated. Gelatinization transition temperatures remained unchanged on alkaline treatment. -- Native, defatted and heat-moisture treated starches were converted to a range of hydroxypropyl derivatives using propylene oxide (at concentrations ranging from 2-25%v/w). At 2%(v/w) propylene oxide, all three starches showed similar molar substitution (MS 0.05). However, at 5-25%(v/w) propylene oxide, the accessibility of hydroxypropyl groups into the starch granule followed the order: heat-moisture treated > native > defatted. This showed that the degree of accessibility of hydroxypropyl groups into the granule interior is dependent upon granule crystallinity. In all three starches, an increase in MS progressively decreased GTT, ∆H and AML . The influence of MS on SF of hydroxypropylated native, defatted and heat-moisture treated starches was due to the interplay that occurs between hydrogen bond disruption (due to hydroxypropyl groups) within the amorphous regions, and the increased interactions that occur between starch chains during defatting and heat-moisture treatment. Pasting temperatures of all starches decreased with increased MS. In defatted starch, Brabender viscosity (at 95°C) progressively increased with increase in MS. However, in native and heat-moisture treated starches, Brabender viscosity (at 95°C) began to decrease (due to granule disruption) at MS levels beyond 0.18 and 0.20, respectively. -- Enzyme digestibility studies showed that both defatting (hot 75% n-propanol, 0-7 h) and heat-moisture treatment (100°C, 30% moisture, 0-16 h) increased the susceptibility of potato starch granules towards hydrolysis by porcine pancreatic α- amylase. These differences were attributed to structural changes that occurred within the amorphous and crystalline regions of the starch granule during defatting and heat-moisture treatment. However, hydrolysis decreased (due to formation of new crystallites) when heat-moisture treatment and defatting were continued beyond 8 h and 9 h, respectively. -- Native, defatted (7 h) and heat-moisture treated (16 h) potato starches were hydroxypropylated (to different levels of MS) with propylene oxide (2 - 20%). The results showed that the reagents (NaOH and Na₂SO₄) used during hydroxypropylation increased the susceptibility of the above starches (native > defatted > heat-moisture treated) towards hydrolysis by α-amylase. Addition of propylene oxide (hydroxypropylation) to alkali treated starches, further enhanced their susceptibility towards α-amylase. However, granule susceptibility towards α-amylase did not increase exponentially with increase in MS. The extent of hydrolysis began to decrease at MS levels of 0.29 (native), 0.28 (heat-moisture treated) and 0.26 (defatted). The decrease in hydrolysis at higher MS levels is indicative of the steric effect imposed by bulky hydroxypropyl groups on the accessibility of α-amylase towards the glycosidic bonds of amylose and amylopectin. -- The retrogradation properties of potato starch gels (stored for 24 h at 4°C and then for 34 days at 40°C) before and after physical (defatting and heat-moisture treatment), and chemical (hydroxypropylation) modification were monitored using turbidity measurements, SEM, DSC, X-ray diffraction and enzyme susceptibility. Turbidity development in native, defatted and heat-moisture treated starch pastes during storage (4°C for 24 h and then at 40°C for 34 days) followed the order : native > defatted > heat-moisture treated. In all three starches, the highest rate of turbidity development was observed during the first 24 h of storage (at 4°C). The above results in conjunction with SEM observations showed that turbidity development is influenced by the interaction between leached starch components (AM-AM, AM-AMP, AMP-AMP), and interaction between granule remnants and leached starch components (amylose and amylopectin). In alkali treated gelatinized native, defatted and heat-moisture treated starch pastes, turbidity development (native > defatted > heat-moisture treated) was influenced by aggregation of granule remnants. In native, defatted and heat-moisture treated starch pastes, both the rate and extent of turbidity development (on storage) decreased after hydroxypropylation. This decrease was due to the interplay of : 1) steric effects imposed by hydroxypropyl groups on chain aggregation, 2) aggregation between small granule remnants, and 3) settling of large granule remnants beneath the path of the spectrophotometer beam. -- The intensity (at 5.2A ) of the 'B' type X-ray pattern of gelatinized pastes of native, defatted and heat-moisture treated starches increased during storage (native > defatted > heat-moisture treated). The same 'B' pattern was also evident (at the end of the storage period) after alkali treatment and hydroxypropylation of the above starches. However, the peak at 5.2A was reduced in intensity after alkaline treatment and hydroxypropylation. The extent of the decrease being greater in the latter. -- Fresh pastes of gelatinized native, defatted and heat-moisture treated potato starches were hydrolyzed by porcine pancreatic α-amylase nearly to the same extent (75.3-77.2%). Storage (at 4°C for 24 h) of the above gelatinized pastes decreased (native > defatted > heat-moisture treated) their susceptibility towards α-amylase. However, storage times longer than 24 h did not seem to have any further influence on the enzyme susceptibility of the starch gels. Alkaline treatment increased the susceptibility of freshly gelatinized starch pastes (native > defatted > heat-moisture treated) towards hydrolysis by α-amylase. However, storage (24 h at 4°C) of alkali treated starch gels, decreased their susceptibility towards hydrolysis by α-amylase. Hydroxypropylation decreased the accessibility of α-amylase towards the glycosidic linkages of freshly gelatinized pastes of native, defatted and heat-moisture treated starches (defatted > native > heat-moisture treated). However, the extent of this decrease was not altered during storage (at 4°C for 24 h). -- The retrogradation endotherm (monitored by DSC) of starch gels (native, defatted and heat-moisture treated) occurred after 2 days of storage (4°C for 1 day and then at 40°C for 1 day). A similar trend was also observed after alkaline treatment. However, hydroxypropylated native, defatted and heat-moisture treated starch gels [at nearly the same MS level (0.10-0.11)], showed a measurable retrogradation endotherm only after 7 days (4°C for 1 day and then at 40°C for 6 days). This showed that hydroxypropyl groups are effective in hindering starch chain realignment during gel storage. Both alkaline treatment and hydroxypropylation decreased the retrogradation enthalpies of native, defatted and heat-moisture treated starch gels.

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