Effect of physical modification on starch structure and physicochemical properties

Vasanthan, Thavaratnam (1994) Effect of physical modification on starch structure and physicochemical properties. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Lipids from wheat, oat, lentil and potato starches were extracted by acid hydrolysis and by selective solvent extraction with chloroform-methanol 2:1 v/v [CM] at ambient temperature, followed by n-propanol-water 3:1 v/v [PW] at 90-100°C. The acid hydrolysed extracts which represented the total starch lipid [TSL] content ranged from 0.1% (w/w) (potato) to 1.13% (w/w) (oat). The combined action of CM and PW resulted in almost complete removal of starch lipids (>98.6%) from most of the starches, the exception being wheat, where the solvent extraction efficiency (%TSL) was 96.3%. The free lipids in the CM extracts (%TSL) ranged from 5.4% (wheat) to 22.1% (lentil), whereas the free and bound lipids (amylose-lipid complex) in the PW extracts ranged (%TSL) from 77.7% (lentil) to 90.8% (wheat). Neutral lipids (NL) formed the major lipid class in the CM extracts of all starches, while in PW extracts there were NL and phospholipids (PL) in potato, glycolipids (GL) in lentil, and PL in wheat and oat starch preparations. There was a great variation among the starches with respect to the major components of the lipid classes in both CM and PW extracts. Monoacyl lipids were most abundant in cereal starches (>78% TSL). The fatty acid composition of NL, GL and PL in CM and PW extracts was determined. -- The physicochemical properties of PW defatted starches of wheat, oat, lentil and potato were determined by monitoring changes in relative crystallinity (RC), Brabender viscosities, swelling factor (SF), amylose leaching (AML), susceptibility towards acid and α-amylase, thermal properties and retrogradation. The RC of potato and lentil starches increased by 21 and 7.8%, respectively, on defatting, while that of the other starches remained virtually unchanged. Defatting eliminated the pasting peak of cereal starches and increased the thermal stability and reduced the hot paste consistency of all starches. However, these changes were larger in potato and lentil. The SF of all starches decreased on defatting, with the decrease being more pronounced in potato and lentil. In comparison with their native counterparts, the extent of AML at different temperatures was higher in defatted wheat and oat starches. The extent of acid hydrolysis of native and defatted starches of wheat and oat were similar throughout a common time interval (2-20 days). However, during the first stage of hydrolysis, defatted granules of lentil and potato were hydrolysed (4 and 7 days, respectively) to a greater extent than were their native counterparts. Defatted granules of all starches were hydrolysed by porcine α-amylase to a greater extent than were native starches. The gelatinization temperature of defatted wheat and oat starch preparations were slightly lower than their native counterparts. Defatted granules of potato and lentil starch gelatinized over broader and higher temperature ranges than did native starches. The results suggested that amylose and amylopectin chains in native granules are more associated with each other in potato and lentil than in the other starches. Defatting did not hasten or delay the onset time of the retrogradation endotherm in wheat, potato and lentil starches. However, the retrogradation endotherm of defatted oat starch appeared earlier than that of native starch. The differences in the onset (To), peak (Tp) and conclusion (Tc) temperatures of the retrogradation endotherm of native and defatted starch gels were only marginal and remained practically unchanged during the time course of retrogradation. The transition temperatures of retrograded gels were lower than those of gelatinized starches. The enthalpies of retrogradation (∆HR) of defatted starches were higher than those of their native counterparts. At the end of 20 days storage, this increase amounted to 4.7, 3.6, 1.7 and 2.2 J/g in defatted wheat, oat, lentil and potato starches respectively. The gel strength (after a storage period of 24h at 25°C) of all four native starches increased upon defatting. The results suggest that the interactions between reassociating starch chains are of a stronger order of magnitude (due to lipid removal) in defatted than in untreated starches. -- A heat-moisture treatment study was carried out with native wheat, oat, lentil, yam and potato starches. The starch samples were heat treated at 100°C for 16 h at moisture contents between 10 and 30 %. The heat-treatment did not change granule size and shape. In oat starch, granules were less compactly packed after heat-treatment. The X-ray diffraction intensities increased in wheat, oat and lentil starches, but decreased in potato and yam. The X-ray patterns of wheat and oat starches remained unchanged, while those of lentil, potato and yam starches became more cereal like. In all starches, the swelling factor and amylose leaching decreased, being more pronounced in potato. Heat-treatment induced complex formation between amylose and native lipids. Differential scanning calorimetry of the heat-treated samples showed a broadening of the gelatinization temperature range and a shifting of the endothermal transition towards higher temperatures. These changes were more pronounced in potato starch. The gelatinization enthalpy of wheat, oat and lentil starches remained unchanged, but those of potato and yam starches decreased on heat-treatment. Heat-treatment increased the 95°C Brabender consistency of wheat starch, but decreased those of oat, lentil, potato and yam starches. In all starches, thermal and shear stability increased after heat-treatment. Acid hydrolysis decreased on heat-treatment of wheat and lentil starches, but increased in oat, potato and yam starches. However, in potato and yam starches the foregoing trend was evident only during the first seven days of hydrolysis. Thereafter, acid hydrolysis was more pronounced in native than in heat-treated starches. The susceptibility towards hydrolysis by porcine pancreatic α-amylase decreased on heat-treatment of wheat and lentil starches, whereas increases were observed for oat, potato and yam starches. The results indicated that the extent of starch chain associations within the amorphous regions and the degree of crystalline order are altered during heat-moisture treatment. The magnitude of these changes were found to be dependent upon the moisture content during heat-treatment and on the starch source. The effect of heat-moisture treatment (100°C, 30% moisture for 16 h) on the thermal and textural characteristics of retrograding starch gels from wheat, oat, lentil and potato were also investigated. The heat-treatment did not hasten or delay the onset time of the retrogradation endotherm in wheat, potato and lentil starches. However, in oat starch, heat-moisture treatment advanced the onset time from 15 days (native) to 6 days. The differences in To, Tp and Tc of the retrogradation endotherm of native and heat-moisture treated starch gels were only marginal, and remained practically unchanged during the time course of retrogradation. However, the enthalpy of retrogradation (∆HR) increased upon heat-moisture treatment in wheat, oat and lentil starches but decreased in potato. Furthermore, in native and heat-moisture treated starches, the ∆HR increased with storage (at 25°C) time. The gel strength (after a storage period of 24h at 25°C) of all four starches increased upon heat-moisture treatment and more marked in potato starch. The results indicated that the changes in thermal characteristics and texture of a heat-moisture treated starch gel on storage is determined by the interplay of the changes in crystallinity, SF and AML on heat-moisture treatment. -- Annealing was studied with native wheat, oat, lentil and potato starches. The starch samples were annealed at various starch/water ratios at 50°C for time intervals ranging from 0.5 to 72 h. Annealing did not change granule size and shape. Oat starch granules were less compactly packed after annealing. X-ray diffraction patterns remained unchanged, and X-ray intensities changed only marginally in all starches. The SF, AML and the gelatinization temperature range (GTR) decreased on annealing. The extent of decrease in SF and AML followed the order: lentil > wheat > potato > oat, while the corresponding order for GTR was: wheat > lentil > oat > potato. The gelatinization transition temperatures (GTT) and enthalpy (∆H) increased on annealing. However, the increases in GTT and ∆H did not begin concurrently during the time course of annealing. Increases in ∆H were slower and were evident only at 1, 2, 6 and 48 h respectively, in lentil, potato, oat and wheat starches. The extent and rate of increase in GTT and ∆H followed the order: potato > lentil > wheat > oat. The magnitude of changes in GTT and ∆H increased with increase in annealing moisture content. The susceptibility of oat starch to enzyme and acid hydrolysis increased on annealing. However, decreases occurred in the other starches (lentil > wheat > potato). Thermal and shear stability of starch granules increased on annealing (potato > lentil > wheat > oat). The results showed that the above changes in physicochemical properties were due to increased interaction between and among starch components during annealing. Annealing (50°C, 75% moisture for 72 h) did not hasten or delay the onset time of the retrogradation endotherm in wheat potato and lentil starches. However, in oat starch, annealing advanced the onset time from 15 days (native) to 6 days. The differences in To, Tp and Tc of the retrogradation endotherm of native and annealed starch gels were only marginal and remained practically unchanged during the time course of retrogradation. The ∆HR increased in all four native starches not only upon annealing, but also with the time of storage. The increase in gel strength was marginal in wheat and oat starches after annealing, but was more pronounced in potato and lentil starches. The results indicated that the degree of chain reassociation in an annealed starch gel is influenced by the double helical content within the annealed granule prior to gelatinization.

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