The effect of acid hydrolysis on granular morphology and physicochemical properties of native cereal starch granules

This study examined the composition (moisture, lipid content, ash content, nitrogen content, amylose content, and starch damage), granule morphology, physicochemical characteristics (X-ray diffraction pattern, degree of crystallinity, degree of swelling, amylose leaching, gelatinization parameters), and the average degree of polymerization (DP) of cereal starches (normal maize, waxy maize, amylomaize V, and VII, rice and oat) in their native state and after acid hydrolysis (2.2N HCI at 35°C). -- The moisture contents of the cereals starches were in the range 10.4 -12.2% (dry basis). The total lipid content in normal maize, waxy maize, amylomaize V, amylomaize VII, rice and oat starches was 0.86, 0.01, 1.21, 1.49, 1.01, and 1.13%, respectively. The surface (unbound) lipids constituted 0.06%, 0.01%, 0.04%, 0.11%, 0.03% and 0.11% of the total lipid in normal maize, waxy maize, amylomaize V, amylomaize VII, rice and oat starches, respectively. The bound lipid content in normal maize, amylomaize V, amylomaize VII, rice and oat starches was 0.76%, 1.16%, 1.33%, 0.98%, and 1.01%, respectively. However, waxy maize was devoid of bound lipid. The ash and nitrogen contents of the cereal starches were in the range 0.09 - 0.45% and 0.01 - 0.05% respectively. The apparent amylose content was 23.7%, 1.1%, 49.0%, 66.9%, 15.2%, and 20.9% in normal maize, waxy maize, amylomaize V, amylomaize VII, rice, and oat starches, respectively. The total amylose content was 26.5%, 1.1%, 61.9%, 78.4%, 20.6%, and 29.3%, respectively, in normal maize, waxy maize, amylomaize V, amylomaize VII, rice and oat starches. The percentage of total amylose complexed by native lipids was 10.6%, 20.8%, 14.7%, 26.2%, and 28.7%, in normal maize, waxy maize, amylomaize V, amylomaize VII, rice and oat respectively. The extent of starch damage was low in all starches (0.3 - 0.7%). -- The average granule diameter of native normal maize, waxy maize, amylomaize V, amylomaize VII, rice and oat starches ranged from 7-8, 5-15,4-16, 6-13, 2-7, and 5-12 μm respectively. Granules of native rice and oat starches were polygonal to irregular in shape. Maize starches granule were polygonal to irregular in shape. Except for native oat starch, the surface of other native cereal starch granules were covered with fissures or pores of varying diameter. -- The average degree of polymerization (DP) in native normal maize, waxy maize, amylomaize V, amylomaize VII, rice and oat starches were, 813, 685, 1247, 1285,1389, and 708, respectively. -- Native normal maize, waxy maize, rice, and oat starches, exhibited the typical 'A'-type X-ray spectrum of cereal starches. Amylomaize V and VII starches exhibited a 'B'-type X-ray spectrum. The relative crystallinity of the native starches followed the order, waxy maize (49%) > rice (36%) > normal maize (34%) > oat (32)> amylomaize (19%)> amylomaize VII (16%). -- The swelling factor (SF) of native starches followed the order waxy maize> normal maize> oat> rice> amylomaize V> amylomaize VII. Amylose leaching (AML) of native starches followed the order rice> normal maize> oat> amylomaize Vll> amylomaize V. -- The gelatinization transition temperatures range (Tc-To) for native starches followed the order; amylomaize VII> amylomaize V> rice> waxy maize> normal maize> oat. The gelatinization enthalpy (∆H) of the native starches followed the order: waxy maize> rice> normal maize> amylomaize VII> amylomaize V> oat. -- All starches exhibited a two stage hydrolysis pattern. A relatively high rate was observed during the first 6 days, followed by a slower rate thereafter. At the end of the 8th day of hydrolysis (corresponding mainly to the degradation of the amorphous region of the granule), normal maize, amylomaize V, amylomaize VII, waxy maize, rice and oat starches were hydrolyzed to the extent of 61.1, 32.6, 28.5, 68.1, 62.0, and 64.4% respectively. Between the 9th and 15th day (corresponding mainly to the degradation of starch crystallites) the increase in the extent of hydrolysis was more pronounced in normal maize (9.3%) than in oat (8.3%), rice (8.1%), waxy maize (7.2%), amylomaize V (3.0%), and amylomaize VII (3.1%) starches. At the end of the 15th day, normal maize, amylomaize V, amylomaize VII, waxy maize, rice and oat starches were hydrolyzed to the extent of 73.4,37.0,32.3,77.3,75.3, and 72.9%, respectively. -- In all starches, the number and size of pores on the granule surface increased after acid hydrolysis. In addition, granules were either deformed or fragmented. In all acid treated starches, DP decreased rapidly during the first 48h. The extent of decrease in DP beyond 48h, followed the order amylomaize VII > amylomaize V > normal maize > rice ~ waxy maize > oat. -- Acid hydrolysis increased X-ray intensities, but had no influence on the X-ray diffraction pattern. Acid treatment decreased (7.4%) the relative crystallinity (RC) of waxy maize starch, but increased (7.7-8.9%) RC of the other starches. -- Granule swelling was monitored by changes in swelling factor (SF). Acid hydrolysis for 24h increased the SF (rice> amylomaize V> amylomaize VII> oat> normal maize). However, granule swelling was not detectable in waxy maize starch. A decrease in SF occurred after 2 days hydrolysis in oat and rice starches, but after 3 days for maize starches. Granular swelling was not detectable in oat, rice and normal maize starches after the 4th, 5th, and 8th, day of hydrolysis, respectively. However, amylomaize V, and VII showed measurable SF even after the 8th day of hydrolysis. Hydrolysis for 24h, increased amylase leaching (AML) in all starches (oat> normal maize> amylomaize V> amylomaize VII> rice). Thereafter, AML decreased gradually. The extent of this decrease followed the order: oat> normal maize> rice> amylomaize V> amylomaize VII. -- In all starches, To (onset temperature of gelatinization) decreased, but Tp (midpoint temperature of gelatinization), Tc (conclusion temperature of gelatinization) and Tc-To (gelatinization temperature range) increased with acid hydrolysis. The extent of increase in Tc-To (after 24h hydrolysis) followed the order: waxy maize> amylomaize V> normal maize> oat> rice. The extent of increase in ∆H (enthalpy of gelatinization) after 24h hydrolysis, followed the order: amylomaize V> rice> oat> normal maize> waxy maize