No abstract

Evidence is presented that corn (maize) and sorghum starch granules have channels that connect a central cavity to the external environment. A mutant sorghum starch with numerous, large surface pores was treated with a methanolic solution of merbromin and sectioned. Light, fluorescence, and compositional backscattering electron microscopy revealed channels connecting an internal cavity to the external surface in most granules. Cavities and channels could also be seen in whole corn and sorghum starch granules treated with merbromin in methanol and viewed by fluorescence microscopy. Treatment of sorghum starch granules with an aqueous solution of merbromin revealed that the molecule penetrated the granule matrix under even slightly swelling conditions. Light microscopy showed cavities in unstained, whole, dry corn and sorghum starch granules mounted in immersion oil.

No abstract

Cereal Chem. 80(1):91-98Flours of two soft wheat cultivars were fractionated into native, prime, tailing, A-, and B-type starch fractions. Starch fractions of each cultivar were characterized with respect to A/B-type granule ratio, amylose content, phosphorus level (lysophospholipid), and pasting properties to investigate factors related to wheat starch pasting behavior. While both cultivars exhibited similar starch characteristics, a range of A-type (5.7-97.9%, db) and B-type granule (2.1-94.3%, db) contents were observed across the five starch fractions. Though starch fractions displayed only subtle mean differences (<1%) in total amylose, they exhibited a range of mean phosphorus (446-540 mg/g), apparent amylose (18.7-23%), and lipid-complexed amylose (2.8-7.5%) values, which were significantly correlated with their respective A-and B-type granule contents. A-type (compared with B-type) granules exhibited lower levels of phosphorus, lipid-complexed amylose, and apparent amylose, though variability for the latter was primarily attributed to starch lipid content. While starch phosphorus and lipid-complexed amylose contents exhibited negative correlation with fraction pasting attributes, they did not adequately account for starch fraction pasting behavior, which was best explained by the A/B-type granule ratio. Fraction A-type granule content was positively correlated with starch pasting attributes, which might suggest that granule size itself could contribute to wheat starch pasting behavior.

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Because, in general, native starches do not have properties that make them ideally suited for applications in food products, most starch is modified by dervatization to improve its functionality before use in processed food formulations, and because food processors would prefer not to have to use the modified food starch label designation required when chemically modified starches are used, there is considerable interest in providing starches with desired functionalities that have not been chemically modified. One investigated approach is property modification via physical treatments, that is, modifications of starches imparted by physical treatments that do not result in any chemical modification of the starch. Physical treatments are divided into thermal and nonthermal treatments. Thermal treatments include those that produce pregelatinized and granular cold-water-swelling starches, heat-moisture treatments, annealing, microwave heating, so-called osmotic pressure treatment, and heating of dry starch. Nonthermal treatments include ultrahigh-pressure treatments, instantaneous controlled pressure drop, use of high-pressure homogenizers, dynamic pulsed pressure, pulsed electric field, and freezing and thawing.

No abstract

To observe granular reaction patterns within modified starch granules, starch derivatives were converted to thallium(I) salts and viewed by scanning electron microscopy compositional backscattered electron imaging. Observation of phosphorylated potato and sorghum starches and a hydroxypropyl analog of waxy maize starch revealed that granular patterns of reaction were influenced by both starch and reagent types. In waxy maize and sorghum starches, flow of reagent into the granule matrix occurred from channels (laterally) and cavities (from the inside outward). In potato starch granules, which do not possess channels, reagent diffused inward through exterior granule surfaces. Phosphoryl chloride (highly reactive) reacted to a large extent at granule surfaces, while the propylene oxide analog (less reactive) appeared to diffuse into the granule matrix prior to reacting.