Atomic force microscopy of pea starch: origins of image contrast

Getting the feel of food structure with atomic force microscopy

This article describes the progress in the development of the atomic force microscope as an imaging tool and a force transducer, with particular reference to applications in food science. Use as an imaging tool has matured and emphasis is placed on the novel insights gained from the use of the technique to study food macromolecules and food colloids, and the subsequent applications of this new knowledge in food science. Use as a force transducer is still emerging and greater emphasis is given on the methodology and analysis. Where available, applications of force measurements between molecules or between larger colloidal particles are discussed, where they have led to new insights or solved problems related to food science. The future prospects of the technique in imaging or through force measurements are discussed.

Expression of an amylosucrase gene in potato results in larger starch granules with novel properties

Expression of amylosucrase in potato resulted in larger starch granules with rough surfaces and novel physico-chemical properties, including improved freeze-thaw stability, higher end viscosity, and better enzymatic digestibility. Starch is a very important carbohydrate in many food and non-food applications. In planta modification of starch by genetic engineering has significant economic and environmental benefits as it makes the chemical or physical post-harvest modification obsolete. An amylosucrase from Neisseria polysaccharea fused to a starch-binding domain (SBD) was introduced in two potato genetic backgrounds to synthesize starch granules with altered composition, and thereby to broaden starch applications. Expression of SBD-amylosucrase fusion protein in the amylose-containing potato resulted in starch granules with a rough surface, a twofold increase in median granule size, and altered physico-chemical properties including improved freeze-thaw stability, higher end viscosity, and better enzymatic digestibility. These effects are possibly a result of the physical interaction between amylosucrase and starch granules. The modified larger starches not only have great benefit to the potato starch industry by reducing losses during starch isolation, but also have an advantage in many food applications such as frozen food due to its extremely high freeze-thaw stability.

Structural variability between starch granules in wild type and in ae high-amylose mutant maize kernels

Starch granule structure within wild-type and ae high-amylose mutant maize kernels has been mapped in situ using light, electron and atomic force microscopy, and both Raman and infra-red spectroscopy. The population of wild-type starch granules is found to be homogenous. The ae mutant granule population is heterogeneous. Heterogeneity in chemical and physical structure is observed within individual granules, between granules within cells, and spatially within the kernel. The highest level of heterogeneity is observed in the region where starch is first deposited during kernel development. Light microscopy demonstrates structural diversity through use of potassium iodide/iodine staining and polarised microscopy. Electron and atomic force microscopy, and infra-red and Raman spectroscopy defined the nature of the structural changes within granules. The methodology provides novel information on the changes in starch structure resulting from kernel development.

Advances in research on structural characterisation of agricultural products using atomic force microscopy

Atomic force microscopy (AFM) has many unique features compared with other conventional microscopies, such as high magnification with high resolution, minimal sample preparation, acquiring 2D and 3D images at the same time, observing ongoing processes directly, the possibility of manipulating macromolecules, etc. As a nanotechnology tool, AFM has been used to investigate the nanostructure of materials in many fields. This mini-review focuses mainly on its latest application to characterise the macromolecular nanostructure and surface topography of agricultural products. First the fundamentals of AFM are briefly explained. Then the macromolecular nanostructure information on agricultural products from AFM images is introduced by exploring the structure-function relationship in three aspects: agricultural product processing, agricultural product ripening and storage, and genetic and environmental factors. The surface topography characterisation of agricultural products using AFM is also discussed. The results reveal that AFM could be a powerful nanotechnology tool to acquire a deeper understanding of the mechanisms of structure and quality variations of agricultural products, which could be instructive in improving processing and storage technologies, and AFM is also helpful to reveal the essential nature of a product at nanoscale.

Natural Food Nanostructures

The development in the early 1980s of new nanoscience tools such as probe microscopy and, in particular, atomic force microscopy, has provided new methods for probing food structures at the molecular level, under near native conditions. The development and use of microscopic techniques in food science has always led to new scientific understanding of food structure and has spawned new technological applications. The availability of probe microscopes has allowed the investigation and solution of previously intractable problems in food science. Such understanding provides a basis for selecting or manipulating the natural nanostructures formed by food molecules, but through rational, rather than empirical selection of new raw materials, or the improvement and new design of food materials through conventional processing methods. Nanoscience thus enables the improvement of natural nanostructures, through the use of standard and accepted selection and processing methods. This approach is illustrated through studies on starch and protein-stabilised foams and emulsions. It is shown how improved understanding of food structure at the molecular scale can be used to select, modify, or design food structures to meet current challenges in regard to nutrition and health. The use of nanoscience to enable the selection of new improved raw materials, and to modify conventional processing methods, provides a basis for designing new functional foods. The status of such products is discussed in the light of the wider debate on nanotechnologies and food.

Structural Transformations during Gelatinization of Starches in Limited Water: Combined Wide- and Small-Angle X-ray Scattering Study

Rice flour (18-25% moisture) and potato starch (20% moisture) were heated with continuous recording of the X-ray scattering during gelatinization. Rice flours displayed A-type crystallinity, which gradually decreased during gelatinization. The development of the characteristic 9 nm small-angle X-ray scattering (SAXS) peak during heating at sub-gelatinization temperatures indicated the gradual evolution into a stacked lamellar system. At higher temperatures, the crystalline and lamellar order was progressively lost. For potato starch (B-type crystallinity), no 9 nm SAXS peak was observed at ambient temperatures. Following the development of lamellar structures at sub-gelatinization temperatures, B-type crystallinity and lamellar order was lost during gelatinization. On cooling of partially gelatinized potato starch, A-type crystallinity steadily increased, but no formation of stacked lamellar structures was observed. Results were interpreted in terms of a high-temperature B- to A-type recrystallization, in which the lateral movement of double helices was accompanied by a shift along their helical axis. The latter is responsible for the inherent frustration of the lamellar stacks.

Spherulitic crystallization in starch as a model for starch granule initiation

The influence of cooling rate and quench temperature on the formation of spherulitic morphology in heated mung bean starch is reported. Spherulites were obtained for a wide range of cooling rates (2.5-250 degrees C/min), provided the system was heated to 180 degrees C and then cooled below 65 degrees C. Branched crystalline structures were also observed, as was a gellike morphology. The dissolution temperature for spherulitic material ranged between 100 and 130 degrees C. A second dissolution endotherm was observed between 130 and 150 degrees C in systems containing gellike material. Spherulites revealed B-type X-ray diffraction patterns. Spherulitic crystallization of starch following phase separation is proposed as a model for starch granule initiation in vivo.