The Gelatinization of Starch

L_arabinanases are recognised, an exo-L~arabinanase that degrades sugar beet L-arabinose, removlllg the (t+ 3)-hnked "-L-arablllofuranosyl reSldues from the substrate to produce a linear (t+ 5)-"-L-arabinan, then deraded, and a L-arabinanase, of the endo-type, which degrades sugar ~eet arabinan to mainly L-arabinose and arabinobiose, and during the course of hydrolysis produces a series of arabinose oligosaccharides which are subsequently further attached. D-Galactanases exist which are specific for two types of {3- D-galactopyranosyl linkages, namely (t+ 3)and (t+4)-{3-D-galactanases. The extent of hydrolysis of the galactomannans by {3-mannanases of the endo-type is influenced by the degree of substitution of D-galactose on the (t+ 4)-{3-D-mannan backbone. The endo-( t+4)-{3-D-xylanases degrade arabinoxylans, arabinoglucuronoxy lans, glucoronoxylans and xylans to mainly D-xylose, xylobiose, and a series of xylose oligosaccharides of mixed constitution containing Larabinofuranosyl and D-glucopyranosyluronic acid residues. PECTIN ENZYMES W. Pilnik and F. M. Rombouts Department of Food Science Agricultural University, Wageningen, The Netherlands Pectic substances (pectin) are complex heteropolysaccharides. Pectinesterases (PE) transform pectin into low methoxyl pectin and finally pectic acid (pectate) by hydrolyzing the ester bonds. They attack the pectin chain from the reducing end or next to a free carboxyl group and then act along the molecule. In this way blocks of free carboxyl groups are formed which make the pectate extremely calcium sensitive. Polygalacturonases (PG) hydrolyze glycosidic linkages next to a free carboxyl group. Their best substrate is pectic acid and there are endo- and exoPGs. Pectate lyases (PAL) split glycosidic linkages next to a free carboxyl group by {3-elimination. There are exo- and endo-PALs with pectic acid and low methoxyl pectin the preferred substrates. Pectin lyases (PL) split glycosidic linkages next to a methylated carboxyl group by {3-elimination. Only endo-PLs are known for which highly esterified pectin is the best substrate. Commercial pectinases are mostly produced from cultures of Aspergillus niger and contain PE, PG and Plo They can break down pectins either by PL or hy the combined action of PE and PG. Assay methods are discussed. Texture changes during ripening and storage of fruit and vegetables are ascribed to native pectic enzymes. Native PE is responsihle for self clarification of citrus juice and, together with PG, for viscosity changes in tomato juice. Microbial pectic enzymes cause phytopathological phenomena and softness in hrined vegetables. Commercial pectinase preparations are used for the clarification of fruit Juices. Enzyme treatment of fruit and vegetable pulps can increase colour and juice yield upon pressing and be used to obtain macerates for baby foods or nectar bases or cloudifying agents from citrus peels. OBSERVATIONS ON THE STRUCTURE OF THE STARCH GRANULE C. T. Greenwood Cadbury Schweppes Limited, London The basic problem in starch chemistry is understanding the nature and structure of the starch granule, i.e. what is its chemical and physical architecture? The fine structure of the three major component, of the starch granule, amylose, amylo-pectin, and intermediate material,are briefly outlined. Some important physical properties of the granule are covered: its morphology, its birefringence and crystallinity, swelling and gelatinization behaviour, and the effect, of degradation by acid and amylolytic enzymes. Granule characteristics depend on the botanical source. Evidence indicates that the branched amylopectin component is probably primarily responsible for the basic crystallinity of the granule, and a model of various types of starch granule based on this concept IS outlined. The behaviour of the starch granule in baking is briefly considered in the context of these concepts. Starch prohahly contributes more to the structure and texture of baked goods than is generally appreciated.

Department of Applied Biochemistry and Nutrition University of Nottingham Recently attention has been directed to exploring the gelatinization of starch granules under both equilibrium and non-equilibrium conditions with a view to providing a physicochemical rationalization of the many and diverse observations relating to this phenomenon. Equilibrium studies have established that the effects of temperature, water and solutes, e.g. maltose, can be adequately expressed by ideas and equations developed in polymer physics, though some of these have necessitated some elaboration. Non-equilibrium studies are of particular interest because most food processes are of this character. It has been postulated that the process of gelatinization of starch can be visualised in terms of: 1. the diffusion of water; 2. a hydration facilitated helix-coil transition or '"melting"; and 3. a swelling process. The dynamics of some of these prucesses have heen explored by a temperature-jump technique which had led to the hypothesis that starch gelatinization can be visualised as a semi-eo-operative process. In addition, it has been found that the slow, time-dependent decay process for differently treated starches and flours appears to mirror important technological differences such as the effects of chlorination and heat treatment, sometimes difficult to detect at the granule level. STRUCTURAL MODIFICATION OF VARIOUS STARCHES BY EXTRUSION COOKING WITH A TWIN SCREW FRENCH EXTRUDER C. Mercier Institut National de la Recherche Agronomique Nantes, France Various starches were treated by extrusion-cooking and effecl'i of different process parameters studied. X-ray diffraction studies and scanning electron microscopic observations revealed destruction of the starch granule by partial gelatinisation. Simultaneously, a re-organisation of the macromolecules was observed by X-ray diffraction with the appearance of the crystalline fraction, the result of complex formation between the amylose and the lipids of starch. Extrusion of potato starch, free of lipids, and waxy-maize starch, free of amylose, led to an amorphous structure but the addition of oleic acid to the potato starch. resulted in complex formation, The amount of soluble starch after extrusion was dependent on the extrusion temperature, the moisture content of the starch hefore extrusion and the amylose/amylopectin ratio. A relation hetween product expansion, water solubility and the fraction of the starch susceptihle to enzyme attack was observed. When extruded at high temperatures, cereal starches were solubilized without any formation of maltodextrins but potato starch produced linear oligosaccharides. Enzymatic studies showed that amylose was split into oligomers, while the amylopectin fraction remained as in the native starch, This different behaviour was reinforced by the fact that in cereal starches, amylose is complexed with lipids and i's unavailable, while in potato starch amylose is amorphous and thus more susceptible to the processing. THE USE OF MODIFIED STARCHES IN THE FOOD INDUSTRY 1.0'Dell Laing National Ltd., Manchester

1. M. V. Blanshard Food Science Laboratories

The structure of natural starches. particularly their amylose content. imposes limitations on their use in modern food processing plants. The development of waxy maize starch, which has an amylopectin content greater than 98%. assisted in correcting this situation but prohlems relating to pH and shear stability, freeze thaw stability, and aesthetic considerations of texture and consistency required further modification. Two closely related types of modification are used simultaneously, cross-linking and esterification/etherification. By varying the extent of cross-linking, a range of starches can be made to suit processing conditions over a wide range, Many processed foods depend upon low pH or heat treatment as a means of preservation and starches can be manufactured to cope with such conditions. Starches which have proved satisfactory in reducing sterilising times in conventional retorting systems have also proved useful in aseptic canning areas where high temperature, short time processing takes place. The high shear resistant properties of some modified starches make them suitable for use in water and oil emulsified food systems, while the

Can. Inst. Food Sci. Tcchnol. J. Vol. 12, No. I, January 1979

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SESSION 3. STARCH IN FOOD PROCESSING THE GELATINIZATION OF STARCH