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Sunday, May 4, 2008

Perspectives of Starch Functionality and Methods of Analysis in Food Systems

Dr. Ralph D. Waniska, Professor
Cereal Quality Laboratory
Department of Soil & Crop Sciences

Texas A&M University
Perspectives on the applicability of analysis methods and how these methods can be used to monitor the critical processing parameters and product characteristics will be discussed. This position paper will review some of the functional properties of starch and how some of these properties are measured. I will not attempt to address all the functional aspects of starch nor all the methods to measure starch functionality.
Starch functions in many ways in food systems. Starch functionality is affected by the type, source, and structure of starch, the matrix of flour particles, pretreatments, processing conditions, storage parameters, etc. Documenting starch structure-process-function relationships are necessary to resolve day-to-day variations that we observe in manufacturing of all starch-based foods.
Review of Starch Functional Properties
Native starch granules function in many food systems by being an insoluble solid (part crystalline, but mostly amorphous solid). Native starch is associated with 0.5-1.0 g water / g starch and enables the preparation of batters and doughs with >50% solids. Some water remains unbound, i.e., mobile or free, even in these high solids mixtures. Starch granules swell reversibly as long as the heat and pressure are lower than gelatinization conditions. As processing conditions, i.e., water content (plasticizer), temperature, and pressure, approach starch gelatinization conditions, molecular motion of starch chains increase. Amorphous regions of starch can begin to be mobile and starch chains can associate to form double helixes, i.e., annealing of starch, as gelatinization conditions are approached. Increased or decreased crystallinity of starch granules can result from annealing depending upon processing parameters before, during and after the gelatinization step (Kuntson, 1990).
Gelatinization of native starch dramatically changes its functionality during processing of foods (Slade and Levine, 1995). Amorphous solids are plasticized (normally with water) and mobilized, crystalline solids are plasticized, melted, and mobilized, and some gelatinized starches are dispersed during the heating process (Table 1)(Waniska and Gomez, 1992). Gelatinization disrupts intra- and inter-molecular associations formed during synthesis and packing of starch molecules in the granule. Gelatinized starch is associated with 5 to 30 times its dry weight with water; it is a hydrated polymer, a hydrocolloid. The rigid, swollen, gelatinized, starch granules cause thickening (sol) or gelling (gel) effects in many food systems.
Table 1. ‘Dispersion’ of Gelatinized Normal Starch During Processing
Process (Sequence)
Initially (excess water)
+ Time + Temp.
+ Time + Temp. + Pressure-Shear
+ Time + Temp. +++Pressure-Shear
* Percent (%) of amylose or amylopectin dispersed and soluble in water.
Values in Table 1 vary with source, type and structure of starch and flour particles; but processing parameters correspond to amounts of starch dispersed and granule characteristics. Initially, amylose leaches from the gelatinized granule but amylopectin can be forced out of the gelatinized granule. More time, temperature, plasticizer, and/or pressure--shear are required to weaken the rigid starch granule, to cause deformations in the granule, and to cause granule contents to disperse. [Note: the term ‘pressure--shear’ represents the range of forces affecting starch during processing, where pressure refers to static forces and shear refers to dynamic forces]. More aggressive processing conditions (higher temperature, higher pressure--shear with sufficient moisture (plasticizer) content) cause gelatinized starch granules to disperse their contents (Table 1)(Waniska and Gomez, 1992). Molecular starch, starch dimers, starch aggregates, microgels of starch, and/or chunks of the gelatinized starch granule goes into the liquid phase of the food, when gelatinized starch granules disperse. The dispersion of gelatinized starch granules corresponds to a substantial decrease in viscosity. Hence, retention of the granular structure of gelatinized starch in food systems is the aim of many food processors.
Polymers of gelatinized starch immediately associate with other components in the food system. These associations can be described as tacky, sticky, or rubbery, especially if amylopectin is the principal dispersed polymer (Slade and Levine, 1995). This is the situation in foods prepared from waxy (mostly amylopectin) starches. Normally, some of the amylose leaches out of the granule and is the principal type of starch between gelatinized granules and other food components. Amylopectin, however, does not leach from the granule as quickly; and thereby, it’s concentration is enriched inside of the gelatinized granule during processing in ample water and less aggressive conditions. More aggressive conditions cause more amylopectin to be dispersed yielding products with a sticky or tacky characteristics.
Reassociation (retrogradation) of gelatinized starch occurs almost immediately after gelatinization. Stable hydrogen bonding between linear segments of amylose occurs post gelatinization in most food processes (Wu, et al., 1992). If the temperature is <115>oC , amylose quickly begins the retrogradation process to form an aggregate or a gel network depending upon concentration and amylose size. In many food systems amylose rapidly (within seconds or minutes) forms a gel network that traps amylopectin. The rapid ‘setting’ of the structure of bread, buns, and rolls are examples of amylose retrogradation. Lower temperatures of food during storage decreases the rate of formation of molecular associations between amylose molecules.
Stable hydrogen bonds between amylopectin molecules post gelatinization occur at <>oC. Many short chains of amylopectin slowly (in hours to days) form double helixes that pack into crystalline structures. Retrogradation of amylopectin gels occur at temperatures as low as -28 oC during weeks of storage.
All starch transformation mentioned above are ‘non-equilibrium’ processes (Biliaderis, 1991). This means that what occurs to starch does not happen instantaneously, and typical processing times may not be sufficient for an equilibrium to be achieved. Rates of these “non-equilibrium processes depend upon the conditions of time, temperature, plasticizer, and pressure-shear. Starches are very large molecules, i.e., 50,000 to 3,000,000 daltons for amylose and 500,000 to 50,000,000 daltons for amylopectin. Each glucose residue has several potential hydrophilic and hydrophobic interactions which translate to millions of inter- and intra-molecular interactions per starch molecule. Mobilizing a large polymer means disruption of thousands of intermolecular associations at about the same time. This is probably the reason that gelatinized starch granules retain some structure many degrees above their melting temperature even in excess water. Increased temperature, plasticizer, and/or pressure-shear, decrease the time of starch transformations, but a uniform extent of transformation is still not assured. Thus, understanding starch functionality requires information about how starch structures (granular and molecular) are affected by processing conditions to yield desired functional attributes, i.e., starch structure-process-function relationships.
Tools to Measure Starch Properties
What starch characteristics / properties are important? What methods provide information on how to optimize processing conditions and/or to maintain the quality characteristics of the product? These are questions that food scientists and starch chemists regularly address during product development, scale up, and manufacturing, and during storage and distribution of products.
Two categories of measurements, i.e., analytical and functional, provide insight into starch properties. Analytical measurements provide details of the chemical and physical properties of starch, but may not predict functionality in complex food systems during processing. Functional starch measurements can be related to processing parameters and product characteristics and shelf-stabilities. The problem with functional measurements are their interpretation, since conditions of measurement of non-equilibrium phenomena may not replicate processing conditions.
Analytical instruments provide information about fundamental attributes of starch. Differential scanning calorimetry (DSC) yields the temperature range and amount of energy required to melt crystalline regions of starch. Both native and retrograded starch contain crystalline regions which can be quantified by DSC. Gel permeation or size exclusion chromatography (SEC) columns can separate and appropriate detectors quantify amounts of amylose and amylopectin. Starch must be quantitatively extracted into heated methyl sulfoxide or cetyl trimethylammonium bromide solutions to yield analytical results. The molecular size and symmetry of amylose and amylopectin can be determined using viscometry and/or laser light scattering detectors connected to the SEC system. The swelling capacity of starch can be quantified using thermal mechanical analysis or by the concentration of molecules too large to enter the gelatinized starch granules. The viscosity of swollen granules can be determined using cone-and-plate rheometers. Several microscopic analysis methods provide information on size and shape of granules and interactions between granules.
Instruments measuring empirical, dynamic, and/or functional attributes of starch can provide information related to process and product characteristics. Simple measures of water solubility and water absorption using times and temperatures similar to normal processing conditions should relate to starch functionality. Measures of damaged starch by solubility in alkaline solution, susceptibility to enzymes, or change in size or shape using microscopy, relate to extent of processing and nutritional potential. Viscosity of starch (flour) as a function of solids contents can be measured using empirical viscometers at fixed or variable temperatures, i.e., temperatures that mimic cooking and cooling of the food. Modern instruments record viscosity using diverse heating and cooling profiles and stirring rates and yield data that should correspond to thermal treatments, pressures, pumping and shearing actions that occur during food processing. Textural measurements, e.g., adhesiveness, cohesiveness, yield stress, viscous flow, and rigidity, of starch sols and gels yield information about starch functionality during processing. These dynamic, empirical methods substantiate the non-equilibrium nature of starch functionality by yielding statistically different data using different measurement conditions.
Analytical methods can be used to measure starch functionality. SEC can quantify amylose and amylopectin solubilized outside of the starch granules (at each step of the food process), yielding amounts and molecular properties of soluble amylose and amylopectin interacting with other food components. DSC (Slade, et al., 1996) and differential mechanical analysis can be utilized in non-theoretical ways to determine pre- and postprocessing effects on starch swelling, melting, dispersion, and retrogradation; and thereby, acquiring functional information about starch functionality in low shear food systems.
Measuring Critical Functional Attributes of Starch
What are the critical functional attributes of starch in the formula, process, and product? Which analytical or functional methods approximate the critical functional attributes of the food system? How should these attributes be measured? I will discuss these concepts using data from several recent papers.
Foods are generally heated to gelatinize some or most of the starch to utilize starch functionalities of increased viscosity and gel formation. Heating foods to temperatures to achieve less than complete melting of starch yields less viscous gels that retrograde slower than foods with completely melted starch (Fig.1)(Fisher and Thompson, 1997). Retorted foods (temperature > 115 oC, to completely melt starch (Fan, et al., 1996a)) contain gelatinized starch that is more evenly distributed in the aqueous phase and which initiates retrogradation more slowly (Fig. 1). Rate and type (annealing vs retrogradation) of reassociation of heated starch depends upon extent of melting and dispersion and the process / storage conditions that promote/inhibit polymer mobility.
Many foods achieve temperatures near than the boiling point of water (100oC). The starch crystals are melted in these foods, but only some of the gelatinized starch is leached or dispersed from the gelatinized granules (Fisher and Thompson, 1997). Hence, the food can contain an amylose enriched aqueous phase and an amylopectin enriched dispersed phase (gelatinized starch granules).
Reassociations of amylose and/or amylopectin occur quite rapidly in many food systems. Starch associations are the structural basis of most bakery products and are considered the cause of staling of these same bakery products. The immediate retrogradation of amylopectin of ae wx corn starch (Fig.1) after cooking (Fisher and Thompson, 1997), suggests that nucleation sites remain in the gelatinized starch or that some chain lengths of amylopectin have a propensity to associate. The rapid association of starch in corn tortillas (Fig. 2) (Fernandez, 1998) causes substantial changes in the swelling potential of gelatinized starch molecules and granules during the first hour after production. The initial viscosity exhibited below 75 oC decreased while the viscosity exhibited above 80 oC increased immediately out of the oven and during the first 24 hours of storage.
Long linear chains of glucose, e.g., amylose and longer chains of amylopectin, associate quicker to form more stable crystals than do the short chains of amylopectin (Goodfellow and Wilson, 1990; Yuan, et al., 1993). Hence, chain lengths of amylopectin, i.e., the nonreducing ends of branched amylopectin structure, affect local, intermolecular, and macromolecular functionality of the starch-based food.
Rate and extent of starch associations arise from differences in starch structure, i.e., chain length, molecular composition, and branching locations [as well as, the normal time, temperature, plasticizer, and pressure--shear processing conditions]. Different chain lengths are observed between and among (Fig. 3) starch sources (Villareal, et al., 1997). Other necessary aspects are where branches occur (frequency, proximity, and location) in the starch molecule and the 3-dimensional conformation of hydrated starches at different stages of retrogradation.
Molecular composition of starch that leaches from the gelatinized starch granules varies form what is retained within the granules (Bello, et al., 1995; Ong and Blanshard, 1995; Eerlingin, et al., 1997). Hence, functionality of leached starch should change as a function of the amount and type of starch distributed into the aqueous phase during processing. Methods to characterize the different properties of starch inside and outside of the gelatinized starch granules need to be utilized to understand the effects of processing parameters, as well as, ingredient functionality.
Gelatinization and retrogradation properties of starch are modified by chemical modification. Some corn endosperm mutants yield starch that functions like cross-bonded, phosphated starch (Yuan, et al., 1993). These observations support the effects of starch molecular structures on functionality and suggest that intermolecular associations can be increased or decreased by the combination of specific starches. Gel strength and retrogradation (Fig. 4) of some starch mixtures are synergist or antagonistic, not just additive properties (Obanni and Be Miller, 1997). Measurement and application of structure-process-function relationships will provide alternative solutions to chemical modification of starch.
Food ingredients affect the hydration, swelling, leaching, dispersion, granule integrity, pasting viscosity, and retrogradation of starch-based foods. Sucrose and lactose delayed pasting and decreased pasting viscosity more than did dextrose (Kim and Walker, 1992; Eerlingen, et al., 1994; Fan, et al., 1996a,b). Sugars' effects on starch functionality varied with the type of starch. Emulsifiers also delayed pasting but emulsifiers’ effects were modified by the amount of sugar present. Very low levels of sulfite decreases the rigidity of gelatinized starch granules and increases dispersion of starch molecules during processing of foods (Paterson, et al., 1996). Even higher levels of sulfite and lactic acid present during corn wet milling, yielded starch granules that had decreased rigidity of gelatinized starch granules and increased dispersion of starch during processing (Shandera and Jackson, 1996).
Processing conditions affect starch functionality in many food systems since every starch transformation is a ‘non-equilibrium’ process. Moisture content dramatically affects extrusion processing conditions and extrudate properties (Whalen, et al., 1997). Initial viscosity development of extrudates prepared using rice flour increased as moisture content went from 15 to 26%; but the pasting viscosity profile abruptly changed when 29% moisture was evaluated (Fig. 5). Pasting curves of extrudates of other cereals yielded the opposite results, i.e., lower initial viscosity development as moisture content increased (data not shown)(Whalen, et al., 1997). Obviously, starch structure and/or the matrix of particles in rice flour behave differently than those of other cereals. Again, documenting starch structure-process-function relationships are necessary to resolve day-to-day variations that we observe in manufacturing of all starch-based foods.
Vibrations, pulsating pumping action, shearing, etc. occur during most food processes and/or during transport. These dynamic physical motions can affect the dissolution of gelatinized starch or their reassociations (retrogradation), since every starch transformation is a ‘non-equilibrium’ process. Pulsations that occur at about a thousand cycles per minute, causes gently gelatinized amylopectin to increase viscosity (Fig. 6) and to become opaque (Dintzis, et al., 1996). Amylose did not participate in the increase in viscosity or opacity. Hence, dynamic measurement of gelatinized starch viscosities yields process information about product stability (and clarity).
Application of dynamic rheometry to food products has been limited due to the ‘theoretical’ basis of the method and problems of holding solid materials without slippage. Compression of solids along with the application of dynamic, oscillatory motions yields nontheoretical information about breads (Fig. 7) (Weipert, 1997). Apparent storage (E’) and loss (E”) moduli can be obtained over a range of shear rates and deformations. Both moduli increased during storage of bread, while both moduli were lower for softer products, i.e., wheat bread compared to rye bread. These apparent rheological values may not characterize theoretical properties of breads, but their application yields information consistent with empirical data from these samples. Documenting starch structure-process-function relationships yield better understandings of fundamental and empirical properties of ingredients, processes and products.
Starch undergoes dynamic transformations and interactions during processing into foods Measurement of these dynamic, non-equilibrium characteristics of starch is critical for the understanding of formulation, processing and product characteristics. Every starch transformation is a ‘non-equilibrium’ process and the processing history of the polymer affects its functionality. Rates of these “non-equilibrium processes depend upon the conditions of time, temperature, plasticizer, and pressure-shear.
Starch chemists and technologists apply analytical and functional methods to model systems. Analytical and functional methods , I predict, will be increasingly applied to complex food systems in theoretical and non-theoretical ways. This will generate more and, hopefully, better information about the critical attributes of starch functionality in food systems.