Chapter 10: Introduction to measurement of physical properties and biological effects of food

1. Chapter 10:Introduction to measurement of physical properties and biological effects of food

2. Examples of important physical properties • Particle size • Solubility • Water binding / holding • Viscosity • Gel strength • Food thermal analysis • Emulsification • Flour and baking quality

3. Role of physical analysis • Important in predicting and understanding the function of food ingredients in: • food processing behavior • effect on final product • food formulation as rapid predictor of consumer acceptability

4. Particle size • Sieving 1000 microns = 1 mm

5. Static light scattering

6. Solubility / Insolubility • Dissolving in water • Centrifuging • Weighing insoluble residue to give % solubility • AOAC dietary fibre method (985.29) measures ‘soluble’ and ‘insoluble’ fibre under ‘physiological’ conditions

7. Water binding / water holding • Soaking sample in water / buffer • Centrifuging to pellet hydrated fibre • Decanting off free water • Measuring bound water by increase in weight of hydrated fibre compared to dry fibre • Expressed as g water / g fiber

8. Rheology and texture • “Rheology is the study of how materials respond to applied force’” (Nielsen 2003, p 505) • force, deformation and flow • considered a component of food texture • homogeneity is important • Rheology evaluates • stress (force per area) and strain (deformation per length) • normal stress “tension or compression” directly perpendicular to a surface • shear stress “acts in parallel to sample surface” • viscosity internal resistance to flow

9. Rheology and texture(cont) • With increased shear rate fluid viscosity changes (time independent) • linear viscosity increase • viscosity diminishes, shear thinning or pseudoplastic • viscosity increases, shear thickening or dilatent • Liquids that thin & thicken with time are thixotropic & anti-thixotropic respectively • detected by monitoring viscosity at constant shear rate in relation to time

10. In general yield stress is required to make fluids flow • “minimum force, or stress required to initiate flow” • Viscoelasticity; materials display solid like (elastic) and fluid like (viscous) behaviour

11. Rheometry-Rotational viscometer, units RPM; • Comparative viscosity using a Brookfield Viscometer • centipoise units represents the energy required by the viscometer to overcome the resistance to stirring of the sample • study of viscosity is part of rheology – set reading • Test fixture (bob) in contact with sample rotates and shears the sample • as the bob moves through the sample fluid the viscosity impedes free rotation this determines shear stress at the bob surface

12. Rheometry-Rotational viscometer, units RPM (cont) • Need to choose test fixture, concentric cylinder, cone or plate • cylinder good for low viscosity fluids, large sample required • cone or plate good for medium and high viscosity samples small sample required • Shear rate at a constant temperature

13. Role of physical properties in the technological functionality of food • Particle size of insoluble dietary fibres related to acceptability of high fibre products • High solubility of whey protein powders required for powdered beverages and nutritional supplements • Specific viscosities required for protein ingredients and starches in food formulations particularly beverages, sauces, toppings

14. Role of physical properties in the physiological effects of food • Particle size of insoluble dietary fibres influences their effect of bowel transit time which is related to risk of bowel dysfunction • Highly soluble fibre are often highly viscous in GI tract and highly fermentable in colon

15. Rheometry-solids compression, extension and torsion • Strength • Measured in gels made from protein, starch and gums by; • energy required to compress gel • energy required to penetrate gel • texture profile analysis

16. Gel strength • Gel strength important in product development to provide correct texture for high consumer acceptance • processed meat product, desserts, confectionery • Oil binding of vegetable protein, starch and fibre ingredients necessary in meat analogues

17. Food thermal analysis • “Techniques that measure chemical or physical changes of a substance subjected to controlled temperature over time”(Nielsen 2003, p 519) • natural polymers such as amylose and amylopectin or actin and myosin • total combustion of food to determine total mineral and caloric content • Differential Scanning calorimetry (DSC) is used extensively in food thermal analysis • involves measurement of heat absorbed or given • endothermic and exothermic

18. Dynamic thermal analysis –Calorimetry - DSC • “Determination of heat absorbed (endothermic) or given (exothermic) when a definite amount of material undergoes a chemical or physical change” (Neilson 2003, p520) • If test and inert reference samples are heated or cooled concurrently under identical conditions • test sample temperature will be either higher or lower than the reference

19. Differential Scanning calorimetry • This technique records difference in energy influx needed for zero temp. difference between sample & reference material against time or temp. • subject to identical heating or cooling regimes • Measures temperature and enthalpy (H) of transition • sample size between 6-12mg • slow rate of heating 1-10C / min

20. Differential Scanning calorimetry http://people.ccmr.cornell.edu/~mseugrad/stage.jpg

21. Gelatinisation Lipid/amylose melting Differential Scanning calorimetry – wheat starch thermogram

22. Analysis of food emulsions • Emulsion, “two immiscible liquids (oil & water) with one liquid dispersed as small spherical droplets in the other” • oil in water; milk, cream, mayonnaise, salad dressing • water in oil; margarine, butter & spreads • Appearance, texture and stability of these products depend on; • composition, microstructure and colloidal interactions

23. Emulsion definitions • Dispersed or internal phase • substance within the droplets • Continuos or external phase • substances of surrounding liquid • Process of converting water & oil into an emulsion is called homogenisation • may be mechanical, ultrasonic or a colloidal mill • For emulsion kinetic stability - days, months • use emulsifiers and / or thickening agents

24. Emulsion stability

25. Emulsifying Capacity-water soluble emulsifiers • Defined as “maximum amount of oil dispersed in aqueous solution containing specific amount of emulsifier with out the emulsion breaking down or inverting” • slowly add oil to aqueous suspension of protein whilst blending • stable emulsion will form • increase in viscosity • no separation of oil and water phase • endpoint is collapse of emulsion • viscosity suddenly drops • oil and water phases suddenly separate

26. Emulsion stability index • Centrifuge emulsion at given speed & time to predict the stability of an emulsion to; • creaming by using low speed • coalescence by using speeds high enough to rupture the interfacial membranes • may not reflect emulsion instability under normal storage conditions • does not take into account chemical & biochemical reactions • Quantitative method • measure emulsion particle size distribution • laser particle size analysis • measured under similar conditions; • pH, ionic strength, composition, temperature

27. Measure emulsion surface tension; • emulsifier adsorption, packing of emulsifier molecules at interface, critical micelle concentrations & surface pressure increase • Surface tensions is measured by tensiometers Nielsen 1999 pp 578 Coultate, 2002

28. Flour quality • Falling number • measure of -amylase activity • breaks down starch = reduced viscosity of heated flour / water suspension • suspension heated to 100C, stirred for 60 sec. • measure time for plunger to fall through suspension (250 sec. acceptable for bread) • high levels of -amylase; • weaken bread structure, soft sticky crumb, difficult to slice, softens dough and reduces amount of water added during mixing

29. Colour test, indication of flour whiteness • indicates colour of endosperm • affects colour of final bread • indicates amount of bran remaining • The Flour Colour Grade (FCG) is produced by; • placing a flour paste in a glass cell & reflecting / measuring light at 540nm. • low FCG corresponds to whiter flour • FCG affected by, variety, fungal contamination & improper grinding and sieving

30. Test baking • Slow, expensive, needs highly trained staff • 1 to 2 kg flour is mixed and baked via standard method • loaves produced are compared to standard control flours • Key loaf performance indicators • volume (seed displacement) • hight (ruler) • visual assessment (under standard light) • colour (trained expert score 1-10) • texture (trained expert score 1-10) • good texture score; dense, fine bubbles with uniform size distribution

31. Test backing Coarse crumb Crust too thick Good texture Side wall collapse Loaf small volume Coring

32. Small Volume • increase yeast level; optimise dough development ; increase dough weight increase proof time • Crust too thick • reduce gluten level; optimise dough development; reduce pan greasing agent; increase humidity in final proof; increase oven temperature • Side wall collapse • adjust level of bread improver; avoid over-proofing; increase baking temperature and / or time; depan immediately once out of the oven

33. Coarse Crumb Texture • use a suitable bread improver at a correct level; Optimise dough development; Adjust floor time / intermediate proof; Check moulder setting and conditions • Coring near crust • adjust moulding technique; Lower level of pan greasing agent; Use cooler bread pans; Avoid dough skinning during final proof; Correct proofing conditions