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

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Presentation on theme: "Chapter 10: Introduction to measurement of physical properties and biological effects of food 1."— Presentation transcript:

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

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

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 3

4 Particle size Sieving 4 1000 microns = 1 mm

5 Static light scattering 5

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 6

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 7

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 8

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 9

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 10

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 11

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 12

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 13

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 14

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 15

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 16

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 17

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 18

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 19

20 Differential Scanning calorimetry 20

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

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 22

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 23

24 Emulsion stability 24

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 25

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 26

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 27 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 28

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 29

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 30

31 Test backing 31 Coarse crumb Coring Crust too thick Side wall collapseLoaf small volume Good texture

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 32

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 33

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