THERMOMECHANICAL PROPERTIES OF VEGETABLE TISSUE AT 30-90 oC Jiří Blahovec

STRUCTURE OF VEGETABLE TISSUE WITH STARCH (Demonstrated on potato tissue)

POTATO CELLULAR STRUCTURE Cell Walls Intracellular Content

CELL WALL MEMBRANE Lipid Double Layer Pore – Based on Proteins Polar hydrophilic side Hydrophobic sides Lipid Double Layer symmetrical Pore – Based on Proteins source of asymmetry

STARCH GRAINS Pukhraj, (b) Kufri Jyoti, (c) Kufri Badshah Dimension, Shape & Composition: amylose x amylopectin Pukhraj, (b) Kufri Jyoti, (c) Kufri Badshah Kaur et al., Food Chemistry 2002

DIFFERENT TUBER PARTS AT ROOM TEMPERATURE (a) pith (b) cortex Starch grains Protein micelles Cell wall Arranged under Karlsson & Eliasson (2003), LWT 36, 735-741.

MECHANICAL PROPERTIES and cell walls

POTATO Deformation Curves in Compression at room temperature Squeezing out ~ 3 %

POTATO Slopes of Deformation Curves Room Temperature True slopes: I Variable, flaccid tissue II about 3 MPa, quasi-elastic III (1.54±0.13) MPa, squeezing out

DYNAMIC MECHANICAL ANALYSIS (DMA)

TYPES OF LOADING

DEFORMATION PARAMETERS Specimen dimensions: length 22 mm width 8 mm thickness 3 mm Distance between jaws 4.4 mm Amplitude 1 mm Frequency 1 Hz Temperature Range 30-90 oC Heating Rate 1 oC/min Tuber tissue from cortex parenchyma Long axis of the specimen parallel to tuber axis

DEFORMATION PARTS Part I Deformation of flaccid tissue (generally inelastic) Turgor pressure ≤ 0 Part II Quasi-elastic deformation Turgor pressure > 0 Cell internal pressure ≤ critical stress for squeezing out of cellular sap water Part III Mechanical dewatering (inelastic)

COMPLEX MODULUS OF ELASTICITY

COMPLEX MODULUS OF ELASTICITY Temperature Derivatives

COMPLEX MODULUS OF ELASTICITY Amplitude

COMPLEX MODULUS OF ELASTICITY Loss Tangent

ELECTRIC CONDUCTIVITY (Role of Temperature) Personius, Sharp (1938) Loss of cell wall semi-permeability

PARTIAL CONCLUSION Mechanical and texture properties are controlled by properties of cell walls at least in part II of deformation curves at critical temperatures

STARCH GELATINIZATION IN POTATO DSC Pith Cortex Stem End Bud End

STARCH GELATINIZATION IN POTATO Combination of DSC and DMA

STARCH CHANGES First Heating POTATO STARCH CHANGES First Heating Arranged under Ratnayake & Jackson (2007) Carbohydrate Polymers 67, 511-529. Tissue DMA Peak

POTATO STARCH CHANGES Shear Rheology X-Ray Diffraction Arranged under Ratnayake & Jackson (2007) Carbohydrate Polymers 67, 511-529. Kaur et al. (2002), Food Chemistry 79, 183-192 Shear Rheology X-Ray Diffraction Partial Crystallinity Crystallinity (%) Nearly amorphous 10 45 Partial Crystallinity 45

POTATO STARCH CHANGES Swelling Arranged under Li & Yeh (2001) F. Food Eng. 50, 141-148

ROLE OF TEMPERATURE Depending on air humidity (drying level) Under Nilsson et al. (1958), Physiologia Plantarum 11, 818-837. Depending on air humidity (drying level) Solutions Starch Critical Temperature

CONCLUSIONS Aftercritical deformation of potato tubers (i.e. at temperatures higher than 60oC) is controlled by starch gelatinization, DMA detects gelatinization in state in which the thermally controlled processes detected by DSC are finished,

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