The glass transition, its nature and significance in food processing

  • J. M. V. Blanshard
Chapter

Abstract

Although the glass transition is a comparatively recent subject of study in physical science and even more so in food science, it is a phenomenon which is widely observed in natural systems. For example, the opening of the pods and the scattering of seeds by many plants relies on dehydration from a rubbery to a highly elastic and brittle glassy condition. Likewise, the formation of spiders’ webs from a rubbery material that can be spun, to one which produces a robust elastic structure is the consequence of a rubber-to-glass transition. On the domestic scene, the ‘ironing’ of a cotton sheet relies on the glass → rubber → glass transition of cellulose. There is also strong evidence that the resistance to dehydration damage and the protection against freezing temperatures observed in both animals and plants arises in many cases from transitions in the prevailing physicochemical system from rubbery to glassy states.

Keywords

Sugar Cellulose Crystallization Sucrose Starch 

Abbreviations

αw

Water activity (see chapter 1)

αp

Coefficient of expansion under constant pressure

αT

Ratio of the relaxation phenomenon at temperature T to the relaxation at the reference temperature T ref

C’g

Concentration of the unfrozen matrix at the glass-transition temperature T’g

Cp

Specific heat at constant pressure

De

Deborah number = λ/θ

δ

Phase angle, where tan δ = E”/E’

E’

Elastic or storage modulus (equivalent to G’ chapter 5)

E”

Loss modulus (equivalent to G”, chapter 5)

H

Enthalpy — heat content per unit mass

k

Rate of reaction at temperature T

kg

Rate of reaction at the glass transition

λ

Characteristic relaxation time, i.e. the time for a reaction/change to take place

η

Viscosity

Mt

Mass of water taken up at time t

θ

Characteristic diffusion time, e.g. the time for water to diffuse into a glassy sucrose layer and make it rubbery

Tg

Glass-transition temperature

T’g

Temperature coordinate of the point of intersection of the extrapolated liquidus curve with the T g curve in a state diagram. It is the T g of the maximally freeze concentrated solution

Tm

Melting temperature

TE

Eutectic temperature, where under equilibrium conditions total solidification would normally be predicted

T2

The relaxation time in nuclear magnetic resonance (NMR) measurements which relates to the protons returning to their natural random spin alignment, after having been aligned by the instrument’s magnetic field

T1

The relaxation in NMR measurements which relates to the time constant with which magnetic spins come to equilibrium with their surroudings

T1p

A varian of T 1 in the rotating frame, which permits measurements at lower frequencies of molecular motion

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© Chapman & Hall 1995

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  • J. M. V. Blanshard

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