Use your knowledge of either glass transition or moisture sorption theory to briefly explain the following phenomena. Wherever possible illustrate your answer with an annotated state diagram or moisture sorption isotherm.
(a) (a) When you boil pasta it goes from brittle to soft. Why doesn't pasta soften with dry heat (in an oven) orharden on cooling after cooking?
The pasta is in a glassy state. Heating the pasta or adding plasticizer (water can increase the molecular mobility and hence encourage a glass to rubber transition softening the product. Looking at the shape of the glass line to the right of the state diagram we can suggest that both heat and water are needed to increase the molecular mobility enough. Oncooling the pasta looses heat but retains enough moisture to remain in the rubbery state.
(b) (b) Hard candies left in a moist environment become sticky on the outside but stay crunchy in the center
Hard candies are glassy but when they adsorb moisture go through a glass to rubber transition and become sticky. The diffusion of water in the candy is slow so the sticky areas areconfined to the outside.
(c) (c) Freshly baked bread packed immediately in a plastic bag will develop a soft crust but left in an open container will develop a crispy crust. (Hint: Freshly baked bread is still steaming).
By loosing heat and moisture (as steam) the bread looses molecular mobility and at the surface perhaps undergoes a glass transition to a brittle crunchy state. Ifthe water is trapped in the crust it will act as a plasticizer and allow the bread to remain in a soft, rubbery state.
(d) (d) To prepare aqueous samples for visualization in an electron microscope it is necessary to freeze them solid. This is often done by dripping the sample into a liquid nitrogen slush and cracking open the solid balls formed for imaging. Why does this process, knownas vitrification, give better samples than simply putting them in a freezer overnight?
The very rapid cooling brings the system into a region of the state diagram below the melting point where it would normally crystallize. However the cooling is so rapid that before ice crystals can form the sample reaches the glass transition temperature (see text for state diagram of low temp foods) andthe entire system becomes glassy.
(e) (e) Milk fat is often fractionated into high melting and low melting fractions that have added value as food ingredients. Fractionation can be done by cooling to below the freezing point of one component of the fat yet above the freezing point of the other. After crystallization is complete the solid fraction is centrifuged out. Would you coolthe fat quickly or slowly? Why?
The rate of cooling will control the number of nuclei and hence the size of crystals formed. It is easier to separate large crystals from a mixture so we should cool slowly to reduce the nucleation rate and hence crystal size. The total mass of crystals formed in this case will be independent of the cooling rate and only dependant n the final temperature.
(f) (f) Cotton candy is made by melting sucrose then spinning the liquid through narrow nozzles into thin strings. What state is the sucrose in cotton candy? How and why does it change when left in a moist environment for even a relatively short period of time?
| Close up of the spinning head in a cotton candy machine. The center is filled with granular sugar which is heatedto melt temperature then spun to force the melt out through narrow holes |
The hot sucrose melts and as it is extruded through the spinner rapidly cools and, before it can crystallize, forms a glass. The glass easily absorbs moisture from the atmosphere (or after you take a bite) and is plasticized. The increased molecular mobility allows the concentrated sucrose to first enter a...