The Kitchen as Laboratory by Cesar Vega

Author:Cesar Vega
Language: eng
Format: epub
Publisher: Columbia University Press

How Water Is Related to Crispness

All products with a crust present common characteristics and therefore challenges because of their mixed structure, normally having a high-water-content, soft, and deformable interior, surrounded by, or attached to, a dry, firm, and brittle crust. The key issue in these kinds of products is how to maintain the crispy character after preparation; in general, the loss of crispness is due to the diffusion of water from the high-water-content part to the low-water-content, crispy part.

Water content in any given sample indicates how “humid” the food is. However, it is more accurate to speak about water activity (aw). The water activity is a simple thermodynamic measure of the dryness of food. Water activity is related to how free, available, or bound the water is. In addition to the water content, water activity values depend on the concentration and type of dissolved substances in the food—mainly sugars and salt, the existence and distribution of pores, and the relation of water to the “thirsty” sites of the food matrix. Proteins, carbohydrates, and starches possess many sites to which water molecules can strongly bind. Critical water activity values are those in which the products become sensorially unacceptable because they completely lose their crispness.

Water diffusion from moist (high aw to dry (low aw layers causes moistening of the components of the crispy parts. At a critical water activity value, further movement of water causes a change from a glassy state (in which the materials behave as hard and brittle) to a rubbery state (in which the materials behave as leathery, soft, or sticky). As the food is transitioning between the glassy and rubbery states, a loss of crispness takes place.

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