NFS386 Lecture 2 - WATER.docx
NFS386 Lecture 2 - WATER.docx

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University of Toronto St. George
Nutritional Science
Debbie Gurfinkel

NFS386 Lecture 2: Water Water  As a constituent of food (part of other foods)  As a beverage Water as a constituent of food Water and Ice  Differences in structures  implications for food quality (freezing affects quality attributes) Water activity  Availability of water in a food system (for chemical reactions)  implications for food quality (e.g. raising keeps for long time; water not available for reactions)  Amount of water available for chemical reactions (e.g. microbial growth) Ice has an ordered hexagonal structure: crystal lattice structure. The structure of liquid water is described by the mixture model: Water retains some hexagonal structure but is more disordered. “Ice like” clusters. When water freezes, it increases in volume. Density of water 1g/mL Density of ice 0.92g/mL Density of water > Density of ice Ice formation has 2 steps 1. Nucleation a. Initiation of ice crystal formation 2. Crystal growth a. Increase in the size of the ice crystal The size of ice crystals depends on the rate of cooling.  As the rate of cooling increases o Number of nuclei increases o Decreased size of crystals because of more nuclei FAST cooling = more small nuclei Freeze concentration: as water freezes, solutes are concentrated in unfrozen zones. Small crystals  dilute solute Large crystals  solute moved to regions outside crystal, more concentrated in certain regions. Freeze concentrations  slow chemical reactions  off-flavors. Freezing will damage the cellular structure of plant/animal foods  Fruits/vegetables/raw meat have high water/moisture content. During freezing, extracellular water freezes first, drawing water by osmosis out of intracellular space. Large crystals/slow  shrinkage of cell Small crystals/fast  less shrinkage of cell Water moves from intracellular  extracellular Thawing  drip loss Properly freeze: cut into thin slices (reduces time cold penetrates), blanch before freezing. Freezing promotes two opposing forces  Low temperatures o Slow chemical reaction rates o Slow growth of microbes, but not completely kill  Freeze concentration increases reaction rates Inadvisable to refreeze food  Increases level of microbes  Food quality Freezer burn: ice at surface undergoes sublimation  Sublimation o ice  water vapor o solid  gas o food dries at surface o prevented by proper packaging Water activity  measure ability of water in food to participate in chemical reactions and support the growth of micro-organisms  predictor of shelf life  depends on water interaction with other food constituents, not just amount of water  increased water activity = increased reactions = increased microbial growth  depends on amount of water in food * interaction with other food How water associates with other food constituents  Bound water o H-bond to polar components of food o Does not behave like solvent o Unavailable for freezing, chemical reactions o Can not support growth of micro-organisms  Bulk water o Physically entrapped in food structure o Behave like solvent o Available for freezing, chemical reactions, supports growth of micro- organisms. Continuum: types of bound water  Constitutional water o Bound to interior of molecule (e.g. Inside fold of protein)  Vicinial water o Bound to ou
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