food2400 UNIT 1

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University of Guelph *

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2400

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Chemistry

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Oct 30, 2023

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docx

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2

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UNIT 1 1. List some unique chemical and physical properties of water and ice Water -high melting and boiling point, surface tension, dielectric content, heat capacity, heat phase transitions, -low density, expand upon solidification -high thermal conductivity compared to other liquid -v shaped ; oxygen is very electromagnetic so it pulls electron from hydrogen – this polorized nature results in associations between neighboring water molecules. Ice -Thermal conductivity higher than water at 0C. conduct heat energy faster than immobilized water. -Ice has a thermal diffusivity approximately nine times greater than that of water -tissues freeze more rapidly than they thaw - As a product freezes, ice forms from the outside in whereas when a product thaws, water forms first on the outside and the centre remains frozen. -less dense than water (91%) -hexagonal pattern 2. Describe some interaction of water with other molecules; bound water - water that tightly associated with molecular structures. - reduced molecular mobility and can’t act as solvent - do not freeze at -40 Non-covalent bonds (hydrogen, ionic, polar) Tendency of water to associate with hydrophilic substances to varying degrees depends on the nature of the non-aqueous constituents, salt, composition, pH, and temperature. Hydrophilic substance interact strongly with water by ion-dipole and dipole -High dielectric constant of water allows it to disrupts electrostatic interaction between ions and dissolve them -ions in water affect or disrupt water structure -Negative ends of the water dipole (oxygen free electron pairs) will orient themselves towards cations and positive ends of the same dipole (hydrogen atoms) will orient themselves towards anions. -Water molecules around a non-polar solute began to loose its entropy. As a result, separated non-polar groups in an aqueous environment tend to associate with each other rather than with the water molecule -Clathrate hydrates are ice-like structures where water (the host) physically entraps low molecular weight compounds known as guests – van der waals force or electrostatic interaction.
3. Define what is meant by water activity Unbound water. Water that is available to participate in degradative activities 4. Explain the 3 regions represented in a sorption isotherm; 5. Explain the differences between bound water and free water in food systems. a. In the first region (Zone A), the water is known as monolayer water. The water molecules are tightly bound to the food molecules in this region. The amount of water in this region is the approximate amount of water needed to form a single layer over the solid. This water acts as a solid instead of a solvent and cannot be frozen at -40 C b. In Zone B, the water is known as multilayer water. The water activity increases from 0.2 to up to 0.85 as the water content increases (Damodaran & Parkin, 2017). The water in this zone interacts and binds to another water molecule or food molecule through hydrogen bonds. This water can act as a plasticizer and has solvent properties (Mine, 2022). c. Zone C is known as bulk water. The water activity is around 0.75 and 0.85. As the water in this phase is loosely bound and can move freely, it promotes the chemical reaction, microorganism growth, and physical changes that are inhibited in Zone A and B (Damodaran & Parkin, 2017). 6. Explain the importance of sorption isotherms to food stability; plots of water activity (a w ) vs. water content (mass of water per unit mass of dry material) in a food at a constant temperature. Useful in predicting stability with moisture content below 50% 7. Define what is meant by hysteresis; and the adsorption and desorption curves on a sorption isotherm are not always superimposable. In the ink bottle theory , it is assumed that if the capillaries have narrow necks and large bodies, they look somewhat like an inkbottle. On adsorption, the capillary will not be completely filled until the water activity corresponding to the large radius (R) is reached. Alternatively, during desorption, the smaller radius (r) of the pore neck controls the emptying of the capillary, so that the water activity is lower.
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