Food 2400 Assignment one food 2400 Guelph University, Assignments of Food science

Download Food 2400 Assignment one food 2400 Guelph University and more Assignments Food science in PDF only on Docsity! FOOD*2400 Assignment #1 Andrea Thayer 1124476 1. a) There are many physical and chemical changes that occur during the freezing process. Freezing occurs when liquid water reaches 0 °C and water’s physical state changes into a solid state. This process is determined by a reduction in water temperature and therefore a decrease in kinetic energy and density in the water molecules changing the structure (De Man, 1999). Crystallization occurs when water transforms from a highly structured liquid to a solid hexagonal lattice pattern that involves less water molecules. Crystallization is a two- step process that includes nucleation and crystal growth. Nucleation is the first step where the nuclei of solid water form ice molecules and crystals grow around it. Heterogeneous nucleation occurs when water is super cooled and small ice crystals are added and as the temperature increases, nuclei develop around foreign particles. Because crystal size depends on the number of nuclei, smaller ice crystals would result in more nuclei and large crystals would have less nuclei. Ice crystals form quicker around freezing point and nucleation happens quicker at temperatures lower than the freezing point. Thus, by increasing the rate of cooling, small crystals will slowly form fine structures and have a high rate of nucleation, and by decreasing the cooling rate, fewer but larger crystals will form faster and have a low nucleation rate (De Man, 1999). By adding nuclei to liquids before freezing can create a finer crystalline structure known as seeding. Recrystallization can occur during the storage of frozen food and cause damage. A rise in temperature melts small crystals and a fall in temperature can cause water to reform these crystals on the surface of others causing damage to the food (Mine, 2021). Glass transition is an additional change that occurs during the freezing process and is important for stability. Glass formation can occur when water-containing food 2. a) Three regions of isotherms: Zone A (monomolecular layer): Water in this zone is the most strongly absorbed and the most immobile water found in food. This water cannot freeze and cannot serve as a solvent. Water activity in this zone is 0-0.05 (Mine, 2021). Zone B (multilayer): Water in this zone is held less strongly to the food. Water is bonded to itself, to the solute water, or through Hydrogen-bonding. This water has solvent properties and water activity in this zone is 0.05-0.76 (Mine, 2021). Zone C (capillary water): Water is the least bound and the most mobile in this zone. It is found in gels and cellular systems and is physically entrapped. It may be frozen, can act as a solvent, and will allow chemical reactions and microbial growth. Water activity in this zone is above 0.76 (Mine, 2021). b) The differences of the isotherms are related to the difference in food composition of the samples. Water activity affects stability of the sample because it’s an indication of how much water is “available” to participate in reactions and for microbial growth and the spoil of food (Mine, 2021). Therefore, foods with high moisture content and higher water activity are the most susceptible and according to the samples, jam, compared to cookies and crackers is the most susceptible to spoil. This is because jam has the highest moisture content due to its hydrophilic properties and high sugar content. The cracker and cookie are less likely to spoil due to having a lower water activity as shown in the graph. The cracker, however, have a lower moisture content than jam but higher than cookies. This indicates higher hygroscopic properties and the composition of the cracker having less margarine than the cookie. Crackers are more stable than jam, but cookies are more stable than crackers because of their low water activity and dryness that make reactions more difficult to occur. Low water activity is needed for soft textured foods to avoid undesired hardness (Kim et al., 1998). c) One factor that can significantly affect the sorption isotherm of a food material is the temperature or change in temperature. When the temperature increases, the moisture content decreases so the isotherm curve shifts to lower values (Kim et al., 1998). 3.a) Upon rehydrating dried apple slices, the slices do not return to their original water content but instead show a phenomenon referred to as hysteresis or the ink-bottle effect. Hysteresis is when food absorbs less water during rehydration compared to dehydration making the sorption isotherms different. Capillaries of food are assumed to be small at the top and larger at the bottom, similar to an ink bottle. During dehydration, the smaller radius (top) controls the emptying of the capillary to lower water activity and during adsorption (rehydration), capillaries will not be filled until the larger radius (bottom) is reached. This results in the food retaining more water during dehydration compared to rehydration (Mine, 2021). b) A factor that can influence hysteresis is the dehydration method that is used. Food that is freeze-dried show more hysteresis after being rehydrated compared to vacuum dried food that show less (Krokida & Philippopoulos, 2007).