Proliferation of Yeasts and Molds in Low Moisture Foods

In foods of low water activity, yeasts and molds proliferate by changing their internal cell moisture concentration form high to low water activity. To do this, they synthesize some water binding molecules (solutes) which can be used at their discretion to trap water from the surrounding environment. The trapped water is then used for biochemical reactions required for growth. E.g. of these solutes include glycerol, glucose, sucrose and sorbitol.
The presence of these compounds also decreases the water activity of the fungi cells, thereby buffering them and enabling equilibrium between the cells and the environment. Increase in solute concentration and decrease in water activity of the cells allow the movement of water from the environment into the cell, thereby enabling growth and proliferation of the yeast and mold.

Chocolate Bloom

Chocolate bloom is the situation whereby fat crystals that were formed initially upon cooling a piece of chocolate, melts, migrates to the surface of the chocolate, and recrystallizes into a more stable form. This gives rise to an unstable appearance that is characterized by a dull color and grayish spots on the chocolate.
Chocolate bloom can be prevented by tempering of the cocoa fat before chocolate manufacture, and by hydrogenation of the emulsifier (usually lecithin) to a more stable form before use in the manufacture of chocolate.
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Natural Pigments in Foods

Chromaphoric compounds are substance that contain chroma. There are functional groups called chromaphores which when added to a saturated hydrocarbon, causes absorption in the UV or visible range, to produce colors. Saturated hydrocarbons are colorless, but when a chromaphore is added to it, a color compound is produced.
E.g. of simple chromaphores include the following:

1. Carbonyl Group
This is a carbon atom bonded to an oxygen atom like in Aldehydes and Ketones.

2. Azo Group
This is a Nitrogen atom bonded to another Nitrogen atom. It absorbs light in the visible UV region. Most synthetic food colors are in the form of azo-dyes. E.g. of an compound with an azo group is Azo Benzene.

3. Nitrate Group
This is a Nitrogen atom bonded between two Oxygen atoms. This chromaphore also absorbs light in the visible UV region.

4. Ethylene Group
This isolated chromaphoric group absorbs light at a wavelength of 190nm. It consists of two double-bonded carbon atoms. The amount of light absorbed by this chromaphoric group as part of a complex compound varies, depending on its molar absorptivity. When a compound contains conjugated ethylene groups, the wavelength for light absorption increases.

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Flavor Development in Plants and Animal Tissues

All flavor components in plants and animal tissues are as a result of cellular metabolism. Since cell structures in plants and animals are different, there exist some similarities and differences of flavor development in these tissues to a certain degree.
A striking similarity is in the production of metabolites by enzymic reactions. Plants and animal tissues undergo physiological reactions at the cellular level which is manifested in food metabolic pathways. The substrates and products of these pathways are compounds that may contribute to the flavor of the tissues. E.g. The production of acetaldehyde in grapes, acyl-amyl acetate in bananas, lactic acid production in meat muscles.
Furthermore, the disruption of cell structure in plant tissues has a profound effect on flavor development that is similar to the effect of membrane disintegration in animal tissues. When cell wall membranes of biological tissues are disrupted, cellular components are released which contribute to the flavor of the tissues. Also during cell disintegration (grinding, milling, macerating, chewing), various compounds which were formerly contained within separate compartments in the cell are liberated. These compounds react with each other and with compounds in the external environment like oxygen, water, saliva, and enzymes, to form a lot of new compounds contributing to the flavor of the food. E.g. The reaction of amylase in saliva with starchy components in tissues during chewing to form sugars which contribute to the sweet taste of cereals and grains.
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Advantages and Disadvantages of Different Extraction Techniques for Isolating Flavor Compounds from Foods

1. Dynamic Headspace Extraction
Advantages: Rapid analysis, sensitivity, minimal equipment investment, ability to analyze sample with traditional GC.
Disadvantages: Sample must be in liquid state, inherent analytical limitation to compounds that do not co-elute with solvent, contamination by culture medium, presence of solvent impurity peaks.

2. Static Headspace Extraction
Advantages: Minimal sample preparation, simplicity, rapidity, use of little or no solvent, inexpensive.
Disadvantages: Restricted to analysis of volatiles and concentrated compounds that elicit strong detector response, does not detect poorly volatile compounds.

3. Solid Phase Extraction
Advantages: Cleaner extract, easier to automate, higher recoveries for polar compounds.
Disadvantages: Incomplete removal of interferences, low recovery of analytes, high variability in results.

4. Solid Phase Micro-extraction
Advantages: Simple, time saving, high screening, high throughput, eliminates environmental hazards.
Disadvantages: High selectivity of SPME fibers towards chemicals, lack of robustness, low reproducibility of results due to ageing of fiber, presents problems in quantitative measurements.

5. Steam Distillation
Advantages: Low cost, simple.
Disadvantages: Time consuming, high temperatures used may alter compounds.

6. Supercritical Fluid Extraction
Advantages: Good quality extract, efficient, selective, minimized product degradation, eliminates solvent residues.
Disadvantages: High cost, technical skills required.

7. Direct Solvent Extraction
Advantages: Simple, no complex equipment, controlled recovery, large selectivity and flexibility.
Disadvantages: Emulsion formation, not efficient, loss of compounds, complicated, laborious, pre-concentration step required.

8. High Vacuum Distillation
Advantages: Low-cost, high throughput, reduces thermal hazard, efficient, purity of distillate.
Disadvantages: Time-consuming, requires technical skill.

9. Dialysis
Advantage: High purification.
Disadvantages: High cost, technical skill required, time-consuming, large samples needed, loss of raw materials, concentration of dialysates required.