Light Microscopy

The most routinely applied microscopy techniques are those collectively categorized under light microscopy. At the heart of these techniques is a microscope equipped with a light source and a series of lenses: the condenser, objective, and ocular. Figure la shows the basic principles of image magnification in a light microscope. A light source provides illumination, that passes through first the condenser, then the specimen, followed by the objective lens producing a reversed upside-down image, which is then further magnified by the ocular lens (6,7). The resolution of light microscopy is between 200 and 500 nm (5). The proper use and limitation of each of these components has been described elsewhere (7).

A specimen's microstructure can only be seen in the microscope when there is some form of contrast. Contrast is

Objective

Specimen

Condenser

Objective

Specimen

Phase plate

Analyzer

Condenser

Illumination source

Polarizer

Phase ring

Polarizer

Figure 1. (a) Schematic of image magnification in light microscopy; (b) phase plates required for phase contrast microscopy (see text for details); (c) polarizers required for polarized light microscopy (see text for details).

usually established through the staining (chemically or physically attaching specific dyes) of particular components. Some commonly used stains in food research are presented in Table 1. Vaughan (4) described in detail the use of various stains to locate specific components in a variety of commodities. An example of the relationship of microstructural changes occurring during processing looked at the effect of mixing on the structure of dough made from different quality flours by observing uranyl-acetate stained gluten fractions (8). Proper dough development required a matrix network of protein strands. For living cells, which may be harmed by stains, a phase ring and plate may be added to achieve phase contrast (7) as shown in Figure lb.

Table 1. Stains of Food Components Used in Light Microscopy Work

Compound

Stain

Starch

Iodine

Protein

Fast green, acid fuchsin

Plant cell walls, muscle tissue

Toluidine Blue O

Fats

Oil Red O, Sudan III

12. Donnelly Marketing, 12th Annual Survey of Promotional Practices, Stamford, Conn., 1990, p. 3.

13. R. Gibson, "Kellogg Shifts Strategy to Pull Consumers In," The Wall Street Journal, Jan. 22, 1990, p. 131.

16. B. Bagot, "1990 Industry Outlooks," Marketing and Media Decisions, 33 (Jan. 1990).

17. W. Wilkie, Consumer Behavior, John Wiley & Sons, Inc., New York, 1990, p. 492.

18. D. Lehmann, Market Research and Analysis, Richard D. Irwin, Inc., Homewood, 111., 1989, p. 3.

John L. Stanton Richard J. George Carol A. Gallagher Saint Joseph's University Philadelphia, Pennsylvania

See also United States food marketing system in the Supplement section.

Why Gluten Free

Why Gluten Free

What Is The Gluten Free Diet And What You Need To Know Before You Try It. You may have heard the term gluten free, and you may even have a general idea as to what it means to eat a gluten free diet. Most people believe this type of diet is a curse for those who simply cannot tolerate the protein known as gluten, as they will never be able to eat any food that contains wheat, rye, barley, malts, or triticale.

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