Clifton E. Meloan

Determination of Moisture

Moisture determination is one of the most important and most widely used measurements in the processing and testing of foods. Since the amount of dry matter in a food is inversely related to the amount of moisture it contains, moisture content is of direct economic importance to the processor and the consumer. Of even greater significance, however, is the effect of moisture on the stability and quality of foods. Grain that contains too much water is subject to rapid deterioration from mold growth, heating, insect damage, and sprouting. The rate of browning of dehydrated vegetables and fruits and of oxygen absorption by egg powders increases with an increase in moisture content.

Refractometry and Polarimetry

When a ray of electromagnetic radiation strikes a flat surface at an angle, the ray may be bent upward (reflected) or bend downward (refracted) as illustrated in Fig. 27.1. Notice that the ray does not go straight through the material.

Preparation of Samples

The care, time, and effort devoted to the preparation of samples for analysis should be commensurate with the information required and the accuracy and precision of the analytical results desired. If the sample is not prepared properly for analysis, or if the components become altered during preparation, the results will be inaccurate regardless of the effort, the precision of the apparatus, and the techniques used in the analysis (Entenman 1961).

Measurement of Color

Color and discoloration of many foods are important quality attributes in marketing. Although they do not necessarily reflect nutritional, flavor, or functional values, they relate to consumer preferences based on the appearance of the product. Color characteristics of foods can result from both pigmented and originally nonpigmented compounds.

X-Ray Methods

Roentgen discovered X-rays in 1895 and within a few years reported several of their basic properties. Among these properties are the following: 1. X-rays travel in straight lines and with the speed of light. 2. X-rays are not charged particles. 3. The generation of X-rays may be accomplished by impinging a beam of high-energy electrons onto a target, the higher atomic weight targets being the most efficient sources for X-rays. 4. X-rays effect a change in photographic emulsions. 5. Electrical charges are dissipated when exposed to X-rays. 6. X-rays induce fluorescence in many materials such as calcium tungstate and zinc sulfide.

Theory of Spectroscopy

In this chapter, we discuss the theory of spectroscopy, that is, the interaction of radiation with matter, which is the basis for many common instrumental techniques of analysis. In succeeding chapters, the various types of spectroscopic analysis and the instruments used in each are considered in more detail. A knowledge of how these instruments work is a prerequisite for their maximum utilization. When an instrument manufacturer sells a piece of equipment, that equipment is guaranteed to meet certain specification and performance levels. However, most instruments are capable of delivering far more than that, and if analysts understand how the equipment works and what it is supposed to do, they can usually extend its practical capabilities manyfold. It is also necessary to have an understanding of how an instrument operates in order to know whether the data obtained from it are correct.

Visible and Ultraviolet Spectroscopy

The energies associated with the visible and ultraviolet (UV) regions of the electromagnetic spectrum range from 160 to about 585 kJ/mol. Such energies are sufficient to cause electronic transitions within molecules and to ionize many substances. The electrons involved are the outer or bonding electrons, and the nomenclature used to describe the process is based on the bonds formed. When two or more atoms unite to form a molecule, the electrons involved can form sigma bonds (α) or pi bonds (π); there also may be some unused or nonbonding (n) electrons.

Objective versus Sensory Evaluation of Foods

The overall quality of food can be divided into three main categories: quantitative, hidden, and sensory (Kramer 1966). Some quantitative aspects of food quality are primarily of interest to the processor, for example, yield of product obtained from a raw material; others are of interest both to the consumer and manufacturer, for example, the ratio of more expensive to less expensive foods or components in a processed food. In some cases the ratio can be evaluated roughly by sensory methods. Hidden quality attributes include the nutritional value of a food or the presence of toxic compounds that, generally, cannot be determined by sensory evaluation. They include, for instance, the vitamin C content of juices, or the presence of trace amounts of pesticides from spraying fruits and vegetables. Sensory attributes of quality guide the consumer in his selection of foods. Such attributes are measured by the processor to determine consumer preference in order to manufacture an acceptable product at maximum production economy. Sensory attributes are measured also in determining the conformity of a food with established government or trade standards and food grades (Pigott, et al. 1988; Zeuthen et al. 1990).

Capillary Zone Electrophoresis

The general approach to capillary zone electrophoresis was first described by Hjerten (1967) using 3-mm bore columns. Later work with 200-μm bore columns was described by Pretorius et al. (1974). Virtanen (1974) and Mikkers et al. (1979) made early advances. However, it was the work of Jorgenson and Lukacs (1981) with 25-μm bore columns and 30 kV applied voltage that made this technique popular. It is a micro electrophoresis system in which the separation takes place in a 10–100 μm internal diameter fused quartz hollow capillary tube from 30 to 100 cm long, each end being immersed in a buffer (Fig. 16.1). The same buffer is used as the electrolyte and up to 300 V/cm dc. are applied through graphite electrodes. UV and fluorescence are common means of detection. Usually two forces act to cause the separation, normal electrophoresis of the charged particles, and electroosmosis, the flow of the buffer in a charged field.

Reporting Results and Reliability of Analyses

The basic purpose of an analytical assay is to determine the mass (weight) of a component in a sample. The numerical result of the assay is expressed as a weight percentage or in other units that are equivalent to the mass/mass ratio. The mass (weight) of a component in a food sample is calculated from a determination of a parameter whose magnitude is a function of the mass of the specific component in the sample.