Work Continues on Carbohydrate, Fiber Analysis
LABORATORY
Despite the popularity of recent diet plans that call for limiting carbohydrate intakes, carbohydrates still serve as a principal source of energy in most diets.
Digestible carbohydrates are the main source of calories for most of the world’s population. In some countries, carbohydrates provide more than 75% of the total caloric intake. Recent consumption figures from the U.S. Dept. of Agriculture indicate that in the United States carbohydrates supply 40–60% of the calories in the diet. The USDA studies also show that per capita consumption of carbohydrates has steadily risen since the early 1970s.
Carbohydrates are one of the major components of organic products, and are found abundantly in plants, animals, and microorganisms. They are the primary products of photosynthesis, the process by which green plants, under the influence of sunlight, convert carbon dioxide in the air into food materials for the plants and, subsequently, animals.
Monosaccharides are the basic carbohydrate compounds. Glucose is a common and important example. It has the molecular formula C6H12O6 and complies with the Cn(H2O)n “hydrates of carbon” generalization from which the family derived its name.
Monosaccharides often occur linked together in pairs (the most common of these disaccharides is table sugar, or sucrose), in small groups (oligosaccharides), in large numbers (polysaccharides), or attached to a large variety of noncarbohydrate compounds to produce glycosides. Polysaccharides are typically polydisperse compounds that vary in molecular weight, sequence, and structure. Those used in the food industry include native and modified starches, dextrins, dextrans, glucans, pullulans, modified celluloses, pectins, and carra-geenans and gums from both microbial and plant seed sources. In foods and beverages, polysaccharides are used as thickening agents, emulsifiers, and emulsion stabilizers, or to add structure to solids. They are also important for their ability to modify fat and water-holding properties and control aroma and flavor release.
Carbohydrates—often in the form of oligosaccharides attached to proteins—serve vitally important roles in the control of key biological processes. This relatively recent field of study is called glycobiology.
Analytical Techniques
Carbohydrate analysis is fairly complicated, but considerable progress has been made in the development of methodologies based on gas/liquid chromatography, high-performance liquid chromatography, and enzymatic analyses. The old methods of determining carbohydrate content of foods “by difference” is giving way to techniques with more specificity.
• Physical Methods. Refractometry is one of the physical methods used to determine the total carbohydrate content in foods. The method is often used in the food industry for online process monitoring, such as carbonated or other beverage manufacturing, and is based on measuring the refractive index of a substance.
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Other physical methods of carbohydrate measurement are hydrometry and polarimetry. Hydrometers measure the density of liquids. They are available in a variety of scales and can be calibrated in percentage weight of sucrose. These readings are referred to as °Brix. Hydrometers may also be graduated in °Baume and are used to measure sucrose concentrations of syrups and molasses.
Since all sugars are optically active to some extent, polarimetry may also be used to measure their concentration.
Modulated differential scanning calorimetry is a fairly new thermal analysis technique that was developed and introduced in the 1990s. More recently, the technique has been applied to the study of carbohydrate systems. Particularly, it is used to analyze the gelatinization and retrogradation processes in starch systems.
• Colorimetric and Spectrometric Methods. A traditional method of carbohydrate analysis is use of standard reagents and simple photometric instruments. Absorbance is measured, and the concentration of carbohydrates is determined after calibration using known substances. For example, simple sugars, oligosaccharides, polysaccharides, and their derivatives give a stable orange color after reacting with phenol and concentrated sulfuric acid. The intensity of the orange color is proportional to the amount of total carbohydrates present, and the absorbance is measured at 492 nm. The level of total carbohydrates in the test solution is determined by referring to prepared standard curves. Other chemical methods used to measure the total reducing sugar include the Somogyi-Nelson and dinitrosalicylic acid methods. Near-infrared transmittance spectrometry is another common method for measuring carbohydrate content.
More often, spectrometric methods such as NMR spectroscopy and mass spectrometry are used. During the 1990s, great improvements were made in the use of mass spectrometry to analyze macromolecules. Two new ionization techniques, electrospray and matrix-assisted laser desorption ionization, coupled with the power of mass spectrometry, permit the determination of molecular weights of macromolecules and biopolymers.
• Enzymatic Methods. These methods are often easy to use and rapid methods to determine carbohydrate content. Reagent strips are used to perform rapid and semi-quantitative analysis for glucose levels in the sample. The strips change color in the presence of glucose, and the intensity of the color differs with the levels of glucose in the strips. The strips require no additional laboratory equipment or reagents. An enzyme kit produced by Boehringer and Mannheim can determine the levels of D-glucose and D-fructose in food samples in several simple steps.
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• Chromatographic Procedures. The strictly chemical methods of carbohydrate analysis have been replaced by various chromatographic procedures. Examples include detection via a pulsed amperometric detector after HPLC separation, and reduction and derivatization prior to GLC separation. Newer developments involve derivatization that allows detection by UV absorbance or fluorometry and separation via capillary electrophoresis.
Gel-permeation chromatography with multi-angle light scattering is a recently developed technique for the characterization of polysaccharides. The instrument measures the molar mass and size of the molecules based on the angular dependence of scattered light. The technique may be used for quality control, as it permits the determination of polydispersity and provides estimates of absolute molar mass and size not obtainable by conventional chromatographic detection methods. The technique is used in the food industry to verify incoming raw materials and evaluate process effects.
The functional properties of starch and polysaccharides depend on their molecular structures. Understanding the molecular structures of these ingredients allows food scientists to design and adjust product formulations. Capillary electrophoresis, an established, efficient carbohydrate characterization method, has recently been used to characterize native and modified starches from different plant sources. For example, laser-induced fluorescence detection and ultraviolet absorption detection are two types of capillary electrophoresis used to generate oligosaccharide maps of starches and polysaccharides.
Dietary Fiber
No single analytical method is available to measure the nutritional or chemical components of dietary fiber in foods. Manufacturers continue to make improvements in the accuracy, precision, rapidity, ruggedness, and cost-effectiveness of analytical techniques. Currently, dietary fiber analysis methodologies are classified into three major categories: non-enzymatic-gravimetric, enzymatic-gravimetric, and enzymatic-chemical methods.
For most foods, the older, non-enzymatic-gravimetric methods do not recover a significant portion of what is considered to be total dietary fiber. Among these methods, the crude fiber method measures fiber as the sum of lignin and cellulose; the acid-detergent method measures fiber as the sum of lignin, cellulose, and acid-insoluble hemicellulose; and the neutral-detergent method measures fiber asthe sum of lignin, cellulose, and neutral-detergent insoluble hemicellulose.
The realization that neither the crude fiber method nor the neutral- and acid-detergent fiber method is satisfactory for measuring dietary fiber and some insoluble dietary fiber in foods led researchers to develop enzymatic-based methods to measure dietary fiber. In the early 1980s, an enzymatic-gravimetric method was developed in which the sum of soluble and insoluble polysaccharides and lignin was measured as a unit, which was considered to be total dietary fiber. The procedure was later extended to determine insoluble and soluble dietary fiber. All three methods use the same basic enzymatic-gravimetric procedure with a phosphate buffer.
Plant cell wall material constitutes much of what is considered to be dietary fiber. About 90% of endogenous plant cell wall material consists of non-starch polysaccharides. The Englyst method determines non-starch polysaccharides after enzymatic removal, precipitation, and acid hydrolysis of starch. Sugars were assayed colorimetrically using older methods, but recently they are measured by chromatographic methods.
The recent low-carbohydrate food trend is sure to generate calls for standards on what constitutes a “low-carb” food. Recent news reports indicate that the Food and Drug Administration may soon define what the amounts, in grams, of carbohydrates allowed in foods advertised as “low” or “reduced carbohydrates” are, and exactly how manufacturers should count these carbohydrate grams. FDA Deputy Commissioner Lester Crawford was recently quoted as saying that he expects that manufacturers will have to change the labels on a substantial number of products as a result of the new definitions. News reports indicate that FDA may rule on this matter by this summer.
by JAMES GIESE
Internet Editor
[email protected]