Current Protocols in Food Analytical Chemistry

WHAT'S NEW AND COMING

Updated March, 2002

RECENTLY PUBLISHED:

Unit A2.5 Measurement of Water Activity by Electronic Sensors (M.S. Rahman and S.S. Sablani, Sultan Qaboos University, Sultanate of Oman). Water activity is an extremely valuable tool for food scientists because of its usefulness in predicting stability during storage. The measurement of water activity by electronic sensors is considered to be the most simple and easiest method for measuring water activity. This method gives a very accurate result as long as calibration is performed and precautions are taken to avoid sensor contamination. Advantages and limitations of different types of electronic sensors are discussed.

Unit B4.1 Overview of Protein Purification and Characterization (R.K. Scopes, La Trobe University, Australia). Purification and characterization of proteins can be an essential part of many processes in the food industry. This unit provides a general discussion of protein purification issues such as the source of protein, detection and assay of proteins, and methods for separation and purification of proteins based on overall size and shape, surface features, net charge, and affinity. Strategies are discussed for soluble extracellular proteins, cytoplasmic proteins, membrane-associated proteins, insoluble proteins, and both soluble and insoluble recombinant proteins.

Unit B4.2 Overview of Conventional Chromatography (A. Williams, Amersham Pharmacia Biotech). Many methods are available for chromatographic purification of proteins, and no single strategy will work for all proteins. This unit provides a general approach to working out the best strategy for protein purification. Parameters discussed include the scale, yield, and purity that are required and the basic stages in purification from the source material to the most pure product. Chromatographic resolution, column capacity, efficiency, and selectivity are addressed.

Unit C2.2 Peptidase Activity Assays Using Protein Substrates (O. Akpinar and M.H. Penner, Oregon State University). Peptidases play integral roles in the food industry. They affect food properties such as gel strength, foam and emulsion stability, and flavor profiles, and are used in applications such as meat tenderization and cheese manufacturing. Peptidase activity can be assayed using native or modified proteins, peptides, or synthetic substrates. This unit focuses on assays based on the hydrolysis of common, commercially available protein substrates – casein, azocasein, and hemoglobin. The assays measure the amount of trichloroacetic acid–soluble peptide generated during a given reaction period. Protocols describe the enzymatic reaction itself, the simultaneous termination of the reaction and separation of the hydrolysis products, and the quantification of products. These protocols are more sensitive in measuring endopeptidase than exopeptidase activity.

Unit D3.4 Emulsion Stability Determination (J. Weiss, University of Tennessee, Knoxville). This unit provides methods for evaluating the stability of macro-emulsions. In the first procedure, droplet size distribution and concentration are measured over the desired test period, corresponding to the required shelf-life of the product. This test can be time-consuming but is reliable and offers high accuracy. In the second procedure, the height of the visible layer boundary is measured over time. This test is typically used on emulsions that destabilize in less than a week, and is useful for determining the effects of ionic strength, pH, and addition of biopolymers or surfactants on stability of the emulsion. Finally, procedures that use test acceleration and ultrasonic or infrared scanning of concentration and droplet size profiles are considered. The commentary provides an in-depth discussion of the mechanisms of emulsion destabilization.

Unit F5.1 Overview of Color Analysis (K. Loughrey, GretagMacbeth). An in-depth discussion of how color is perceived and analyzed, and the applications of these methods to food chemistry, is presented. The discussion begins with the observer situation, comprising the light source, the object, and the human observer. The use of spectrophotometers versus colorimeters is discussed, as are methods that have been developed for quantifying color using tristimulus values, including CIE X,Y,Z, Hunter L,a,b, CIELAB, and CMC.

FORTHCOMING:

Unit B1.3 Spectrophotometric Determination of Protein Concentration (M.H. Simonian, Beckman Coulter). The concentration of a sample protein in solution can be determined by spectrophotometric methods. Absorbance can be measured at 280 or 205 nm. Both methods are simple and can be used for crude lysates or for purified or partially purified protein. Although absorbance at 205 nm has a lower limit of detection, it is less commonly used because it is more sensitive to interferences. Alternatively, a spectrofluorometer or filter fluorometer can be used to measure the intrinsic fluorescence of a sample. This method is simple and can be used for dilute samples, but is only appropriate for purified proteins.

Unit B2.2 Evaluation of the Progress of Protein Hydrolysis (M.A. Navarrete del Toro and F.L. García-Carreño). The degree of protein hydrolysis is defined as the percentage of hydrolyzed peptide bonds. The techniques described in this unit measure the degree of hydrolysis by determining the number of free amino or free carboxyl groups released. The ninhydrin reaction, TNBS reaction, fluorescamine reaction, and formol titration all evaluate the release of free amino groups over time. These methods include both spectrophotometric and potentiometric techniques. The pH-stat method is also presented as a kinetic technique that is useful for assaying protein-catalyzed hydrolysis on an industrial scale.

Unit D1.5 Analysis of Tocopherols and Tocotrienols (Z. Xu, Louisiana State University). HPLC methods with a fluorescence or UV detector have been developed to quantify tocopherols and tocotrienols. These methods provide high sensitivity and specificity and simple sample preparation compared to conventional colorimetric or polarimetric and GC methods. Methods for normal-phase and reversed-phase HPLC are compared. Sample preparation methods, which generally include saponification, heating, and liquid/liquid extraction, are also presented.

Unit G1.5 Analysis of Citrus Oils (T.S. Gentry, Cornell University). Most citrus flavors are associated with odor compounds found in essence oils. There are several classes of flavor compounds including terpenes, aldehydes, esters, and alcohols. The quality of these oils determines their functionality and market value. This unit presents a number of methods for qualitative and quantitative evaluation of citrus oils, including gas chromatography, pycnometry, refractive index determination, and polarimetry. Additionally, methods are presented for quantifying the amount of oil present in whole fruit, wet peel, press cake, dry peel, pellets, press liquor, and molasses. Finally, methods are given for determination of total aldehydes and volatile esters in citrus oils.