Thermal Food Processing

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UHT Thermal Processing of Milk 325 Valero and coworkers 137 studied the evolution of volatile components of commercial whole and skim UHT milk during 4 months of storage. Sensory analyses showed a decrease in quality during storage either in whole or skim milk, and after 3 months of storage there was a slight stale flavor. The concentration of volatile compounds, such as aldehydes and ketones, provides discriminant information about storage time and storage temperature of whole and partially skimmed UHT milk. Some volatile compounds, such as acetone, 2-butanone, 2-pentanone, 2-heptanone, 3-methylbutanal, pentanal, hexanal, heptanal, dimethyl disulfide, toluene, and limonene, are reportedly useful to differentiate milk subjected to different heat treatments. 138 In whole milk, 2-pentanone and 2-heptanone occurred during heat treatment, and dimethyl sulfide and dimethyl disulfide were responsible for the cooked note. Skim milk, instead, showed a different pattern of volatile compounds with hydrocarbons representing the main group. 138,139 During storage, UHT milk develops off-flavors due to the release of lowerchain-length fatty acids by psychrotrophic microorganisms. Indeed, in UHT milk samples, stored at 22 and 37°C and monitored over 33 days, volatile and free fatty acids showed a sharp increase after 13 to 14 and 20 to 21 days, respectively, due to lipases from psychrotrophic spores. 140 Hence, good-quality raw milk should be used for UHT milk to prevent rancid flavor development. 140 Actually, it is known that milk with a high acid degree value has an acceptable lipolyzed flavor. 58 Iwatsuki and coworkers 141 examined the effect of pasteurization temperature on sensory properties of UHT milk (treated at 120, 130, or 140°C for 2 sec); the sensory analysis of the milk was conducted by an expert panel. Using multiple regression analysis, it was found that the palatability of UHT milk is judged mainly on aroma and aftertaste, freshness, and intensity of milk flavor. Results of sensory and principal component analyses showed that milk body, related to milk flavor, viscosity, and fattiness, becomes stronger with increasing UHT temperature. 141 Sensory properties and palatability of UHT milk (heated at 130°C for 2 sec) were also evaluated as a function of homogenization pressure. 142 Evaluation of sensory properties by an expert panel and principal component analysis indicated that milk homogenized at lower pressure has greater fattiness, body, milk flavor, sweetness, and thickness. Palatability, as judged by a panel of housewives, improved when milk was homogenized at low pressure. In terms of physicochemical properties, the authors found that with increased homogenization pressure, fractionation of fat globules occurs, resulting in an increase in viscosity, degree of whiteness, and refractive index for the UHT milk. 142 10.7 CONCLUSIONS Alterations in the chemical composition of milk due to thermal processing are extremely complex. They depend on the composition of the raw milk, time and temperature of thermal processing, other processing conditions (e.g., homogenization), and time and temperature of storage. Some heat-induced chemical
326 Thermal Food Processing: New Technologies and Quality Issues alterations affect milk nutritional quality (e.g., lysine loss, isomerization of alltrans retinol), some the physical appearance (e.g., casein proteolysis), and others the organoleptic characteristics. As regards safety of UHT milk, contamination with heat-resistant bacteria, aflatoxin, and chemical compounds remains of some concern. The problem here is not so much with thermal processing, but with the quality of the raw milk, provided processing and domestic handling procedures are properly performed. Continuous monitoring seems warranted. The growing scientific interest in heat-induced modifications of chemical, biological, and sensorial characteristics of UHT milk is a response to the growing consumer demand for information on food quality and food safety. This aspect will be more and more important in the future, when development of milk products, as other foods, will be specifically planned on consumer concerns. A comprehensive description of the heat-induced changes will be necessary to allow more complete milk labeling, extending to nutritional and functional components. This will be a new, important challenge for the food chemist. ACKNOWLEDGMENTS The authors thank Sandra Bukkens for editing of the manuscript and her valuable suggestions. REFERENCES 1. Council Directive 92/46/EEC, June 16, 1992. Official Journal of the European Communities L 268. 2. MM Hewedy, C Kiesner, K Meisser, J Hartkopf, HF Erbersdobler. Effect of UHT heating of milk in an experimental plant on several indicators of heat treatment. Journal of Dairy Research 61: 305–309, 1994. 3. J O’Brien. Heat-induced changes in lactose: isomerization, degradation, Maillard browning. In: PF Fox, Ed. Heat Induced Changes in Milk, Special Issue 9501. Brussels: International Dairy Federation, 1995, pp. 134–170. 4. L Pellegrino, I Resmini, W Luf. Assessment (indices) of heat treatment of milk. In: PF Fox, Ed. Heat Induced Changes in Milk, Special Issue 9501. Brussels: International Dairy Federation, 1995, pp. 409–453. 5. E Troyano, I Martinez-Castro, A Olano. Kinetics of galactose and tagatose formation during heat-treatment of milk. Food Chemistry 45: 41–43, 1992. 6. E Marconi, MC Messia, A Amine, D Moscone, F Vernazza, F Stocchi, G Palleschi. Heat treated milk differentiation by a sensitive lactulose assay. Food Chemistry 84: 447–450, 2004. 7. J De Block, M Merchiers, R Van Renterghem, R Moermans. Evaluation of two methods for the determination of lactulose in milk. International Dairy Journal 6: 217–222, 1996. 8. International Dairy Federation (IDF). B-Document 198. 1991. 9. EEC Document VI/5726, Revision 2. 1992.
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Thermal Food Processing
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87. Polysaccharide Association Stru
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133. Handbook of Frozen Foods, edit
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Published in 2006 by CRC Press Tayl
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Various alternative thermal process
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Professor Sun is a fellow of the In
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Antonio Martínez Instituto de Agro
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Contents Part I: Modeling of Therma
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Chapter 17 Pressure-Assisted Therma
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1 CONTENTS Thermal Physical Propert
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Thermal Physical Properties of Food
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2 CONTENTS Heat and Mass Transfer i
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Heat and Mass Transfer in Thermal F
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5 CONTENTS Modeling Thermal Process
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Modeling Thermal Processing Using C
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6 CONTENTS Thermal Processing of Me
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Thermal Processing of Meat Products
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7 CONTENTS Thermal Processing of Po
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Thermal Processing of Poultry Produ
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Thermal Processing of Dairy Product
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Thermal Processing of Ready Meals 3
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14 CONTENTS Ohmic Heating for Food
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Ohmic Heating for Food Processing 4
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15 CONTENTS Radio Frequency Dielect
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Radio Frequency Dielectric Heating
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16 CONTENTS Infrared Heating Noboru
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Infrared Heating 495 0.78 1.4 Far-i
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Infrared Heating 497 A black body a
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Infrared Heating 499 Spectral atten
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Infrared Heating 501 Axial Distance
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Infrared Heating 505 TABLE 16.1 Com
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Infrared Heating 525 55. CM Liu, N
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640 Index Wax beans, 398, 411 Web s
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