Thermal Food Processing

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Thermal Processing of Dairy Products 271 acids (with a concomitant decrease in pH), isomerization (e.g., to lactulose), production of compounds such as furfural, and interactions with amino groups of proteins (Maillard reaction). In the Maillard reaction, lactose or lactulose reacts with an amino group, such as the ε-amino group of lysine residues, in a complex (and not yet fully understood) series of reactions with a variety of end products. 16,17 The early stages of the Maillard reaction (condensation reactions between the carbonyl group of lactose and the -amino group of lysine of milk proteins) result in the formation of the protein-bound Amadori product, lactulosyllysine, which then degrades to a range of advanced Maillard products, including hydroxymethylfurfural (HMF), furfurals, and formic acid. The most obvious consequence of the Maillard reaction is a change in the color of milk (browning), due to the formation of pigments called melanoidins, or advanced-stage Maillard products. Extensive Maillard reactions can result in polymerization of proteins, 17 and also alter the flavor and nutritive quality of dairy products, in the latter case through reduced digestibility of the caseins and loss of available lysine. Moderately intense heating processes applied to milk primarily cause the isomerization of lactose to lactulose. Lactulose, a disaccharide of galactose and fructose, is of interest as a bifidogenic factor and also as a laxative. More severe treatments (e.g., sterilization) will result preferentially in Maillard reactions. Products of heat-induced changes in lactose, such as lactulose and HMF, may be used as indices of heat treatment of milk. 18 9.5 OVERVIEW OF THE MICROBIOLOGY OF MILK Milk from a healthy cow is secreted free of microorganisms, but it becomes contaminated with many microorganisms during direct contact with various sources (Figure 9.1). 19 Three main types of microorganisms are the key factors for assessing quality and safety of heat-treated milk: • Thermoduric/heat-resistant bacteria • Bacteria that enter through postprocessing contamination • Bacteria that can grow at refrigeration temperature Raw milk has a short shelf life even under highly hygienic refrigeration conditions 20 and requires processing as soon as possible after arrival in the dairy plant to ensure the production of high-quality processed milk and dairy products. Heat treatment is the most common and single most effective industrial processing procedure for reducing bacterial numbers in milk, thereby enhancing its shelf life and ensuring the absence of pathogenic organisms. The quality and safety of heat-treated milk depend on raw milk quality, processing conditions, postprocessing contamination, and storage temperature. 14 The former will be discussed here, while the latter factors will be considered later in this chapter. Even if produced under highly hygienic conditions, raw milk contains a heterogeneous population of bacteria, some of which are pathogenic and others of which can cause spoilage during milk storage. Fortunately, pathogens are
272 Thermal Food Processing: New Technologies and Quality Issues Human Coliforms Salmonella Enterococcus Staphylococcus Water Coliforms Pseudomonas Coryneforms Alcaligenes Feed Clostridium Listeria Bacillus Lactic acid bacteria Air Streptococci Micrococci Coryneforms Bacillus Yeasts and moulds Soil Clostridium Bacillus Pseudomonas Mycobacterium Yeasts and moulds Bedding Clostridium Bacillus Klebsiella Inside udder Streptococcus Micrococcus Corynebaterium Milking Machine Micrococcus Streptococci Bacillus Coliforms Faeces Escherichia coli Staphylococcus Listeria Mycobacterium Salmonella Outside udder and teats Micrococcus Staphylococcus Enterococcus Bacillus FIGURE 9.1 Sources of contamination of raw milk. (from Frank, J.F. and Hassan, A.N., in Encyclopedia of Dairy Science, Roginski, H., Fuquay, J.W., and Fox, P.F., Eds., Academic Press, London, 2002, pp. 1786–1796. With permission.) generally not determinants of the shelf life of thermally treated milk and dairy products. 21,22 This is due to their low number in raw milk, their inability to grow at refrigeration temperatures, and their destruction by pasteurization. In exception to the above generalization, one potentially pathogenic bacterium in raw milk, B. cereus, can be a significant spoilage agent due to the ability of its spores to survive pasteurization and the ability of some strains to proliferate in pasteurized milk at refrigeration temperature. This bacterium can produce enzymes such as a proteinase, which hydrolyzes casein to produce an intensely bitter flavor, and phospholipase, which hydrolyzes the phospholipids of the milk fat globule membrane and causes instability of the fat emulsion. In unhomogenized milk, this can cause the defect known as bitty cream, but this is not observed in homogenized milk. B. cereus in milk seldom causes food poisoning, as the contaminated product develops a strong, unacceptable flavor, 14 and also because toxigenic strains are seldom psychrotrophic. In addition, large numbers of this bacterium are required to produce food poisoning; Ryser 23 reported that most outbreaks of B. cereus poisoning were caused by contamination levels of at least 10 6 cfu/g. In addition to B. cereus, other common types of spore-forming bacteria frequently present in raw milk are Bacillus licheniformis and Bacillus stearothermophilus, although the types and numbers of organisms vary considerably
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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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74 Thermal Food Processing: New Tec
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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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UHT Thermal Processing of Milk 321
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11 CONTENTS Thermal Processing of C
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Thermal Processing of Canned Foods
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12 CONTENTS Thermal Processing of R
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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 503 the calculated
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Infrared Heating 505 TABLE 16.1 Com
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Infrared Heating 507 Rotating drum
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Infrared Heating 513 Degradation ra
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Infrared Heating 519 Temperature (
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Infrared Heating 521 and thawing ti
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Infrared Heating 523 15. T Takeo. F
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Infrared Heating 525 55. CM Liu, N
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638 Index Tea, 451, 510 Temperature
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640 Index Wax beans, 398, 411 Web s
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Thermal Food Processing

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