Microbial Food Spoilage — Losses and Control Strategies a Brief Review of the Literature
Total Page：16
File Type：pdf, Size：1020Kb
FRI BRIEFINGS Microbial Food Spoilage — Losses and Control Strategies A Brief Review of the Literature M. Ellin Doyle, Ph.D. Food Research Institute, University of Wisconsin–Madison Madison, WI 53706 Contents Introduction................................................................................................................................. 1 Detection of Spoilage.................................................................................................................. 2 Spoilage Organisms .................................................................................................................... 3 Yeasts.................................................................................................................................. 3 Molds.................................................................................................................................. 3 Bacteria............................................................................................................................... 4 Modeling Spoilage ...................................................................................................................... 5 Factors Affecting Food Spoilage and Shelf Life ......................................................................... 6 Dairy Products .................................................................................................................... 6 Cereal and Bakery Products................................................................................................ 7 Vegetables........................................................................................................................... 7 Fresh Meat .......................................................................................................................... 7 Processed Meat ................................................................................................................... 8 Fish .................................................................................................................................... 8 Fruits and Juices.................................................................................................................. 9 Control of Spoilage Microorganisms .......................................................................................... 9 References................................................................................................................................. 10 INTRODUCTION Economic Research Service estimated that more than ninety-six billion pounds of food in the U.S. were lost Food spoilage is a metabolic process that causes by retailers, foodservice and consumers in 1995. foods to be undesirable or unacceptable for human Fresh produce and fluid milk each accounted for consumption due to changes in sensory characteris- nearly 20% of this loss while lower percentages were tics. Spoiled foods may be safe to eat, i.e. they may accounted for by grain products (15.2%), caloric not cause illness because there are no pathogens or sweeteners (12.4%), processed fruits and vegetables toxins present, but changes in texture, smell, taste, or (8.6%), meat, poultry and fish (8.5%), and fat and appearance cause them to be rejected. Some ecolo- oils (7.1%) (79). Some of this food would have been gists have suggested these noxious smells are pro- considered still edible but was discarded because it duced by microbes to repulse large animals, thereby was perishable, past its sell-by date, or in excess of keeping the food resource for themselves (21;144)! needs. There are also environmental and resource Food loss, from farm to fork, causes consider- costs associated with food spoilage and loss. If 20% able environmental and economic effects. The USDA Corresponding author: M. Ellin Doyle, Ph.D., [email protected] July 2007 http://fri.wisc.edu/docs/pdf/FRI_Brief_Microbial_Food_Spoilage_7_07.pdf Food Research Institute, University of Wisconsin–Madison 2 FRI BRIEFINGS: Microbial Food Spoilage: Losses and Control Strategies of a crop is lost, then 20% of the fertilizer and irriga- teins are attacked, producing volatile compounds tion water used to grow that crop was also lost. with characteristic smells such as ammonia, amines, Shelf life of a food is the time during which it and sulfides. These odors start to develop in meat remains stable and retains its desired qualities. Some when there are about 107 cfu of bacteria/cm2 of meat spoilage is inevitable, and a variety of factors cause surface and are usually recognizable at populations of deterioration of foods: 108 cfu/cm2 (46). endogenous enzymes in plants oxidizing phe- Early detection of spoilage would be advanta- nolic compounds (browning) or degrading geous in reducing food loss because there may be pectins (softening); interventions that could halt or delay deterioration, insects infesting foods and rodents chewing on and on the other hand food that had reached the end foods; of its designated shelf life but was not spoiled could parasites, when visible for example in meat or still be used. Numerous methods for detection of fish, rendering food undesirable; spoilage have been devised with the goals of deter- microbes (bacteria, molds, yeasts) growing on mining concentrations of spoilage microbes or vola- and metabolizing foods; tile compounds produced by these microbes. How- light causing degradation of pigments, fats, ever, many of these methods are considered inade- and proteins (off-flavors and odors) or stimulat- quate because they are time-consuming, labor-inten- ing pigment production (greening of potatoes); sive, and/or do not reliably give consistent results. Some representative papers using different methods temperature; both excessive heat and freezing are described below. physically affecting texture of foods and break- ing emulsions; Traditional methods of estimating bacterial populations do not provide results quickly enough to air, particularly oxygen, oxidizing lipids pro- allow for interventions. Microbial population levels ducing strong off-odors and flavors; can be measured by real time PCR in liquids, such as moisture: too little causing cracking, crum- Gluconobacter in an electrolyte replacement drink bling, or crystallization whereas excess causes (56) and yeasts in wine and fruit juices (23;71;145). sogginess, stickiness, or lumping. Other methods for detecting bacteria include ATP These factors are interrelated, as certain temperatures bioluminescence and electrical impedance assays but and oxygen and moisture levels increase the activities some food matrices may contain interfering sub- of endogenous enzymes and of microbes. Rodent and stances (46). insect damage may provide an entry point for Detection of volatile compounds produced by microbial growth. spoilage bacteria can be a less invasive and more Food spoilage is a broad topic that cannot be rapid means for monitoring spoilage. Biogenic completely addressed in one review article. This amines (putrescine, cadaverine, histamine, and paper will emphasize spoilage caused by microor- tyramine) are commonly produced during spoilage of ganisms and will consider spoilage of foods that peo- high protein foods and can attain levels that cause ple purchase or consume. For example, spoilage of illness, particularly in spoiled fish. HPLC methods bread will be considered but not deterioration of have been used to quantitate different amines in fish wheat plants in the fields or wheat grains in storage. (9), chicken (10), and cheese (75). Combined con- Non-microbial spoilage such as loss of water (shriv- centrations of these amines are expressed as a bio- eling of greens or carrots) or changes induced by genic amine index that is related to the extent of food degradative enzymes in plants (yellowing of broc- spoilage and to the concentrations of spoilage coli) will not be covered. Pathogenic organisms, e.g. organisms. Listeria that cause human illness, will not be consid- Electronic noses were first developed about ered even though they may also cause some spoilage. twenty years ago and have undergone many refine- ments since (24). They consist of a set of sensors that react with different volatile chemicals and produce an DETECTION OF SPOILAGE electrical signal. An odor profile can be analyzed by using pattern recognition files. Such systems have Spoilage is manifested by a variety of sensory cues been used to detect spoilage in beef (12;121), bakery such as off-colors, off-odors, softening of vegetables products (103;104), fish (2;70), and milk (69). Other and fruits, and slime. However, even before it be- techniques being developed to detect microbes or comes obvious, microbes have begun the process of chemicals associated with spoilage include: (a) FT-IR breaking down food molecules for their own meta- (Fourier Transform-Infrared Spectroscopy) used with bolic needs. Sugars and easily digested carbohydrates beef (45) and apple juice (95); (b) visible and short- are used first, plant pectins are degraded. Then pro- wavelength near-infrared spectroscopy to detect Corresponding author: M. Ellin Doyle, Ph.D., [email protected] July 2007 http://fri.wisc.edu/docs/pdf/FRI_Brief_Microbial_Food_Spoilage_7_07.pdf Food Research Institute, University of Wisconsin–Madison FRI BRIEFINGS: Microbial Food Spoilage: Losses and Control Strategies 3 microbial load in chicken by diffuse reflectance (94); target, and there are some yeasts that grow on the (c) ion mobility spectrometry for detecting trimethyl- surfaces