Chapter 40 Microbiology of Food

Intrinsic Factors – control growth of microbes in food composition pH presence and availability of water physical structure presence of antimicrobial substances

Putrefaction – anaerobic breakdown of proteins Carbohydrate – fungal , bread, jams, fruits, vegetables Fat protein bacterial proteolysis and anaerobic breakdown of proteins, yielding foul-smelling amine compounds, cadaverine, putrescine – meat, dairy products pH impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food Low ph yeasts and molds Neutral alkaline bacteria

Water availability in general, lower water activity inhibits microbial growth water activity lowered by: drying addition of salt or sugar Hypertonic environment

Table 40.2

Physical structure grinding and mixing increase surface area and distribute microbes promotes microbial growth outer skin of vegetables and fruits slows microbial growth

Antimicrobial Substances coumarins – fruits and vegetables lysozyme – cow’s milk and eggs phenolic compounds – herbs and spices -Cinnamon, mustard. allicin – garlic

Extrinsic Factors temperature relative humidity atmosphere lower temperatures retard microbial growth relative humidity higher levels promote microbial growth atmosphere oxygen promotes growth

Microbial Growth and Food Spoilage results from growth of microbes in food alters food visibly and in other ways, rendering it unsuitable for consumption involves predictable succession of microbes different foods undergo different types of spoilage processes toxins are sometimes produced

Figure 40.2 Lactococcus -acids Lactobacillus Yeasts and molds Dominant- breakdown lactic acid pH increases Bacteria breakdown proteins Curds and whey Left: fresh milk Right: curdled mild. The curdled milk has undergone a natural 4-step sequence of spoilage organisms activity, resulting in separated curds and whey. Figure 40.2

Figure 40.3 Left: fresh milk Right: curdled mild. The curdled milk has undergone a natural 4-step sequence of spoilage organisms activity, resulting in separated curds and whey. Figure 40.3

Toxins ergotism toxic condition caused by growth of a fungus in grains rye Claviceps purpurea – fungus grows on grains rye Ergot Hallucinations, constriction of capillaries

disrupt synthesis and metabolism of sphingolipids fumonisins Figure 40.5

aflatoxins intercalate into DNA, causing frameshift mutations The letter designations refer to the color of the compounds under ultraviolet light after extraction from the grain and separation by chromatography. The B1 and B2 compounds fluoresce with a blue color and the G1 and G2 appear green. The two type of M aflatoxins are found in the milk of lactating animals that have ingested type B aflatoxins. Figure 40.4

Table 40.3

Controlling Food Spoilage Table 40.4

Removal of Microorganisms usually achieved by filtration commonly used for water, beer, wine, juices, soft drinks, and other liquids

Low Temperature refrigeration at 5°C retards but does not stop microbial growth microorganisms can still cause spoilage with extended spoilage growth at temperatures below -10°C has been observed

Pasteurization kills pathogens and substantially reduces number of spoilage organisms different pasteurization procedures heat for different lengths of time shorter heating times result in improved flavor

Water Availability dehydration freeze-dried foods is commonly used to eliminate bacterial growth food preservation occurs as a result of free-water loss and an increase in solute concentration

Microbial Product-Based Inhibition bacteriocins e.g., nisin used in low-acid foods to inactivate Clostridium botulinum during canning process

Food-Borne Diseases two primary types food-borne infections food intoxications

Food-Borne Infection ingestion of microbes, followed by growth, tissue invasion, and/or release of toxins Salmonellosis

Food Borne Infections Salmonellosis Listeriosis gastroenteritis from ingestion of contaminated meats, poultry or eggs Listeriosis pregnant women, the young and old and immunocompromised individuals most vulnerable responsible for the largest meat recall in U.S. at risk people should not eat soft cheeses, refrigerated smoked meats, deli meats and undercooked hot dogs

Food Borne Infections… Escherichia coli diarrhea caused by enteropathogenic, enteroinvasive and enterotoxigenic types E. coli 0157:H7 is thought to have acquired enterohemorrhagic genes from Shigella, including genes for the shigalike toxin

Production of Breads involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions maximizes CO2 production, which leavens bread

Table 40.9

Other Fermented Foods… sauerkraut (sour cabbage) – from wilted, shredded cabbage – Lactobacillus plantarum pickles – from cucumbers

yogurt Lactobacillus bulgaricus, Lactobacillus acidphilus, Streptococcus lactis Cup of milk – warm Add a spoon of yogurt, mix, cover, leave it at room temperature overnight Bacteria ferment lactose – acids coagulate the milk protein casein. Milk becomes solid.

Cheese Various species of Lactobacillus bulgaricus, lactococcus lactis Curd formation from milk Watery part (whey) removed ripened ( a year) Peniclillium camemberti, P. Roqueforti Swiss cheese Propionibacterium makes holes by making gas

Microorganisms as Foods and Food Amendments variety of bacteria, yeasts, and other fungi are used as animal and human food sources probiotics microbes added to diet in order to provide health benefits beyond basic nutritive value Lactobcillus acidophilus in cattle feed reduces Number of toxic E.coli in the intestinal tract of cattle

Benefits of probiotics immunomodulation control of diarrhea anticancer effects possible modulation of Crohn’s Disease in beef cattle, decrease E. coli in poultry, limit colonization of gut by the process of competitive exclusion

Wine production Saccharomyces cerevisiae added to grape juice. Fermented 3 to 5 days at 20 to 28oC.