Chapter 17: The Microbiology in Food and Water

Illness may be a "common source", associated with consumption of one thing by many people

- Salmonellosis "food poisoning"; botulism For example, typhoid fever, a serious disease caused by Salmonella Typhi (see Microbes in Focus 18.2), is classically considered a waterborne disease, but it can be spread through contaminated food. On the other hand, salmonellosis, a milder salmonella-caused illness, is most frequently foodborne, although outbreaks of salmonellosis resulting from contaminated water have been reported FIGURE 17.21 Salmonellosis is a common foodborne disease a. Salmonella spp. are commonly found in the intestinal tracts of chickens and can contaminate eggs and meat. b. Thorough hand washing with hot soapy water is recommended after handling these raw foods.

Illness may spread through either food or water

-Cholera FIGURE 17.22 Cholera—A waterborne disease a. Vibrio cholerae-contaminated water following the 2010 earthquake in Haiti. b. Haitian girl receiving ORT, a solution of sodium salts in water to replace that lost from severe diarrhea. Vibrio cholerae, the bacterial agent of cholera, attaches to intestinal cells following ingestion of contaminated water. Once attached, it secretes a potent enterotoxin that causes ion imbalances in intestinal cells, causing significant water loss across cell membranes, resulting in massive diarrhea and dehydration. ORT replaces lost fluids and restores ion balance.

17.2 Food Preservation •What techniques are used to minimize food spoilage?

-Multiple methods can be used to prevent food spoilage: •Reduction of water activity (a w) of food •Control of temperature •Increase in acidity of food •Addition of chemical preservatives •Irradiation of food •Use of modified-atmosphere packaging (M A P) •Hurdle technology

Goals for treating wastewater (sewage) include

-Reducing total organic carbon (TOC) -Removal or inactivation of harmful microbes in wastewater -Reduction of inorganic compounds (especially ammonium, nitrogen, and phosphorus levels) to prevent eutrophication of natural waters. -- A newer goal is to reduce persistent organic pollutant (POP) levels. - These compounds, even in low levels, may affect fish and mammal reproduction and development.

Drinking-water purification

-Screening removed large objects. -Flocculation, sedimentation, and sludge treatment are similar to wastewater treatment steps. -Filtration removes many microbes from the water -Disinfection steps destroy remaining microbes. -Water is usually tested for indicator organisms (likely to cause disease) such as fecal coliforms prior to release. -- May be tested through the most probably number (MPN) or biochemical identification methodologies FIGURE 17.30 Process Diagram: General schematic of drinking water purification Untreated water may be screened to remove large objects before it enters the flocculation tank. A chemical such as alum is added to promote floc formation. The floc-laden water moves to a sedimentation tank where the floc settles as sludge, which is removed and treated, similar to that for a wastewater treatment plant. The clarified water is filtered through thick layers of sand, activated charcoal, or other specific filtration media. Filtered water is disinfected and stored, ready for distribution.

Vinegar Manufacture Vinegar is also used in the food industry as a pH control agent, a preservative, and a flavoring agent. Ketchup, sauces, salad dressings, pickles, and mayonnaise all contain vinegar. Main component is dilute acetic acid.

-Small amounts of vinegar are useful for flavoring food. -Larger amounts can be used as a preservative. -Produced by fermentation of ethanol by acetic acid bacteria into acetic acid. --> The source of the ethanol can imbue different flavors to different vinegars. --> Methods vary in efficiency. - Trickle/Quick method uses acetic acid bacteria growing on inert support material for fermentation. FIGURE 17.20 Quick or Trickle method of vinegar production The ethanol-containing liquid is introduced onto a bed of inert support material such as wood twigs or shavings. As the fluid passes through the bed by gravity, the adherent biofilm of acetic acid bacteria metabolizes the ethanol to acetic acid. Air is constantly diffusing up through the bed to ensure the bacteria have sufficient oxygen. The acidic liquid drips through the grate at the bottom and is recirculated as necessary to obtain a 4 percent minimum acetic acid concentration. The process requires approximately four to five days.

•Some food "spoilage" is actually desirable. Changes to milk to form cheese, yogurt, butter, etc

17.1 Fact Check 1. List the intrinsic and extrinsic factors that affect microbial growth in food. Check out I and E chart.

•How can microorganisms spoil food? -Perishable foods •Easily support microbe growth •Green salad items, fresh meat -Semi-perishable foods •Don't spoil as quickly •Nuts, potatoes -Non-perishable foods •Remain edible for long periods •Flour, sugar, dried beans

17.1 Food Spoilage How can microorganisms spoil food? What is spoilage? In terms of spoilage-- what are the three categories and what are examples of each? What determines if a food is more likely to spoil? •Likelihood of spoilage is related to intrinsic (food properties) and extrinsic (storage environment) factors. -The likelihood that a food will spoil is related to its qualities as a microbial growth medium, which in turn depends on the characteristics of the food and its surroundings (Table 17.1). Intrinsic factors relate to characteristics of the food itself, such as its nutrient content or pH. These also include structures that protect the food from microbial invasion, such as rinds or shells. We simulate that protection when we wrap our foods in plastic to protect it from spoilage. Extrinsic factors relate to the conditions under which a food is stored. For example, cold temperatures in a refrigerator will slow the growth of human pathogens that grow best in warmer temperatures of the body.

Colloquially, foodborne illness is often referred to as "food poisoning," a reflection of the fact that the first food illnesses linked to microbes truly were poisonings. Staphylococcal food illness was recognized in 1914 and definitively linked to an enterotoxin produced by Staphylococcus aureus in 1930. In reality, food poisoning may be an intoxication or an infection.

A foodborne intoxication refers to illness caused by ingestion of microbial exotoxins as is the case with S. aureus food poisoning. Illness from S. aureus enterotoxin is usually short-lived, but a more lethal example of food intoxication occurs after consuming foods in which C. botulinum has produced the toxin that results in deadly botulism. Anaerobic growth of C. botulinum and/or other contaminating bacteria in cans produces gas, which is evidenced by bulging of the can (Figure 17.23). Because the toxin will be present in the food, symptoms may appear relatively quickly after ingestion, sometimes within 30 minutes and usually within six hours.

Fruits are typically more acidic than vegetables. Because of this, mold spoilage is more common on fruit. The slower-growing fungi are generally more tolerant of lower pH and can grow on fruits that are too acidic to support bacterial growth. For example, lemons spoil extremely slowly but will eventually be covered by mold if abandoned in the refrigerator. Faster-growing bacteria outcompete molds at near neutral pH, as found in many vegetables.

A variety of microorganisms can degrade the carbohydrate, protein, or fat components of milk. Souring caused by Lactobacillus sp. and Streptococcus sp. can cause formation of curds and a sour taste. Some of these reactions are desirable and produce cheese. Contamination of milk by Clostridium sp. or intestinal bacteria like Escherichia coli produces gas. Growth of Alcaligenes sp. or Klebsiella sp. causes the milk to become slimy. This variety of reactions reflects the nutrient-rich nature of milk. Microorganisms that spoil fruits and vegetables are often found in soil; such foods are unlikely to be spoiled by intestinal microorganisms. However, fresh produce can sometimes be contaminated with incorrectly treated manure that has been used as fertilizer in the field or untreated rinse water containing intestinal pathogens from sewage.

Botulism

Although improvements were made to commercial appertization processes, the commercial canning industry nearly foundered in the early part of twentieth century because of problems with the disease botulism. Botulism is a paralytic and deadly illness caused by botulinum toxin produced by Clostridium botulinum, which is normally an innocuous bacterial inhabitant of soils and aquatic sediments (see Microbes in Focus 21.2). Because C. botulinum is a strictly anaerobic, endospore-forming chemoorganoheterotroph, the low-oxygen and high-nutrient content of canned foods provides ideal conditions for germination of spores and growth of the organism. Botulinum toxin is the most toxic natural substance known. Early commercial canning procedures sometimes did not eliminate C. botulinum endospores from foods entirely. As a result, the industry was plagued with periodic outbreaks of botulism, resulting in severe illness and regular fatalities among those who consumed the contaminated food. Intense consumer reaction to botulism outbreaks led to research by commercial canners in the 1920s to establish protocols designed to vanquish endospore survival in canned products.

•Reduction of water activity can be achieved by drying food out or by adding solutes (sugar/salt).

Because microorganisms need available water to flourish, removing water from food can prevent spoilage, sometimes indefinitely. In freeze-drying, which is also known as lyophilization, the food is first frozen and then dried under a vacuum, often with subsequent storage in watertight packaging. The process is expensive but efficient and can be useful for special purposes. Freeze-dried foods are compact and lightweight, making them convenient for transport. Campers often carry freeze-dried food in a backpack and add water on site to rehydrate it before consumption. Drying reduces aw by removing water. An alternative for decreasing the amount of available water in foods is to bind water molecules by the addition of solute, such as sugar or salt, making water unavailable to microorganisms. For thousands of years, humans have practiced food preservation by the addition of large amounts of sugar or salt (NaCl) to food. Neither option is viewed as particularly healthy today, but this strategy is still used in foods like jams, jellies, and salted meat

Control of Temperature is a useful way of preserving food. -Cooling or heating can be used (pasteurization). -Usually cheap and highly effective. -May be combined with pressure to eliminate endospores (canning). -May alter taste or texture of food, though.

COOLING In industrialized countries, surely the most prominent food preservation method is cold temperature in the form of refrigeration. Like sugaring or salting, this practice has deep roots; the ancient Romans reportedly used ice and snow to pack seafood. Modern household refrigerators are usually set at about 4°C, whereas commercial chillers may operate at 1-2°C. However, cold storage is not foolproof. Some bacteria can tolerate refrigerator temperatures; such psychrotolerant microorganisms can grow and spoil food even while the food is refrigerated. An example is Listeria monocytogenes, introduced in the opening story of this chapter. Fortunately, most human pathogens do not grow well at refrigerator temperatures, instead preferring warmer temperatures of a living body. FREEZING Extending storage by freezing is possible for some foods, although not all. Ice crystal formation during the freeze-thaw process can damage food tissue, producing a mushy thawed product that bears little resemblance or taste to the original fresh material. In addition, aw is reduced by the conversion of liquid water to ice crystals. It is important to monitor how long frozen food has been stored, because the cold, dry atmosphere of the freezer will lead to physical defects such as freezer burn. Moreover, some slow microbial growth may be possible in small pockets of fluid that do not freeze because of their high solute content. Frozen foods will remain unspoiled for months, but not indefinitely. HEATING High-temperature processes also have a role in food preservation. Most obviously, thorough cooking of a food will kill most or all of its microbial community, although once cooled, the food item is subject to recontamination. Canning is a high-heat process that involves heating a food to 100°C or above for an extended period of time, usually 20-100 minutes and under pressure of 10-15 psi. Traditional canning processes often use a temperature of 121°C.

Conclusion

Due to handling and waster runoff/contamination, microbes will always have a chance at getting to your food and water We will have multiple ways of detecting eliminating, and preventing contamination. We will also have ways of exploiting microbes to enhance the quality and variety of our food in a safe manner.

Fermentation of Milk Products •Fermentation of milk products oLactic acid bacteria (L A B) are used to produce: •Cheese •Yogurt An amazing array of microorganisms, and variants on metabolic pathways, contribute to food fermentations, and the exact biochemistry that is taking place in each of the thousands of fermented products found globally is still not completely known. However, we are aware of some of the main metabolic pathways and groups of microorganisms involved. One group that is particularly prevalent in food fermentation is the lactic acid bacteria (LAB). Certain LAB, such as Lactococcus lactis (Microbes in Focus 17.2), are capable of fermenting the milk sugar lactose, a disaccharide of glucose and galactose, and are used in producing fermented dairy products such as cheese, buttermilk, koumiss, kefir, sour cream, and yogurt. In commercial production of yogurt, milk is first centrifuged to remove some of the fat (Figure 17.13). Milk solids are increased by evaporation concentration or the addition of powdered milk solids to enhance the rich texture of the final product. The preparation is pasteurized, and a defined starter culture is added. Lactococcus thermophilus and Lactobacillus delbrueckii subspecies (subsp.) bulgaricus are used because they produce volatile aromatic compounds that produce the distinctive aroma and flavor of yogurt.

FIGURE 17.13 Process Diagram: Steps in making yogurt Milk fats are reduced by centrifugation, and milk solids are increased. Pasteurization minimizes populations of microorganisms present in the whole milk, and a defined starter culture of LAB is added to initiate fermentation. Lactic acid production coagulates milk proteins to thicken the milk. Fermentation proceeds until 0.9 percent acidity and pH 4.4 are reached. KNOW THIS PROCESS Other LAB are involved in producing fermented vegetables and meats. Leuconostoc mesenteroides is active in sauerkraut and Lactobacillus brevis in fermented meats.

Product defects may occur when the wrong microorganisms dominate a fermentation.

FIGURE 17.15 Process Diagram: Steps in cheese making Starter cultures are added to pasteurized, homogenized milk to induce acid production, but they also impart flavor, aroma, and texture. Acid production coagulates milk protein and forms curds. Addition of citric acid or rennin facilitates coagulation. For loose cheeses, the curds and liquid whey are mixed together, ready for consumption. For creamy and hard cheeses, the curds are separated from whey and pressed. Ripened cheeses are aged by storage, allowing microbial growth to continue. Unripened creamy cheeses are not aged. know how to make various cheeses!

Sometimes, things can go wrong. -The wrong microbe gets in the mix and produces the wrong fermentation product. --> Lactic acid instead of ethanol in wine --> Production of CO2 gas instead of lactic acid. - Infection of microbes by lytic bacteriophages can stop a desired fermentation from occurring. - Starter-culture microbes may also lose plasmids with desirable genes for fermentation.

FIGURE 17.19 Cultivating koji for sake brewing A Toji, or brewing artisan, is cultivating koji in a traditional manner. In this case, steamed rice is inoculated with Aspergillus oryzae spores (left). The inoculated rice is then placed into wooden containers to cultivate the koji mold (right). Individual manufacturers have their own proprietary versions of koji, some of which are centuries old. Soy sauce and sake use koji starter-mold cultures and Aspergillus species for fermentation.

During secondary treatment, a trickling filter is often used. -These are simple, passive, and inexpensive. -Crushed material is used as a base support for a growing biofilm of microbes. -As the material trickles across the surface of the support, the biofilm uses it as a food source.

FIGURE 17.27 Trickling filter a. Overview of a trickling filter system. Wastewater is sprayed evenly over a filtration bed of rock, shown in b. about 2 m deep or over plastic matrices 12 m deep. The nutrients in the percolating wastewater support a microbial biofilm on the bed matrix, as shown in c. Biofilm formed by microbes that have colonized the extensive inner surface area of a small 2.5-cm-diameter plastic matrix (manufactured by Headworks BIO, Inc.) forming the bed of a treatment system. Microbial metabolism removes pollutants from the effluent.

•Irradiation of food can prevent spoilage. -This does not cause food to become radioactive itself. -Strength of microbe elimination depends on type of radiation (but all cause damage to microbe DNA). •Non-ionizing radiation (UV): Surface level only, not strong •Ionizing radiation (gamma/X-rays): Stronger, more penetrating what is this and why are differences in wavelengths? Should people be concerned about radiation when discussing this type of food preservation? Ionizing radiation refers to radiation of short wavelength with sufficient energy to remove electrons from molecules in a medium, producing ions. In cells, ionizing radiation produces oxidative damage and toxic free radical generation, all of which directly or indirectly damage nucleic acids. Ionizing radiation in the form of X-rays or gamma rays can be used to pasteurize or even sterilize foods, particularly in regions where refrigeration is not widely available. Ultraviolet (UV) radiation has a wavelength range of 10-380 nm. UV radiation in the wavelength range of 240-280 nm is the most lethal form of non-ionizing radiation. It causes the formation of damaging thymine dimers in deoxyribonucleic acid (DNA), which prevents transcription and DNA replication. UV radiation does not penetrate materials very well, so its use is limited in the food industry to surface sterilization only. UV lights may be installed in rooms where cheeses or sausages are stored for aging or ripening to reduce the microbial load in the air. UV light may also be used to reduce viable microorganisms on surfaces such as warehouse shelving or in equipment such as plastic milk bags used for aseptic filling of liquid foods.

FIGURE 17.9 The electromagnetic spectrum Ionizing radiation on the short-wavelength end of the spectrum includes some ultraviolet (UV) light (less than 124 nm), X-rays, and gamma rays. The only form of non-ionizing radiation used for food irradiation is longer wave (240-280 nm) UV light.

Foodborne infection requires consumption of the organisms themselves. When these living pathogens are ingested, they actively multiply within the body, typically in the small or large intestine. In the body, these organisms may also produce a toxin that initiates symptoms when the level becomes high enough.

Foodborne infections may appear the day after ingestion of the offending food. Bacteria that commonly cause foodborne infections include Salmonella, Shigella, E. coli, and Campylobacter. Thorough cooking of food usually prevents foodborne infections.

Addition of Chemical preservatives can also be used to prevent food spoilage (often by reducing pH). -Natural antimicrobial preservatives include: •Bacteriocins (nisin) •Lactic acid •Acetic acid (vinegar) -Artificial preservatives include: •Sodium benzoate •Proprionate •Sorbates •Sulfur dioxide •Nitrites what are the types of preservatives?

For example, sodium benzoate helps preserve products such as fruit juices, pickles, ketchup, salad dressings, and soft drinks. Significantly lowering pH inhibits growth of even yeasts and molds that can generally tolerate relatively low pH. Acetic acid is one of several weak organic acids used as food preservatives. In 1908, sodium benzoate was the first chemical food preservative permitted by the U.S. Food and Drug Administration (FDA). Propionates and sorbates have also been used in some types of food for decades. All of these acids have "generally recognized as safe" (GRAS) status awarded by the FDA, permitting their widespread use. biocontrol through the use of microbial products called bacteriocins, produced by some bacteria that have been used in food fermentations for millennia. Bacteriocins are small proteins that do not affect the organisms that produce them, but exert a negative effect on other closely related bacteria.

FIGURE 17.2 Water activity (aw) Foods with a high aw value, approaching 1.0, have readily available water and can support microbial growth. Foods with a low aw value, approaching 0, contain very little available water; these foods are resistant to microbial growth.

Fruits and vegetables are rich in carbohydrates but low in protein. As a consequence, their spoilage patterns reflect metabolic pathways that microorganisms use to degrade carbohydrates, resulting in accumulation of acids and gas from fermentation of sugars

Pasteurization Goal: Kill 99-99.9 % of pathogens, spoilage bacteria, and fungi.

Heat can affect the flavor or texture of some food products adversely, making them unsuitable for high-heat processing. Heat processing, or pasteurization, was developed by Louis Pasteur to avoid such problems. Pasteur's original goal was to develop a process to prevent wine from souring without causing irreparable damage to the wine from heat. He found that mild heating could prevent souring without greatly affecting the flavor of the wine. His success was primarily due to killing the majority of alcohol-metabolizing acetic acid bacteria and other spoilage microorganisms in the wine. Today, pasteurization uses a variety of techniques to destroy microorganisms without cooking the food product. Pasteurization processes include heating, irradiating, or applying high pressure.

Fermentation with Mold •Fermentation with mold -Extensively used in Asia •Miso = Fermented soybean paste •Tempeh = Fermented soybean •Soy sauce •Sake know the difference from with bacteria and examples. KNOW THIS

However, mold fermentation of solid substrates is used extensively in Asia to produce food products, such as miso, a fermented soybean paste used as a base for soups and sauces. FIGURE 17.18 Process Diagram: Production of soy sauce Prepared wheat and soybeans are inoculated with koji—a mixture of cultivated mold strains. The koji fermentation step encourages growth of the aerobic mold throughout the substrate to maximize extracellular enzyme production. The resulting breakdown products from starches and proteins are used by LAB and yeast in the next anaerobic moromi mash fermentation stage. The end product contains ethanol, low pH (lactic acid), and salt, all of which are inhibitory to growth of microbial contaminants.

•Use of modified-atmosphere packaging (M A P) -Vacuum packing takes oxygen out of the package. •Many microbes need oxygen to survive and multiply. -Sometimes, packaging is also flooded with C O 2 to achieve the same effect. -Red meat may be packed in a high-oxygen environment to reduce growth of harmful anaerobic microbes and keep the meat a desirable red color. What is an easy name for MAP?

Many foods are rapidly spoiled by the activities of aerobic microorganisms. One way to slow such activity is to remove the oxygen (O2) required by the growing microorganisms. This strategy is employed in modified atmosphere packaging (MAP), or vacuum packaging. MAP is frequently coupled with refrigerated storage. In vacuum-packed foods, microbial respiration and respiratory activity of fresh vegetables and fruits leads to O2 depletion and carbon dioxide (CO2) accumulation within the sealed package. In other MAP processes, the atmosphere surrounding the food is altered and maintained by gassing the package or container with an appropriate gas mixture before sealing. For fresh fruits and vegetables, aerobic microorganisms are largely responsible for spoilage. MAP packaging for such produce typically uses high levels of CO2 (greater than 10 percent), low levels of O2 (less than 10 percent), with N2 making up the balance.

How can we keep our water supplies safe? FIGURE 17.24 The water cycle Recycling of Earth's freshwater supply occurs through the water (hydrologic) cycle. Aquifers and surface water are sources of drinking water. Excessive removal from these sources can prevent their renewal.

Many ways of improving food safety. Water covers much of the Earth, but - ONLY 3% is FRESHWATER. - Not all of that 3% is in a form ready to be consumed - Waster can build up and run off into water (sewage)

Increase in Acidity of Food in foods can prevent spoilage. -Pickling places foods in vinegar or allows fermentation to naturally drop the pH over time. -Many microbes cannot grow (or die) in a low-pH environment. FIGURE 17.8 Common pickled foods Pickling involves the production or addition of lactic or acetic acid in food to reduce the pH below 4.6, which is inhibitory to microbial growth.

Microbial spoilage occurs most readily in a pH ranging from 6 to 8, where many microorganisms prefer to grow. Food is preserved from spoilage if the pH can be made more acidic or more alkaline without impairing the food's appeal. Humans ingest very few alkaline foods due to their bitter taste; egg white is one such alkaline food. We don't seem to mind acidity, however, and perceive it as tartness. However, too much acid in a food can be perceived as sour. In a process known as pickling, the pH of a food can be reduced by storing or marinating it in a chemical such as dilute acetic acid (vinegar) or by allowing lactic or acetic acid production to occur naturally through microbial fermentation. Pickling achieves a pH of less than 4.6, a condition that deters the growth of most microorganisms

Wastewater treatment is a multistep process.

Pre-treatment: Physical removal of particulate matter by sedimentation and oils by skimming. (of large objects) Pretreated wastewater flows into large tanks where it is typically retained for several hours to separate these materials. Primary treatment: Physical removal of sediments and grease that form primary sludge. Oil and grease and any material less dense than the water float to the surface and are removed by skimmers. Suspended materials that are denser than the waterfall to the bottom of the tank and are removed by a raking apparatus that scrapes the material from the tank bottom into a hopper. Secondary treatment: Uses trickling filter/activated sludge unit to form complex biofilms that break down organic compounds over time Tertiary treatment: Not always used; filtration method Disinfection: Chlorination, UV light exposure, or ozonation FIGURE 17.26 Process Diagram: General schematic of a wastewater treatment plant In an effective treatment plant, wastewater moves through a pretreatment screening, followed by physical primary treatment and biological and physical secondary treatment. Effluent from these treatments is subjected to final disinfection before being released to a natural water system. Treated, collected solids (sludge) can be added to soils.

Activated sludge systems may be used instead of a trickle-filter system. -Depends on formation of flocs: -- Clumps of biomass of absorbed material and biofilm microbes - Aeration tank mechanically mixes sludge - Settling tank/ clarifier collects sludge to remove it from the effluent. Activated sludge systems are used for municipal wastewater treatment plants. An activated sludge process relies on the formation of flocs—clumps of biomass consisting of adsorbed material and biofilm microorganisms. Activated sludge is comprised of accumulated flocs, the formation of which is critical to the efficient function of the activated sludge unit. The word activated refers to the effect that the perfusion of oxygen throughout the otherwise thick and oxygen-free mixture has on the resident microorganisms in the sludge. An activated sludge unit is composed of two parts: an aeration tank, which mechanically mixes the sludge and purges it with air or sometimes pure O2, and a settling tank or clarifier that collects sludge

Recall from Chapter 6 that aerobic metabolism is more efficient than anaerobic metabolism, so biofilm microorganisms exposed to oxygen will be more effective in removing materials from the effluent. FIGURE 17.28 Aeration tank of activated sludge system Effluent enters the aeration tank where a constant stream of air oxygenates and mixes the wastewater to promote the aerobic growth of floc-forming microorganisms responsible for the removal of organic and inorganic matter.

•Understanding how food and water can be affected by microbes is important to humans. -Food spoilage can reduce the amount of food we have available for consuming. •Food preservation methods can make food last longer without spoiling. •Use of microbes to modify food can produce new varieties (cheese, yogurt, vinegar, etc.). -Microbes can contaminate food and water, causing human disease. •We must have ways of recognizing contaminated food and water and/or eliminating contamination to make such items safe for consumption.

Sandwiches of packaged luncheon meats are a lunchtime staple for many people. Sitting in their plastic-sealed containers on the supermarket shelf, these meats appear anything but dangerous. If the meat has been contaminated during packaging, however, the microbes that survive and thrive can sicken or kill those who eat it. Such was the case in Canada in August 2008 when meats contaminated by slicing machinery at a Maple Leaf Foods facility in Toronto, Ontario, were distributed to supermarkets nationwide. Fifty-seven people fell ill with listeriosis after eating meat that had been contaminated by a bacterium called Listeria monocytogenes. Of these, 22 people died, almost 40 percent of those who became ill. This high percentage of deaths is not unusual because L. monocytogenes is one of the more dangerous sources of foodborne disease. In the United States in 2017, more than 4,000 outbreaks of listeriosis were reported, causing 102,908 illnesses and 126 deaths. Listeria monocytogenes has a short history as a pathogen, being first identified as the cause of illness in a 1981 outbreak in Halifax, Nova Scotia. For most of us, the risk posed by L. monocytogenes is very low. For the very young, the elderly, pregnant women, and people whose immune system is not working properly, the situation can be disastrously different. Pregnant women may suffer spontaneous abortion. Others can suffer from vomiting, nausea, cramps, diarrhea, severe headache, constipation, and fever. The bacteria can access the bloodstream to reach the brain and spinal cord, eyes, and lungs. The widespread pattern of the 2008 Canadian outbreak is an unfortunate consequence of our modern method of food preparation. Often, food prepared at one or several large facilities is shipped to distant locations. When the system works, it provides an economical method of getting food to many people. When something goes wrong, however, the consequences can be national or even international in scope.

Anaerobic sludge digester - Last step of secondary treatment -Digests remaining organic wastes in anaerobic processes by microbes: Can take 2 to 4 weeks to break down sludge Can operate in mesophilic mode (35 to 37C) or thermophilic mode (50C, speeding up reactions) -Biosolid material (undigestable) is dehydrated/incinerated or used as fertilizer. Completion of secondary treatment of wastewater involves the anaerobic sludge digester—essentially a large anoxic bioreactor operating in a semicontinuous mode—that allows final anaerobic digestion of remaining organic wastes

Sludge is typically held from two to four weeks in the digester, which can be designed to operate in a mesophilic mode, functioning optimally at 35-37°C, or thermophilic mode, functioning optimally at about 50°C. The latter has the advantage of more rapid reaction rates and a greater degree of pathogen kill. FIGURE 17.29 Anaerobic sludge digester Anaerobic microbial digestion of organic material in sludge takes place under anoxic and heated conditions. The majority of the digester is underground to help maintain temperature. The roof is suspended so that it may be vented in response to accumulated CO2 and CH4 (methane) within the reactor. Methane can be collected as a fuel source.

•How are microorganisms used in food production? 17.3 Food Fermentation -Fermentation can be used to produce a number of different types of foods. •Often well-characterized starter cultures of microbes are added to food to begin the fermentation process. •The process is monitored and controlled for a desirable outcome of flavors and textures. •Many foods are produced in this way―some that you may not have thought of before (see the table on the next slide for some examples!) Read and know other examples!

The concept of a starter culture or inoculum was established in these early societies. Starter cultures are preparations of microorganisms that are added to food to aid in the production of fermented products. For example, taking leftover dough from a previous bread-making session and adding it to a new dough ensures that bread will rise

FIGURE 17.7 Flash-heating pasteurization Flash-heating allows continuous flow-through. In order to kill contaminating microorganisms, a small volume of raw liquid flows for a specified but very short period of time through metal plates or tubing (left) heated to a very high temperature. Liquid then passes through a cooling unit (right) to quickly lower the temperature to prevent excessive heat denaturation of nutrients. The product is then packaged.

The original process of milk pasteurization involved heating milk in batches to a temperature of at least 62.8°C and holding it there for 30 minutes. This is termed the low-temperature hold (LTH) method (Table 17.3). Modern dairies often use flash-heating, which forces a small volume of milk between metal plates or pipes heated to very high temperatures (Figure 17.7). Small volumes reach the required temperatures much more quickly and evenly than large volumes. Exposure to high temperatures for only a very short time will kill the required number of microorganisms. Another advantage of flash-heating is that nutrients, such as vitamins, are not exposed to long periods of heat that may destroy them (as occurs in the LTH method).

17.4 Foodborne and Waterborne Illness -Food contaminated with microbes can cause illness in humans and animals. -This is quite common. -Usually referred to as foodborne intoxication -Due to microbial toxins in food -May be due to actual microbes in food (referred to as a foodborne infection)

Top either infectious agents responsible for foodborne illness estimates for the United States.

•What is hurdle technology? -Using multiple levels of antimicrobial control in food. -The levels often work synergistically, giving better overall protection than a single method alone.

You may wonder if a sealed pouch made of relatively impermeable plastic filled with food and depleted in O2 might prove a suitable habitat for Clostridium botulinum, a strict anaerobe. Indeed, if MAP technology is used with foods where the presence of C. botulinum is possible, then additional preservative methods must also be applied. This is an example of hurdle technology, which employs multiple constraints or "hurdles," each of which the microorganism in question must overcome in order to proliferate. -A pH of < 4.6, as for pickling or marinating in acidic solutions -An aw of < 0.96, as in salt-curing (see Figure 17.2) -A high NO2 or O2 gas concentration, as for MAP packaging -A temperature of < 4.4°C, as for freezing or refrigeration

Moist foods are more perishable than dry foods. However, water content alone is not the key factor in spoilage. Rather, one must consider the amount of water that is available and accessible to microorganisms. You may recall from Section 7.2 that this is called water activity (aw). Most fresh foods have an aw above 0.99, which is sufficient to support the growth of most microorganisms. Most spoilage bacteria require an aw above about 0.91 (Figure 17.2). Foods that are dry or possess a high-solute content, usually of sugar or salt, have reduced aw values. Fungal spoilage organisms, such as molds, can grow at lower aw of approximately 0.8. Foods with lower aw values are generally quite stable and have long shelf lives. Although spoilage bacteria require an aw above about 0.91, one notable salt-tolerant bacterium is Staphylococcus aureus, which can grow at an aw around 0.86. This can be significant for foods such as ham with a relatively high salt content that reduces available water, because some strains of S. aureus secrete microbial enterotoxins that act on the intestine, leading to foodborne illness (Microbes in Focus 17.1).

fresh foods aw above 0.99 spoilage bacteria: aw above 0.91 DRY FOODS reduced aw values fungal, mold grow at aw =~ 0.8 foods with longer aw values have long shelf lives FIGURE 17.1 Spoilage of fruit by microorganisms a. Fungi produce spores that are easily dispersed and can survive in an inert form for relatively long periods. When they land on a nutrient source, such as food, the spores germinate and the fungus proliferates. b. Bacterial soft rot produces discolored and water-distended lesions that develop and expand rapidly. Other microorganisms may invade, and a soft, slimy mass of lysed cells and bacteria fills the inside of the fruit.