No one doubts the importance of food in our lives. As you learned earlier, all active living organisms must have a constant source of energy. This energy may be supplied by materials a cell or organism has stored internally (e.g. our fat or carbohydrate) or it may come from an external source in the environment. Food (NUTRITION) supplies two major components of life, ENERGY and THE CHEMICAL BUILDING BLOCKS OF LIFE. Energy is required for the various enzymatic reactions that require an input of energy for the reactions they catalyze. For example, the movements of the muscles in our legs during a race or in our intestines as we digest our latest meal or to draw air into our lungs for breathing all require energy. Those of you who race or otherwise run for pleasure, know that it is recommended that you stock up on carbohydrates the day before a race so that you will have a ready supply of available fuel stored in your liver to supply the ATP your muscles will require. The glucose, in the form of the polysaccharide glycogen in your liver, is oxidized to provide the energy, as ATP, that your muscles need to keep moving. When you run out of a ready supply of glucose you "hit the wall" and have to begin using other sources of energy.
Food also supplies the structural material required for living organisms to make new macromolecules for repair of damaged structures or for new construction, such as the manufacturing of offspring. That is, food supplies living organisms with the raw materials necessary for cell construction, as well as other essential components of life such as vitamins and minerals. A balanced and sufficient diet must contain all the calories (units of energy) required to maintain life and the materials for cell maintenance and construction. In this country most of us have never suffered any sort of serious food shortage, nor do we even know anyone who has. Sadly, however, there are pockets of poverty, ignorance and life-style choices that result in some Americans being malnourished to varying degrees. In contrast to our food affluence, educated people know that much of the world's population goes to bed hungry and suffers from constant nutritional deprivation of one kind or another. This condition ranges from full starvation to perpetual lethargy to permanent brain damage of millions of people, mostly children. It is further common knowledge that any increase in the world's population requires a commensurate increase in food production or starvation becomes inevitable. There is no way around this cruel, uncompromising formula.
Microbes play a crucial role in food resources. Microbes are responsible for the direct loss of much food through food spoilage and through the destruction of the crops and animals from whence the food comes. If you doubt this, just search the back regions (if you dare) of your refrigerator for those ugly remains of last month's forgotten leftovers (that furry pizza piece from the party last month). Conversely, microbes are responsible for manufacturing, via their biochemical activities, much of the favorite food we humans enjoy. Further, the microbes, again through their biochemical activities, preserve foods so that we can enjoy them at a later date. Finally, through their activities, microbes are vital to maintaining the fertility of the soil; so much so that we would soon starve if the soil microbes were to vanish.
To fully understand food issues, it is necessary to appreciate how much our ideas about food are the result of social training and experience. For example, everyone is aware that different social groups have different food preferences. A brief walk through downtown Pullman (is there any other kind?) will take you past several ethnic eating establishments, offering a tempting variety of cuisine which most of you have probably sampled one time or another. Even within our own country we have regional food preferences such as "Southern Cooking" or "Midwestern Cuisine" etc. However, the different preferences in food across the world are enormous, including Africans that live off of fresh cattle blood and yogurt (they're very healthy), Asians that drink urine-flavored brews, to Eskimos who enjoy rotted fish. Many of you like buttermilk and cottage cheese, but the English consider the latter to be "spoiled milk". Conversely, many English and Scots relish a pheasant that has been hung out at room temperature for several days until it is rather "ripe" (the nose tells you when it is ready). Our local meat counters offer animal intestine and testicles and some students order their pizza covered with small fish that contain their entire gut contents and others enjoy raw fish and raw oysters. Ethyl alcohol, which is the metabolic waste, or urine equivalent, of yeast, is considered nectar-of-the-Gods by people all over the world. Other peoples relish ants, grasshoppers or bees and look forward to the harvest of these culinary delights with the same enthusiasm Americans hold for that Thanksgiving turkey. The bottom line in all cases is that whatever we label "FOOD", provides us with the energy and nutrients we require to maintain life.
Food has played a critical role in history. Archeological evidence suggests that many, perhaps most, ancient civilizations disappeared as a result of losing the ability to feed themselves. The most common reasons cited for this disaster are climate and ecological changes, combined with overpopulation. Conversely, the simultaneous growth in population and the industrial revolution were fueled by new discoveries in agriculture that made it possible to feed many more people a good diet (well fed people do more work and work smarter). In history you have been taught about how the "SPICE TRADE" was the driving force behind the intense burst of exploration that inspired Columbus, among others, to make their long and perilous voyages of discovery. Actually it was the microbes that really provided the impetus for those journeys.
We, in the US, have only had widespread refrigeration available for approximately the last 50 years. When I was a young boy, I remember following the ice-man with my friends as he delivered 50 pound blocks of ice to our "iceboxes" on hot summer days. We children would grab ice chips from the rear of his truck and suck on them (remember, there was no TV in those days). A 50 lb. block of ice, when placed in an insulated icebox would keep the meat, milk and butter cool for several days and retard their spoilage. It wasn't until the 1940's that my home had an electric refrigerator. In the middle ages foods like meat, milk etc. spoiled quickly, particularly on warm days. However, even spoiled meat is nutritious in spite of its rank odor and bad taste, and it beat starvation by a long stretch. Thus people, even the wealthy, frequently ate meat in various stages of active decay and pretended to actually like it. However, they found that if you added SPICES to this rotting meat the strong flavors the spices imparted covered up the rotten aroma and minimized gagging during dinner. Therefore, spices became the "had to have item" for every host who liked to throw parties and impress his friends. Since spices only came from the far east by camel and small, leaky boats, those that survived the journeys were able to command top prices for these prized, gourmet, barfing-preventing items. The large profits involved stimulated an intense interest in finding quicker and safer routes to the source of these valuable spices (does "worth-its-weight-in-gold" mean anything to you?), hence the exploration explosion of the 1400 to 1600s. One might even say that the MICROBES and not Columbus discovered America.
Another role of microbes in the middle ages was that of producing miracles. It seems that the damp, dank churches of the middle ages were perfect incubators for the growth of the bacterium Serratia marcescens in the sacramental wafers. Under these conditions the bacterium produces a bright red pigment that resembles BLOOD, thus the appearance of blood-covered holy bread; clearly a miracle in the eyes of the people of that time. (Reference: ASM News May 1994).
FOOD PRESERVATION
As supermarkets were rare during most of human evolution, food was in short supply and fresh food was even more limited. Early man may not have been exactly rocket scientists, but they could tell the difference between really-rotten and not so-rotten food. So when someone discovered a way to preserve food while it still had a reasonably decent taste and odor they were likely considered a hero. What follows is a description of some of the old and new food preservation methods.
HEAT STERILIZATION
One of the problems with war is that soldiers insist on being fed regularly (remember, they hold the weapons). In the early 1800s, Napoleon found that the joy of his life, his large army, could no longer feed itself by stealing from the local peasants and thus his plans to conquer the world were stalled. His solution was to offer a reward for anyone who could figure out how to preserve food so he could take it along with his army, thus keeping them and him happy (the only unhappy ones being those he conquered). In 1810 a man by the name of APPERT found that if he put food in a bottle, jammed a cork tightly in it and placed it in boiling water for an hour or so the contents didn't spoil. BINGO!!, he won the prize, Napoleon got his war, and learned just how seriously cold a Russian winter could get. This procedure, known as STERILIZATION, eventually developed into the canning process. In the process of sterilization all living organisms are destroyed, including bacterial spores. As you will learn later, the most deadly biological toxin is produced by the spore-forming bacterium, Clostridium botulinum. C. botulinum is an obligate anaerobe that can grow in seal containers like cans and jars, therefore the canning process is specifically designed to destroy the C. botulinum spore. This is achieved by heating food to a minimum of 123o C or 253.4o F for 15 minutes.
In the home, sterilization is carried out using a PRESSURE COOKER. Many of you probably have seen your grandmother, or perhaps your mother, using this container to sterilize home-canned food. The pressure cooker works as follows:
A pint or so of water is placed in the bottom of the pressure cooker.
The food to be sterilized is placed in the container with the lids loose.
The top is placed on tightly and the water is brought to a boil until all the air is vented through the outlet port.
Then a weight is placed on the outlet port. This weight is adjusted so that steam will only escape once the pressure has reached 15 pounds per square inch. At this pressure the temperature will reach 123o C at sea level.
Once this temperature is reached and steam begins to bleed from the port, heating is continued for a period of time necessary to bring all the food in the containers to 123o C for 15 to 20 min.
The heat is turned off and the contents are allowed to cool.
Finally, the pressure cooker cover is removed, and the jar lids tightened immediately to prevent contamination from entering.
In the microbiology laboratory and commercial canning companies sterilization is achieved by using large containers that operate exactly the same as the home pressure cooker. The laboratory instrument is called an AUTOCLAVE. In commercial canning processes the sterilization containers may be as large as rooms and the food is often wheeled in on large carts.
Can you explain the difference between PASTEURIZATION and STERILIZATION. If not go "directly to jail and do not pass GO or collect your $200".
As described above, except for Eskimos and other inhabitants of the far north, cooling has only emerged as a common means of preserving food since the mid 1800s when the ice-making machine was discovered. Prior to that time it was common in northern climes for people to cut large blocks of ice from local lakes and to store them in insulated warehouses for use during the summer months to cool their beer and other food items.
Cooling as a food preservative is utilized at two levels, 7 to 4o C and -20o C or lower. The higher temperature is commonly used in home refrigerators. At this temperature, the growth of microbes is slowed down but not stopped. Indeed, some microbes (#psychrophiles) grow optimally at these temperatures. The failure to prevent spoilage at this higher temperature is attested to by anyone who has attempted to use milk older than two weeks in a refrigerator or who has left fruits and vegetables in a 'fridge' for extended periods. At the lower temperature the food is frozen. As microbes are unable to grow in frozen material, freezing is one of the most successful means of preserving food with minimal change in flavor or loss of nutritional value. The major draw back to the use of cooling is that (a) it is expensive and (b) it also preserves many pathogens that happen to be present in the food when it was cooled. As a matter-of-fact the storage of living material at temperatures of -70o C or lower is the best way of maintaining cells in a state from which they can be subsequently cultured. Such material as sperm, ova, embryos (human and other forms of life), all types of microbes and tissue cells can be frozen and stored for years with little loss of viability providing the procedure is carried out properly.
DRYING
Drying as means of preserving food may very well be the oldest method of preservation known to man. Almost certainly it was an accidental discovery made by our primitive ancestors living on the hot plains of Africa. Most likely, our ancestors frequently came across carrion (a sort of road kill) that had dried in the arid conditions. Being hungry, they ripped off the dried meat and chowed down. It didn't take them long to recognize that it wasn't spoiled, that it was light and that it stayed unspoiled as long as it remained dry. Some budding hairy-Einstein soon realized that fresh meat could be dried by placing it in the hot sun and the human race was off to the races, so to speak.
Drying is employed today as a common means of food preservation by all peoples living in warmer climates. Generally the food, such as fresh meat, is cut into small strips and placed on rocks exposed to the sun, or hung over sticks by a campfire. The pieces must be small so that the food dries fast enough to prevent spoilage. In the case of meat, one trick is to hang it high enough so the flies can't get to it and lay their eggs in it. As the water evaporates and the food dries, the OSMOTIC PRESSURE (the result of hydrophilic molecules binding water molecules) increases to a point where microbes are unable to compete with the water-binding material in the food for the remaining water. Since microbes are unable to grow without free (available) water, the food is safe from spoilage, even though it may retain significant bound-water. In some cases (beef jerky) the food is salted prior to drying. The salt is inhibitory to many microbes and contributes to the high osmotic pressure that prevents microbial growth.
SALTS AND OTHER CHEMICALS
The use of salt as a food preservative is probably as old as drying, if not older. All mammals need salt and they will travel long distances to obtain it. Our human ancestors certainly visited the ocean or salty lakes to collect the salt that had dried on the shore. Occasionally animals or fish must have died in pools of salty water and then dried in the sun leaving their desiccated carcasses impregnated with salt. Again our hungry ancestors were unlikely to turn down a potential meal and they must have quickly recognized that the salted food was unspoiled and remained so as long as it was impregnated with salt. The salted food served a dual role as a source of nutrition and of sodium chloride, and as it dried it was easier to transport. Before canning, salted meat was the staple food on ships that traveled any significant distances away from land (hence the term "ol salt").
NITRATE (NO3) AND NITRITE (NO2) (SALTPETER):
Nitrate and nitrite salts are used in many foods today as both a preservative and to prevent meat from browning. The bacterium #Clostridium botulinum is an obligate anaerobe in that the presence of even a tiny amount of free oxygen prevents its growth. Yet, C. botulinum readily grows in prepared meats like sausage. Nitrate and nitrite are OXIDIZING AGENTS that are chemically similar to oxygen. As such they, like free-oxygen, inhibit the growth of C. botulinum in foods. In addition, they prevent certain substances in meat from becoming REDUCED, which causes them to turn the meat brown, suggesting that it may be poor quality. In recent years scientists have discovered a link between nitrate/nitrite and the formation of #carcinogens. As a consequence of this the FDA has required the removal where possible of these chemicals from foods or the lowering of their concentration to the minimal level. The use of nitrate/nitrite poses a classical cost/benefit conflict. That is, is the cost (cancer) of using these substances in our food supply balanced by the protection against death by botulism poisoning? Each of us should decide that ourselves don't you think?
SULFITE (SO2) and VITAMIN C:
Most of you have observed the "BROWNING" of fruits and vegetables; the apple, peach or banana you eat turns brown before your very eyes, even as you chow it down. Generally, people feel that "brown" food items are spoiled or at least of lower quality. The browning results from the actions of enzymes in the fruits and vegetables that rapidly react with oxygen to produce brown-colored chemicals that protect the damaged food from microbes; i.e., the brown chemical is inhibitory to many microbes. Sulfite is a powerful "REDUCING" chemical that BLOCKS THE BROWNING RESPONSE, and it is inexpensive, & effective in tiny amounts. Therefore it is common to rinse fruits and vegetables in restaurants in solutions containing SULFITE. This insures that items that were prepared several hours before will remain "fresh-looking" all day long on the customer's plates. At the concentrations used, sulfite is not toxic, but a small percentage of people are highly allergic to sulfite and an exposure to even a tiny quantity of it on lettuce etc. may be sufficient to induce a violent #allergy attack. This is why restaurants often have signs telling their customers that they are using sulfite on their foods. Another powerful reducing agent that serves the same purpose is vitamin C (ascorbic acid). This vitamin also is inexpensive, is effective in small amounts, plus it is beneficial to those who ingest it. However, because it is more expensive than sulfite and it tends to decay faster, it is not universally used.
ORGANIC ACIDS:
As you recall, all microbes require an #optimum pH or acidity in their environment to grow. If there is too much acid or base, a microbe will not grow. As the by-products of many microbial fermentations include the production of chemicals like ACETIC ACID (vinegar), LACTIC ACID, and PROPIONIC ACID it is not too surprising to find that humans, and other life, can actually use these substances as nutrients. However, when they are added to foods in sufficient quantities to lower the pH below that which will support the growth of most food-spoilage microbes, they can serve as natural food preservatives. Again, our ancestors recognized that "SPOILED" foods such as milk and certain vegetables, retained their nutrition upon becoming acidic and remained eatable (preserved) for long periods. Thus was born choice food items like yogurt, sauerkraut, pickles, cheese and buttermilk. Artificial acids, like benzoic acid, inhibit the growth of some molds, thus it is added to breads and other bakery products that require long shelf live. In many foods, like the sauerkraut you made in lab, salt is combined with acids to preserve food.
ANTIBIOTICS:
Most common antibiotics are inexpensive, stable, safe and effective in small quantities. With their ability to kill or inhibit many microbes, antibiotics might seem the perfect food preservative. However, all is not what it seems. Using antibiotics for food preservation is like using 100 dollar bills for toilet paper; it gets the job done but it is not the best use for that item. As you've learned , we are in grave danger from infections produced by antibiotic-resistant microbes. The use of antibiotics in preserving food and in animal feeds has been demonstrated to increase the spread of antibiotic resistance between pathogens. Although some action has been taken to limit the use of antibiotics for these purposes, it is still done in many places.
RADIATION:
Atomic radiation is becoming widely used in the preservation of food, although its use remains controversial and frightening to many people. In 1997 the FDA approved radiation as a means of preserving meats. Many of the prepared meals available on the supermarket shelves at room temperature have been sterilized by radiation. Atomic radiation is lethal to all life when used in high doses. To sterilize food by this technique, the food is placed in a protected room and exposed to a high dose, usually of gamma radiation, from radioactive wastes refined from atomic power plants. A dosage that had been determined to be lethal to all microbes, including bacterial spores, is used.
Current studies indicate that increased use of irradiation to destroy contaminating microbes would slightly increase the cost, but it is suggested that the increase in cost would be offset by the reduced loss of stored foods. Use of radiation to eliminate Salmonella enteritidis contamination from eggs is under consideration.
A new for of radiation involving high energy electrons has been approved and foods sterilized in this way will be on the store shelves early next year. This is NOT radiation from radioactive material, but involves the use of "fast electrons". How well people will accept foods sterilized in this way remains to be seen
Available in http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap18.html, as
'FOOD MICROBIOLOGY, FOOD BORNE DISEASES'. Adapted and illustrated to be posted by Leopoldo Costa.
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