Microbial growth also varies with other environmental and physiochemical conditions. Growth rates are generally highest on substrates that provide a well-hydrated, well-balanced mix of carbohydrates, proteins, and lipids and sufficient quantities of essential minerals Sterner and Elser These conditions are most readily met on fresh meat, fish, seafood, fruits, and some vegetables.
Microbial growth rates are lower, and may approach zero, when the composition of food deviates from such ideal mixtures Blackburn , Pitt and Hocking Water content is especially important. Microbes growing on fresh animal and plant tissues are in approximate osmotic balance, because the water content of active bacterial and fungal cells and of these substrates are similar Pennington and Douglass The dehydration of food causes osmotic physiological stress and reduced growth rates for the microbes Blackburn , Pitt and Hocking Some foods are naturally preserved by having low water content and high concentrations of osmotically active compounds.
In this condition, they can resist microbial growth and remain viable and nutritious for many years. Many nuts contain high concentrations of fats and oils but relatively little water and carbohydrate Pennington and Douglass In addition, many of the herbs and spices that have been used for millennia to preserve food produce secondary compounds that are distasteful, toxic, or antibiotic Mitscher , Swain For example, the common herb thyme Thymus vulgaris contains thymol—a monoterpene—which is a powerful inhibitor of microbial growth figure 2 ; Marino et al.
The effect of increasing concentrations of essential oils of the common herb thyme in retarding spoilage by different species of bacteria. Microbes were cultured in culture broth, and the time elapsed to grow to a threshold population density was recorded.
The lines were fit using ordinary least-squares regression. Source: The data are from Marino and colleagues and Shils and colleagues Abbreviation: ppm, parts per million.
Depending on temperature, water content, nutrient composition, and the presence or absence of antibiotic compounds, foodstuffs remain nutritious and nontoxic to humans for periods from a few hours to many years. Food scientists use shelf life to quantify the length of time a food can be stored and remain suitable for human consumption or commercial sale, but the storage times can vary by orders of magnitude depending on the identity of the foodstuff, environmental conditions, and methods of preservation figure 3.
At one extreme, fresh fish, meat, shellfish, and many fruits and vegetables can be stored for only a few days, even under refrigeration see supplemental table S2. Foods that naturally contain little water, an unbalanced nutritional composition, or possess antibiotic compounds or protective layers last longer.
At the other extreme, dry seeds and frozen foods can be stored for years. The shelf life of representative food items, with and without the use of storage technology. The time to spoilage varies widely over untreated food, from less than a day in fish to over a month in root vegetables such as potatoes to many years in grains such as wheat that have been naturally dried on the stalk. The increase in shelf life that results from the use of storage technology varies widely by the technology used but can be orders of magnitude different.
See supplemental table S2 for additional information and data sources. Many of the food-processing techniques used to retard spoilage and extend shelf life date back over at least tens of thousands of years figure 3. When they could, hunter—gatherers avoided spoilage by eating food soon after harvest and by keeping animals and plants alive until they were eaten Bailey Nevertheless, most early cultures inhabited temporally and spatially fluctuating environments, so they collected food during times of abundance and stored it for times of scarcity.
They understood enough about the causes of spoilage and the variation in susceptibilities among different foods to develop simple, robust techniques for processing and storing foods. Through millennia of observation and experimentation and depending on geographic location and cultural history, humans developed many methods to extend the shelf life of common foodstuffs. They learned how to manipulate osmotic conditions through the addition of sugars, salts, or lipids e.
They learned to use the secondary metabolites in various herbs and spices not only to mask the odor and taste of partially spoiled food but also to slow microbial growth and retard spoilage. Because microbial growth rates increase with higher temperatures and water availability, preventing spoilage has always been a major problem in tropical climates. It is no coincidence, therefore, that a wide variety of herbs and spices are used in the cuisines of tropical cultures throughout the world Billing and Sherman , Sherman and Billing The development of agricultural societies and city—states resulted in high population densities, with only a proportion of the population directly involved in food procurement Weiss et al.
By cultivating and domesticating wild plants, farmers were able to produce more food than they could themselves consume. This led to the diversification and specialization of labor, with some members of the population devoted to other tasks, such as toolmaking, animal husbandry, defense, and religion. Diets based on cereal grains or tubers were rich in carbohydrate but poor in protein, which were supplemented in various ways by different cultures: by fishing, hunting, keeping domestic animals, and consuming wild pulses Cordain et al.
When milk from domesticated mammals became an important part of the diet of both sedentary and nomadic cultures, the shelf life of this highly perishable product was extended by separating the high-lipid cream and churning it to make butter, which is much more resistant to microbial spoilage. In some cases, specific microbial cultures were added to milk to make fermented products, such as yogurt and cheese, that extended storage time and allowed humans with adult hypolactasia to consume them McCracken Lengthening storage times using beneficial microbial cultures and controlled fermentation also became an important way to preserve fresh vegetables e.
For the most part, the storage technologies used by agriculturists were modest modifications of the methods developed by foraging societies. For example, farmers in temperate climates harvested ice in the winter to keep stored food cold into the summer months, and agricultural societies in Mediterranean climates used ethanol, vinegar, brine, and olive oil to preserve a variety of foodstuffs.
Changes in food-storage technologies accelerated with the transition from agricultural to industrial—technological societies. The concentration of an ever-increasing proportion of the population in cities means that an ever-decreasing proportion of farmers and fishers must produce all the food and that larger harvest areas and longer supply lines are needed. The increasing distance between harvesters and consumers means that spoilage must be prevented for longer periods, typically days to weeks, because food is transported over distances of hundreds to thousands of kilometers.
The technological advances of the industrial age revolutionized the storage of many foodstuffs, allowing a greater variety of items to be preserved, but these new technologies often require large energy inputs to achieve increases in shelf life figure 4. Canning—using a combination of heating to kill microbes and sealing the food in hermetic containers to prevent recolonization—was pioneered by Appert in and developed commercially by Donkin in Featherstone Refrigeration using compressed gas was pioneered in the early nineteenth century Reif-Acherman Freezing—the natural extension of refrigeration—was commercialized by Birdseye in and rapidly applied to preserve a wide variety of foodstuffs Archer Energy use for food storage in kilocalories per kilogram [kcal per kg].
Increased use of energy does not necessarily prolong shelf life. Much preservation still relies on ancient principles, which use little energy and can still preserve food for long periods.
The most conspicuous exception is compressed gas refrigeration, especially freezing, which requires continual energy input. The bounding ellipses show the storage time and energy inputs for different food types for that storage type.
Despite the impressive innovations and technological advances that accompanied the industrial revolution, most preservation of foodstuffs still relies on the principles discovered by ancient cultures: retarding microbial population growth by using low temperature, dehydration, wood smoke, unbalanced nutritional composition, osmotic stress, or organic chemicals. Modern preservation techniques often combine multiple methods e.
Even with the most modern techniques, the majority of the caloric requirements in contemporary industrial societies are typically met by cereal grains, which are still preserved primarily by simply keeping them dry. Food transport and storage are intimately interrelated, because transporting food over increasing distances requires preventing spoilage en route.
Advances in transportation technologies have played a major role in feeding the growing and increasingly urbanized human population. As cities grow, so do their ecological footprints and their dependence on more distant environments Wakernagel and Rees , Burger et al. They are increasingly dependent on larger areas to produce enough food and on longer supply lines to import foodstuffs harvested on distant farms, grazing lands, oceans, lakes, and rivers.
Because of spatial heterogeneity in soil types, climate, and aquatic productivity, these larger foodsheds allow for a more diverse and nutritious diet than was available to pre-industrial agricultural societies, although industrial agriculture has led to an overall homogeneity of commercially grown old- and new-world crops Khoury et al.
Advances in food transport have been achieved by some technological innovations that shorten transport time by increasing speed and others that decrease spoilage en route. Travel speed has increased by orders of magnitude over human history, ranging from a few kilometers per hour for the human- and animal-powered conveyances of hunter—gatherer and early horticultural societies to more than km per hour for jet planes of modern urban—industrial societies figure 5 a.
This increase in travel speed is primarily due to access to fossil fuels and to the successive inventions of the steam engine, the internal combustion engine, the jet engine figure 5 a , and associated infrastructure. For each mode of transport, the maximum possible speed has increased continually but at a diminishing rate. More importantly, however, the commercially practical speed increased rapidly and then plateaued at approximately 40 km per hour for boats and km per hour for railroad trains by the eighteenth century, 90 km per hour for automobiles trucks by the midtwentieth century, and km per hour for airplanes by the late twentieth century figure 5 a.
We suggest that this leveling off occurs at optimum economical speeds that reflect fundamental trade-offs due to physical and engineering constraints for each medium water, land, or air and source of power animal, steam, internal combustion, or jet engine.
Because energy use is a significant contributor to the economic cost of cargo transport, the economically optimal speed will be close to the speed that maximizes the energetic efficiency of cargo transport—the amount of energy required to transport a given mass of cargo across a given distance Karman and Gabrielli , Winebrake et al. The speed a, in kilometers [km] per hour and conveyance size b, in kilograms [kg] of human transport capacity, by medium, over time. The transport technologies powered by fossil fuels are outlined in black.
See supplemental table S3 for sources and calculations. Although the commercially practical speed has remained relatively constant for at least the last 50 years, continual innovations—mostly in engine and conveyance design and cargo capacity figure 5 b —have substantially increased energetic efficiency figure 6.
There is a general trade-off between speed and efficiency both within and across transportation technologies. Ships have always been more efficient than trucks, trains, or airplanes, and by far, the most energetically efficient means of long-distance transport is the cargo ship. Both the speed of a cargo jet and its per-capita energy cost of transport are approximately 20 times greater than those for a container ship figure 6. To take advantage of the energetic efficiency of water transport, larger and more efficient sea vessels and the associated changes in infrastructure, such as ports and canals, are currently under construction Panama Canal Authority , Beaubien The relationship between energetic costs in joules [J] per kilometer [km] per kilogram [kg] and speed kilometers [km] per hour for transportation fueled by animal metabolism the black dots and transportation fueled by extrametabolic processes typically fossil fuels in water blue , land green , and air red domains.
In all cases, the mass used to calculate the energetic costs represents both the transportation vehicle plus its fully loaded cargo. See supplemental table S3 for additional information and sources. Not surprisingly, the kind of vehicle used to transport foodstuffs depends primarily on economic optimization within physical and biological constraints.
These depend mostly on the cost of fuel, shelf life, and distance moved figure 7. Nearly all contemporary cargo ships, trains, trucks, and airplanes are powered by fossil fuel—mostly some form of petroleum—so the cost of oil figures large Notteboom and Cariou , Notteboom and Vernimmen Because of the trade-off between speed and efficiency, foodstuffs with short shelf lives have high transport costs.
At one extreme are fresh seafood, meat, and some fruits and vegetables, which spoil so rapidly that airplanes are used to minimize travel time across long distances. Additional energy may be used to run compressor cooling for refrigeration or freezing during transport in order to retard spoilage. At the other extreme are cereal grains and other dried foods with long shelf lives. For these food types, travel time is not an issue, and transport by ship or rail is most economical over long distances.
There are additional complications, however. For example, the journey of a food item from source to table almost always involves multiple modes of transport because of trade-offs among energy, speed, cargo size, distance traveled, and constraints to infrastructure. Each data point represents a particular food item, treated with a specific storage technology, and transported using a specific transportation technology.
This knowledge can also help us to work out how to preserve food for longer. Extrinsic factors are factors in the environment external to the food, which affect both the microorganisms and the food itself during processing and storage. Extrinsic factors include temperature, humidity and oxygen. Different microorganisms grow over a wide range of temperatures. Some microorganisms like to grow in the cold, some like to grow at room temperature and others like to grow at high temperatures.
This is of paramount importance in food safety, because if you know the temperature growth ranges for dangerous microorganisms it helps you to select the proper temperature for food storage to make them less able to grow and reproduce. The humidity of the storage environment is an important factor for the growth of microorganisms at the food surfaces. If you store food in a dry atmosphere , microorganisms are less able to grow than if the food is stored in a humid moist environment.
Therefore, dry conditions are better for food storage than moist conditions. Many microorganisms need oxygen in order to develop and reproduce: these are called aerobic microorganisms. A good example is Escherichia coli , a faecal bacterium which grows readily on many foods. If you keep food in a low oxygen environment, aerobic bact eria cannot grow and multiply. Conversely, there are some microorganisms that grow without oxygen, called anaerobic microorganisms.
An example of this is Clostridium botulinum , the bacterium causing botulism, which can survive in very low oxygen environments such as tinned foods. Intrinsic factors exist as part of the food product itself. For example, meat has certain characteristics that may promote the growth of certain microorganisms.
The following common intrinsic factors affect the growth and multiplication of microorganisms in foods. Environments that are acidic have pH values below 7; those that are alkaline have pH values above 7.
Most microorganisms grow best at close to the neutral pH value pH 6. Only a few microorganisms grow in very acid conditions below a pH of 4. Bacteria grow at a fairly specific pH for each species, but fungi grow over a wider range of pH values. For example, most meats naturally have a pH of about 5. At this pH meat is susceptible to spoilage by bacteria, moulds and yeasts; however the pH of meat can be lowered by pickling, which makes it less favourable as an environment for microorganisms to grow in.
Microorganisms need a moist environment to grow in. The water requirements of microorganisms are described in terms of water activity represented by the symbol a W , a measure of how much water is present. Most foodborne pathogenic bacteria require a W to be greater than 0. Think of some foods that store well when they are dry but become contaminated quickly when they are wet.
You may have thought of different examples: the one that we thought of is rice. When rice is dry it will store for a long time, but when it is cooked and wet it will go bad quite quickly and cause food poisoning. In order to grow, multiply and function normally, microorganisms require a range of nutrients such as nitrogen, vitamins and minerals. Microorganisms therefore grow well on nutrient-rich foods. The natural covering of some foods provides excellent protection against the entry and subsequent damage by spoilage organisms.
Examples of such protective structures are the skin of fruits and vegetables such as tomatoes and bananas Figure 8. Bacteria are a major source of microbial contamination of food, i. Viruses, parasites and fungi are also able to contaminate food and cause foodborne illnesses in humans. Microorganisms can enter food through different routes. Look at Figure 8. The most common routes of entry are discussed below. Microorganisms are found everywhere in our environment. Many types can be found in air and dust , and can contaminate food at any time during food preparation or when food is left uncovered Figure 8.
Imagine a kitchen where food is prepared and stored in rural communities, and think how easily microorganisms in the air and dust could contaminate the food. Many microorganisms present in soil and water may contaminate foods. Microorganisms also grow on plants and can contaminate food if care is not taken to remove them by washing or inactivate them by cooking. Soil is a particularly rich source of Clostridium bacteria. Water may be contaminated by faeces. Plants may also be contaminated by faeces if untreated sewage has been used as a fertiliser.
The intestines of all humans and animals are full of microorganisms, some of which are beneficial but others are pathogenic. Bacterial pathogens such as Salmonella, Campylobacter and Escherichia coli strain OH7 are common examples. Contamination of foods by faecal material is the major cause of food poisoning events. Escherichia coli abbreviated to E.
The strain called E. Many foodborne microorganisms are present in h ealthy animals raised for food, usually in their int estines, hides, feathers, etc. Meat and poultry carcasses can be contaminated during slaughter by contact with small amounts of intestinal contents. Animal hides are an important source of contamination of the general environment, the hands of meat worker s, and skinned meat carcasses.
Hides are a primary source of E. Hides become contaminated either because the outside of the hide is dirty, or because once removed from the animal, the inside of the hide is a good breeding place for microorganisms. Animal feeds are a source of microorganisms, especially Salmonella , which can contaminate poultry and other farm animals.
The organisms in dry animal feed spread throughout the local environment and may get on to animal hides, hair and feathers, as well as on people who handle the feeds. The term food handler can be applied to anyone who touches or handles food, and this includes people who process, transport, prepare, cook and serve food. The microorganisms transmitted to foods by food handlers may come from the hides of animals, soil, water, dust, gastrointestinal tracts and other environmental sources.
In food preparation at home, foodborne microorganisms can be introduced from the unwashed hands of people who are infected by bacteria and viruses, and who cook and serve the food to family members. Food utensils are cutting boards, knives, spoons, bowls and other equipment used in food preparation, which may become contaminated during food processing and preparation. For example, in families where there is no access to running water, the food utensils may not be properly cleaned, stored and handled, and may become a major route of food contamination.
Cross-contamination of food is the transfer of harmful microorganisms between food items and food contact surfaces. Prepared food, utensils and surfaces may become contaminated by raw food products and microorganisms. These can be transferred from one food to another by using the same knife, cutting board or other utensil without washing it between uses. A food that is fully cooked can become re-contaminated if it touches raw foods or contaminated surfaces or utensils that contain pathogens.
For example, you should never:. An unsafe temperature for food storage is a major factor in food contamination. Many microorganisms need to multiply to a very large number before enough are present in food to cause disease in someone who eats it. However, if bacteria can have warm, moist conditions and an ample supply of nutrients, one bacterium can reproduce by dividing on average every half an hour and can produce 17 million bacteria in 12 hours!
So, if you leave lightly contaminated food out overnight, it will be highly contaminated and infectious by the next day. Poor personal hygiene of food handlers is another major factor in food contamination. The most important contaminants of food are the microorganisms excreted with faeces from the intestinal tract of humans. These pathogens are transferred to the food from faecal matter present on the hands. We have already mentioned failure to wash hands after visiting a toilet as a source of food contamination.
Can you suggest other times when food handlers should wash their hands? Hands should be washed before starting work on preparing food, and after touching any food, surface or equipment that may be contaminated e.
Bad personal habits like scratching your hair and nose with your fingers also contributes to food contamination. Sneezing and coughing spreads contaminants and microorganisms through the air and onto uncovered food, and onto surfaces and hands that can transfer the infectious agents into food.
Foods can be damaged and also contaminated by pests. Many stored grains are lost through the damage done by pests, including termites mist , beetles, locusts, cockroaches , flies and rodents such as rats and mice. Pests can damage and co ntaminate foods in various ways, such as boring into and feeding on the insides of grains, or tunnelling into stems and roots of food plants.
For example, weevils cause large losses of stored grains, especially in warm and humid conditions such as in lowland areas of Ethiopia. Pests also dama ge the protective skin of foods allowing microorganisms to get inside the food and causing it to rot more quickly.
Pests can pollute food with their excreta, and with bodies and body fragments when they die. They also transfer microorganisms on to food while walking on it Figure 8. Overcooking is another such reason for spoilage of foods.
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