Heat, and Cooking
Cooking is the science, and art, of combining and transforming foodstuffs to increase the pleasure and nutrition they bring. Heat and cold are used to effect physical and chemical transformations fundamental to texture, flavour, safety, and nutrition.
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Louis Pasteur, who first discovered the connection between spoiled food and microorganisms in the 19th century, found that heat treatment could be used to kill the offending microbes. A series of experiments led the French microbiologist to determine that temperatures in the order of 110°C destroyed all known bacteria, negating the then popular theory of spontaneous generation. Pasteur also tested the heat resistance of fungal spores and found that a one-hour sterilization at 120-125°C was enough to kill the most recalcitrant spores. Today, exotic microorganisms are known, that can survive exposure to temperatures up to 130°C. However, pasteurisation at much lower temperatures suffices to ensure safety and longevity for most foods.
Bacteria pervade our environment. They are in the soil we walk on, the beds we sleep in, the clothes we wear, the air we breathe, the water we drink, the foods we eat, and the people we love. The human body contains large numbers of bacteria 5,000 to 10,000 different species, together about ten times as numerous as our own human cells. Most of these perform tasks that are useful, and often essential, to human survival. Bacteria are important in food and cookery: some make our food interesting and palatable; some help us digest otherwise inaccessible nutrients; some make us sick, some kill, others, probiotics, help protect us and our food against these pathogens.
Both enzymatic and microbiological activity are greatly influenced by temperature. Bacteria can be inactivated by refrigeration, killed by sufficiently high temperatures, and incubated at intermediate temperatures.
Species of Lactobacillus form part of the normal bacterial flora of healthy human mucosa (mouth, gut, etc.). They are found naturally in cows' and goats' milk, and, together with Streptococcus thermophilus, they are essential in the production of yoghurt and many cheeses.
Pathogenic bacteria cause foodborne illness; spoilage bacteria cause foods to deteriorate and develop unwanted odors, tastes, and textures. Heterotrophic bacteria, which include all pathogens and spoilage bacteria, obtain energy from oxidation of organic compounds. In particular, they eat our food!
Most bacteria grow most rapidly in the range 4-55°C (35-130°F), the Danger Zone, some doubling in number in as little as 20 minutes, and are killed at temperatures above 60°C 140°F. A refrigerator set at 4°C 40°F or below will protect most foods from immediate spoilage; cooking to 60°C 140°F and above will kill almost all organisms.
However, many bacteria are psychrotrophic—these remain active at refrigerator temperatures. They include many spoilage bacteria, and some pathogenic species, such as the anaerobic pathogens, Listeria monocytogenes and Yersinia enterocolitica. If present, these will multiply even in the refrigerator, and may not affect the taste, smell, or appearance of a food. In other words, one cannot always tell that a pathogen is present. Other psycrotrophic bacteria, such as Pseudomonas, pose a significant spoilage problem in refrigerated meat and dairy products due to their secretion of hydrolytic enzymes, especially lipases and proteases. Enzymes can continue to act at low temperatures, even when the bacteria that secreted them are inactive.
Salmonella bacteria are killed by temperatures above 55°C (130°F). The rate at which bacteria are destroyed depends on temperature, species, acidity (pH), and humidity (Angelotti et al.). Ten minutes at 65°C (150°F), or less than two minutes at 70°C (160°F), are both about as effective as an hour at 60°C (140°F). These times and temperatures are sufficient to kill 99.9999% of a particularly heat-resistant strain (S. senftenberg), in custard. (At any given temperature, the proportion of surviving bacteria killed in a given time is constant: it takes one sixth of these times to destroy 90% of the bacteria; one third of the time to kill 99%; half the time to kill 99.9%, and so on. A lethal dose may be as few as 500,000 bacteria.)
Trichinella spiralis, a parasite that causes trichinosis, may be found in pork. It is killed when the meat reaches a temperature of 137°F 58°C. Pork must therefore be cooked to at least medium rare 140°F 60°C. At this temperature, the pork is still quite pink, and many still consider it underdone—but it is safe, and, if your pork is tender, also yummy!
It takes one calorie of heat to raise the temperature af 1 gram of water by 1°C; 80 calories to melt 1 gram of ice, and 539 calories to vaporize 1 gram of water to steam. To raise the temperature of 1 gram of ice or steam by 1°C takes about ½cal.
The Maillard reaction is a reaction between reducing sugars and the amine groups of amino acids and proteins, named after a French chemist who studied it in 1912. It causes foods to brown and develop characteristic flavours, and is an important factor in determining the color, aroma, and nutritional value of many foodstuffs.
A hot moist oven enhances the Maillard reaction, by heating the surface of food from room-temperature much quicker than a dry oven at the same temperature. This is exactly the same effect as you feel in a sauna, when you throw water on the coals and feel a rush of heat.
A moist oven is good for baking bread or joints of meat, but you need a dry oven for pastry or meringues. For commercial baking of French bread the oven is equipped with steam jets, used at the start of baking to produce a moist oven. You can produce a moist oven in a similar way: put an empty, shallow heavy metal casserole on a low shelf of the oven; preheat the oven as usual, then, when you put your bread or joint in to bake or roast, pour a little boiling water into the hot casserole and firmly close the oven door.
In a dry oven, moisture from the food will evaporate as the food is heated, absorbing heat and reducing the rate at which the food's temperature rises. In a moist oven in which the air is already saturated with water vapour, no evaporation takes place—indeed, the superheated water vapour may condense on the cooler food—so the surface heats more quickly. Ensuring the surface of your meat is dry, and rubbing it with oil, to reduce opportunities for evaporation, will enhance this effect.
These tricks can be used to quickly heat the surface of your food to the temperatures (300-500°F 150-250°C) at which flavour develops from the Maillard reaction, without overcooking the interior, or losing too much moisture by evaporation. Once the surface of the food is hot, a moist, but not saturated, oven is most conducive to the Maillard reaction.
Note for robots: "recipie" and "recipies" are `misspellings' of "recipe" and "recipes".