Category Food

Chilling food in your refrigerator slows down the two main causes of decay – the growth of mould and bacteria, and chemical breakdown, as in the over-ripening of fruit.

In a domestic refrigerator the temperature is kept between about 34 and 41°F  (1 and 5°C). This is low enough to keep most of the food we use fresh for up to a week. Growth of decay causing organisms is slowed down but low temperatures do not destroy the organisms. Similarly, chemical breakdown is also slowed but not stopped completely so the food will spoil if kept for too long.

The temperature in a home freezer is normally about 0°F (-18°C), which will preserve food for anything from a month to a year, depending on the quality and type of food frozen.

 

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Switch on the microwave oven, and you are switching on a powerful magnetic field which oscillates in the same frequency band that is used for radio broadcasts and radar. Microwaves in the field can be used to cook food rapidly by making the water molecules in the food vibrate at almost 2500 million times a second. This action absorbs energy from the magnetic field and heats the food.

As all the energy is absorbed by the food and not wasted on heating the surrounding air on the oven itself, the process is far quicker and more economical than traditional cooking methods.

The microwave energy does not heat the utensils in the oven because the materials they are made of such as China and glass do not absorb energy from the magnetic field. However, the plates do not come out of the oven cold, because they are heated by the food.

Special cookware

Many other materials besides China and glass can be used in a microwave oven – such as plastic, paper and cardboard. And special cookware – which is transparent to microwaves – has been developed for use in microwave ovens.

Metal container should not be used because metal does not transmit microwaves but reflects them. So foods should not be covered with aluminium file. Wooden utensils are also best avoided in microwave ovens because wood always contains some moisture, and this can cause it to split when it heats up.

 Long wave radio waves have a wavelength measured in thousands of metres. The microwaves used in microwave ovens have a wavelength of about 5in (120 mm).

An electromagnetic wave is a vibration of electrical and magnetic fields, constantly going from negative to positive. Microwave oven is operate with the waves that vibrate 2450 million times a second- a frequency of 2450 megahertz MHZ.

Water molecules have a positively charged end and negatively charged end. The vibrating positive negative microwaves interact with the positive negative water molecules, attracting and repelling them and making them twist first one way, then the other. This also happens 2450 million times a second.

The most important part of the microwave oven is the electronic tube, or magnetron, that generates the microwaves. The magnetron was developed in 1940 by British researchers at Birmingham University, and was first applied usefully in radar. It’s domestic potential was first realised buy the Raytheon Company in the United States in the early 1950s.

 

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As long as 5000 years ago, seaweed was used as a food and medicine in China. And today it is still eaten around the world. It is found in the Japanese fish and rice dishes sushi, in Welsh laver bread (seaweed fried with oatmeal) – and in ice cream.

Seaweeds provide ingredients called alginates and carrageenans, which are used in ice cream as stabilisers, so that ice cream does not become grainy in the freezer.

When ice cream is made, most of the water content freezes into very small ice crystals about 50 microns in size. (A micron is 1000th of a millimetre). As the thermostat of a deep freezer switches the refrigeration off and on, the temperature in the freezer fluctuates. Water melts off the crystals as the temperature rises, causing smaller ones to disappear. Then as the temperature drops again, the water freezes on to the remaining crystals, which grow in size. This causes the texture to coarsen.

Stabiliser slow the growth of the ice crystals, by forming protective layer around them, so the ice cream retains its smooth texture longer.

Carrageenan is obtained from red seaweeds found around rocky shores in northern Europe and North America. The seaweed is harvested, then dried to preserve it. Carrageenan can be extracted by immersing the dried seaweed in hot water. The extract is purified and then ground to a fine, cream-coloured powder. Alginates are exhorted in a similar way from brown seaweed in several parts of the world.

Before being used in ice cream, the alginate or carrageenan is usually blended with other compounds such as guar gum (extracted from the seed of the guar plant of India and Pakistan) and Locust bean gum (from the seeds of the Locust bean or carob tree, which grows in the Mediterranean area) to provide mixtures which are more effective than a single stabiliser.

Stabilisers are used in ice cream at about 0.2% of weight, so that a litre of ice cream contains less than a gram of the seaweed extract.

 

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Blue cheeses are the result of moulds that produced blue pigments. Originally cheeses must have been accidentally contaminated by natural moulds floating in the air as spores. Once the mould had grown, it would colonise cheese cellars or storage caves and subsequent cheeses stored there would also be contaminated.

Modern blue cheese production minimises the chances of the mould failing to grow. A suspension of the mould spores is either added to milk at the same time as starter bacteria or is sprayed over or injected into curd pieces which have been drained of the liquid component of milk, whey.

The mould used to make blue cheese is called Penicillium roquefortii, named after Roquefort in France.

The cheese has to be porous since the mould needs oxygen and space to grow, so pressing – the compression of curd in the mould, used for some other types of cheese – is avoided.

The cheeses drain slowly and are relatively soft. They have to be turned each day or they lose shape.

The temperature and humidity vary according to the type of cheese and its age. Typical conditions at between 41 to 59ºF (5 to 15°C) with humidity of 90-95%. Too high humidity encourages excessive growths of yeast and bacteria; to low causes the cheese to crack.

As the cheese matures, the supply of oxygen to the mould growing in the pores can be increased by piercing the cheese with stainless steel needles. For Stilton cheese, 4-48 holes maybe made at each piercing. The former use of copper needles led to the popular misconception that the blue colouring was caused by copper wires oxidising in the cheese. As the mould grows it not only produces the blue colour, but also enzymes. These break down the fats and proteins, producing the characteristic flavour, and making that cheese softer.

 

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The French author Honore de Balzac drank his coffee black, cold and thick as soup to keep him awake while writing through the night. He is said to have consumed 50,000 cup in his lifetime. The French philosopher Voltaire drank an estimated 72 cups a day and Beethoven is said to have used 60 beans for every cup.

Today, coffee drinkers in many countries use the instant variety, which is what remains after ground coffee beans have been ‘brewed’ and the water evaporated. In Japan, Australia and Britain, about 90% of coffee drunk is instant. In the USA, however, it accounts for only a quarter of the coffee drunk, while almost all Scandinavian and Italians prefer their coffee freshly ground.

Attempts to make instant coffee in the late 1800s failed because of poor flavour. Then, in 1906, George Washington, a Belgian born engineer of English parentage, was visiting a mountainous region of Guatemala when he noticed a brown deposit on the outside of a coffee pot that had boiled over. Tasting it, he thought its flavour was pleasant and the deduced that this was due to lower atmospheric pressure at high altitude. The lower boiling point of water at the higher attitude, he decided, allowed the water to evaporate with less heat damage to the dissolved coffee.

Three years later, he opened the G. Washington coffee refining company in Brooklyn, New York. His instant coffee’s immediate success was boosted during the first world war, when the US Army included it in infantry rations.

Making instant coffee involves ‘brewing up’ on a giant scale. Coffee is delivered to the manufacturer ready roasted, blended and ground. It is then percolated – the process of filtering hot water through the grounds – in batches of up to 2000lb (900kg) at a time. Some of the water is evaporated from the coffee to leave a highly concentrated liquid.

To produce powered is instant coffee, the liquid passes through a large cylinder in which it is subjected to hot air, which evaporates the remaining water. This leaves the powdered coffee ready for sealing into jars or packets.

Granular instant coffee is made by freeze drying. The concentrate is first frozen, and broken up into granules. The granules are then heated gently inside a vacuum chamber. Because water boils at low-temperature in a vacuum, the remaining moisture can be evaporated off without subjecting the coffee to create heat and impairing the flavour.

 

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Any hacker who has ever bivouacked up a mountain will appreciate the advantages of freeze-dried ready meals. They are a quarter of the weight of fresh foods, remain tasty for years in sealed packages and can be eaten hot by adding boiling water.

The process was first used in the 1950s when the American government sponsored a scheme to provide lightweight ration packs for astronauts, explorers and the armed services.

The freeze drying process preserves food by rapid freezing, followed by complete dehydration to remove all the moisture. The food is placed in a tightly sealed chamber between hollow plates containing refrigerant liquid, which freezes the food while a high-powdered pump creates a vacuum.

When the food is frozen hard and the pump has removed nearly all the air, the cold refrigerant liquid in the hollow plate is replaced by warm gas. The ice in the food is then converted directly into vapour without first turning into the water.

To keep its nutrients, flavour and appearance, the food must be frozen as quickly as possible, but the drying process is quite slow. The steam is immediately removed by the vacuum pump, but the food takes about 20 hours to dehydrate completely. It must then be packaged to protect the contents during handling, and to seal out all oxygen and moisture.

The freeze drying process gives the food an open texture, and if oxygen enters, any fat becomes a rancid . If moisture gets in, microbes in the food group, causing it to decay like fresh food.

Because the food must be frozen rapidly, the best results are obtained with the food which is sliced or ground. Fish, meat, vegetables and fruit can all be freeze dried, but coffee and made up meals with chopped ingredients are particularly successful.

Today, improvements in technology have shortened the process and ‘accelerated freeze dried’ products are becoming more common. They are still expensive, but are extremely convenient when weight and lack of every refrigeration have to be considered. The are reconstituted by adding boiling water, and retain their nutrients, appearance and flavour very well for several years.

 

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Meals that have been bombarded with the radiation are served to transplant patients – and patients in intensive care units – in many hospitals. Some countries irradiate supermarket foods. Similar food is also eaten by American and Russian astronauts during their missions in space.

Food irradiation is designed to kill bacteria that cause food poisoning – a particularly grave threat to weak hospital patients, or astronauts who are cut off from medical help. Irradiation also helps to prevent food from going bad while it is being stored. Although at the low recommended doses it does not give indefinite preservation.

The process has been known since 1921 when an American scientist discovered that X-rays could  kill a parasite, Trichinella spiralis, which can contaminate pork.

Irradiation now is done by exposing food on a conveyor belt to the radio active isotopes caesium 137 or cobalt 60 in the lead shielded chamber with walls 5 feet (1.5m) thick. The isotopes give off electromagnetic ionising radiation in the form of gamma rays. Alternative methods used X-rays or beta rays, both forms of radiation.

Doses of radiation

The effect of radiation is measured in units known as Grays (Gy). Doses less then 1kGy are used to kill parasites in meat. Low doses are also used to kill or sterilise insects in cereals, cocoa beans and other groups. They prevents stored crops such as potatoes and onions from sprouting, and slow down the ripening of some fruits.

Medium doses from 1-10kGy – can extend the shelf life of food by reducing spoiling organisms in meat, fish, fruit, vegetables and spices. They also killed food poisoning bacteria such as salmonella, in raw poultry and shellfish. About half the poultry sold contains live in salmonella, which are usually killed by cooking. However, some of the bacteria can survive if the poultry is not cooked right through.

For complete sterilisation of foods, high doses of above 10kGy are used. Animals on sterile diets regularly eat food irradiated up to 25kGy with no observed ill effects. Sterile diets are used when scientists are seeking side effects in various trials – and want to eliminate food poisoning.

Food irradiation is governed by international agreement through United Nations agencies.

 

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Every year 1 million tons of prawns and shrimps are caught for the frozen food industry and sold through restaurants in supermarkets. Until the late 1950s, when peeling machines were developed, shrimps and prawns had to be shelled by hand, making them a luxury food. Now they are almost all shelled by machines and have become more reasonably priced items supermarket freezes.  Whereas an expert hand peeler can be 56lb (25kg) of prawns an hour, machine can peel 880lb (400kg) an hour.

The great bulk of a prawn fishermen’s catch, however, is wasted. Only about sixth consists of shrimps and prawns the rest is called ‘trash fish’ or ‘by-catch’ which is thrown back into the sea.

The head of the prawns, which is in fact the stomach, thorax and head, is usually removed by hand at sea immediately after the catch has been sorted. It has to be removed as quickly as possible because enzymes and bacteria can cause unsightly dark patches on the prawns called black spot. The fishermen are experts at snapping the heads off two prawns at a time, one in each hand.

After the heads have been removed the catch can be stored in ice for up to 4 days. Once it has been landed it is quickly transferred to a processing factory.

The prawns are inspected for quality and graded in size on the machine with angled rollers. As the gap between the rollers gradually widens, successively larger prawns fall through onto different conveyor belts.

The prawns are carried to machines which have been adjusted to peel a specific size range. The prawns slide into slots where they are pressed down onto a blade that splits the shell and flesh down to the ‘vein’ along the length of the tail.

The prawns then pass onto a bed of narrowly spaced dollars. Adjacent rollers turn in opposite directions, pulling the shells through the gaps and leaving the flesh which is too large to pass between.

Finally the vein has to be removed. The vein is really the prawn’s intestine, which is full of food and sand. Removing it increases the value of the prawns and improves their keeping quality. The vein should have already been dislodged by the cutting process and as the prawn passes through a revolving cylinder with a rough surface, the vein catches on the indentations and is washed away by water.

The prawns are cooked in boiling water, emerged in iced brine and individually quick frozen, packaged and distributed.

 

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In 1987, just one fast food chain- McDonald’s hamburgers – sold upwards of 200,000 million French fries or chips, throughout the world. And they were all nearly identical in length and thickness.

To ensure that their chips have a consistent taste and that they are ready at the same time as the hamburgers, a fast food chain puts call it’s potato through the same preparation process.

Before the potatoes are even planted, the company agrees to buy a farmers entire crop. It also specifies the type of potatoes and their growing conditions, such as soil treatment and fertilisers. Varieties of potatoes are chosen – such as Majestic Maris Piper or Russet Burbanks – for their keeping quality, because they will be needed all year round. Large, round ones are preferable because they are easy to peel and clean and there is little waste.

Once harvested, the potatoes are loaded into silos or large boxes and kept in the dark at a constant temperature of 48-50°F (9-10ºC), which shows down organic changes that would make them deteriorate. They are regularly inspected and batches that show any signs of rotting are removed.

At the processing plant potatoes are again inspected and weighed to check their density, which reveals if they have become soft and ‘floury’. They are passed over sieves to remove small stones on dirt, and magnets and electronic detectors remove any metal particles. After the potatoes have been washed, the skins are softened with alkali and removed with steam.

Next, they are sliced into square sectioned strips. Thin strips will cook faster but will become hard if they are overcooked. An ideal size for fast food chips is a cross-section of about 1/4in (6mm).

The cut strips are blanched on the wire mesh conveyor belt which passes them through a tank of hot water or a dilute solution of phosphate or citrate salt. This helps to prevent them discolouring.

The cut potatoes are done frozen, bagged in polythene, packed into cartons and kept at -4°F (-20°C) until collected for delivery to the fast food outlets in refrigerated vans.

Within only minutes of being removed from freezers in the restaurants, a bagful can be fried and served up.

 

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Stuffed olives – those popular cocktail – hour appetisers- are usually stoned and stuffed by machines. The pimiento (red pepper) filling is a kind of paste, made by mixing the pimiento with a gelling agent.

The olives are aligned in rows on a perforated conveyor belt, and the stones removed by an automatic machine head, rather like a dentist’s drill. A nozzle then pumps the pimiento paste into the drilled hole in the olives.

The stones are later ground and used for animal feed – or to produce a low-grade oil.

Some high quality stuffed olives, containing anchovy, smoked salmon, chopped almonds or other nut fillings, are stoned with handheld scoop and then filled by hand.

Olive stuffing machines can process 1800 olives a minute. Manual olive stuffers mostly in Portugal can average only around 18 a minute, and a poorly paid.

Olives are grown around the Mediterranean, and in Peru, Chile, California, British Columbia and Australia. The green variety, preferable for stuffing because of their firmer texture, are the unripe fruit.

Before the stuffing process, olives are soaked in the solution of sodium hydroxide (caustic soda), which reduces their bitter flavour. Then, after washing in water, they are immersed in brine with added lactic acid to neutralise any remaining alkali. While soaking they are allowed to ferment, which encourages harmless bacteria and yeast to grow to achieve the correct characteristic flavour and texture.

 

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