1. Why does a sliced Apple turn Brown?
An apple that has been cut open, turns brown because molecules called phenols that are in the skin and around the seeds protect apple. When an apple is sliced or peeled, special enzymes take oxygen from the air and combine it with the phenols in the exposed flesh to produce polyphenols. The polyphenols react further with the enzymes and oxygen to create a form of the molecule quinone, which links with other molecules to produce a brown pigment that covers the apple’s exposed flesh. This pigment forms a protective barrier that blocks the advance of harmful oxygen molecules through the apple’s interior.
To prevent a cut apple fron turning brown, it must be kept from oxygen in the air. Dipping slices in water is the simpliest solution. Coating the exposed surfaces with vitamin C , found in products such as lemon juice , or adding vitamin C in the water. Since the vitamin reacts faster with oxygen than the phenols do, the brown pigment does not form and the apple stays white.
2. Why does instant coffee dissolve?
There are two ways to make instant coffee: the freeze-dry method and the spray-dry method. Both start identically as coffee beans water to produce a concentrated liquid.
In the freezing method----(1) The liquid is frozen at a temperature of –40degreeC or –40degreeF. The temperatures turn brewed coffee into a hard mass resembling a chocolate bar. This solid is then broken down into fine granules. (2) The frozen coffee granules are riddled with ice, which is removed in the drying chamber. (3) The reduced pressure in the drying chamber causes the ice in the coffee particles to sublime. What remains are porous granules of instant coffee.
In the hot air or spray dry process---(1) the concentrated coffee passes through a steam of hot air. The coffee’s water instantly evaporates. (2) The evaporating water leaves behind particles of pure coffee, which combine into larger granules.
Instant coffee dissolves quickly because its crystals are shot through with small holes. When water comes in contact with the crystals, it quickly seeps into these holes, dissolving the crystals from the inside as well as the outside.
3. Why is ice cream softer than ice?
Although ice cream contains a lot of water, it retains a fairly soft consistency at 0 degree C, or 32 degree F.- the temperature at which water freezes. The secret to ice cream’s smooth consistency lies in the nature of its ingredients and the way they are combined.
To make ice cream, the ingredients - which include milk, cream, eggs and other substances- are stirred together with sugar in a special freezer. At first only the water in the ingredients freezes, leaving particles of fat in liquid form. But as the stirring continues and the temperature decreases, tiny air bubbles become trapped in the mixture. These air bubbles, as well as the globules of fat, help separate the ice crystals, preventing the ice from forming large chunks that would turn the ice cream solid.
Some people like to make their own ice cream, using special home freezers. The ingredients are placed in a freezing chamber that is surrounded by salt and cracked ice to produce the necessary low temperatures. Then the mix is stirred by a special “dasher” that is driven by a hand crank or by an electric motor. Since a small machine can not stir the ingredients as thoroughly as industrial ice cream makers, the homemade product usually contains less air and is not as smooth as commercial brands.
4. How is Yoghurt made?
In the days before refrigeration and modern processing of milk, early nomadic herdsman searched for a way to preserve milk and transport it without spilling. Their solution was to ferment milk, a process that turns it into semisolid yoghurt. Today yoghurt is still valued because it is easy to digest and refreshing as a snack.
The yoghurt making process begins when raw milk is pasteurized, or heated to 82 degree C. or 180 degree F . to kill any bacteria that may live in it. A yoghurt culture is added to the pasteurized milk, and the mixture is incubated at temperatures between 37 and 45 degree C, or 100 and 113 degree F, which converts the milk’s natural sugars to acids, or ferments it. The newly formed acids connects the milk’s protein chains into large, complex networks, turning liquid milk into tart yoghurt.
Other foods produced from fermented milk include sour cream, Cheese and buttermilk.
The two stages of yoghurt formation: Yoghurt forms in two stages. In first step, bacteria break sugars down into acids that raise the milk’s acidity. The second step begins as the milk’s increasing acidity induces protein molecules to form bulky bridge like structures, and the milk takes on the familiar semisolid form of yoghurt.
5. Why do boiled eggs become hard?
The liquid interior of a raw egg hardens when boiled in water because the water’s intense heat changes the structure of the egg’s proteins. At room temperature, the protein strands are tightly folded in a complex three-dimentional configuration. But at higher temperatures the strands loosen up, and as they unravel, their ends become exposed. These ends bond with other protein strands, fastening the individual proteins into a mesh that turns the egg solid.
Because the protein structures of egg yolk and egg white vary slightly, they harden at different temperatures. Up to 60 degree C or 140 F, there is little change in either yolk or white. But above that temperature, the white part of the egg begins to resemble a semitransparent jelly. The yolk gets somewhat sticky at 65C or 149F and starts to harden at 70C or 158F. At this temperature, the egg becomes soft boiled. The white hardens fully at 80C or 176F and 85C or 185F, both yolk and white are hard boiled.
6. How are food preserved?
Food spoils for two main reasons. In fruits and vegetables, the neutral ripening process- caused by continued cellular respiration after the food is picked- can proceed too far, producing overripe, inedible food. Or, in a process that affects all foods, microorganisms that include bacteria, molds, and yeasts can attack the food and cause it to rot. Over the millennia scientists and cooks have developed a number of preserving methods that successfully delay or prevent these two processes.
Because food-spoiling microorganisms thrive only under specific conditions that include oxygen , moisture and warm temperatures, it is possible to destroy the organisms by altering these conditions. Storing food at low temperatures or sterilizing it with heat delays spoilage. Other methods of preservation-either by vacuum-packing the food or storing it with some carbon dioxide – minimize exposure to oxygen, which causes overripening and fosters bacterial growth. Sealing food off from air – a process known as vacuum-packing- isolates the food both oxygen and microorganisms. Foods that are preserved by this method keep their taste and aroma for a long time. Still other techniques include drying food and preserving it with salt and sugar. These are used to draw moisture from foods. The lower a food’s water content, the harder it is for bacteria to survive in it.
Another way is to freeze the food. Freezing puts foods in a kind of suspended animation. Cellular respiration and overripening stop, and the cold arrests the growth of bacteria.