Category Career Queries

Ornaments could be made of pure gold but would get easily pressed out of shape. This is because gold is a soft metal, though it is heavy.

The two terms ‘dense’ and ‘hard’ do not mean the same. Gold has high density, over two times that of iron, because its atoms are heavier. Hardness can be easily demonstrated by ‘scratch’ test.

  A steel knife cannot cut glass, but a diamond tipped steel knife could, because diamond is harder than glass and steel is not. Gold is easily scratched. Haven’t we seen goldsmiths assessing the purity of gold by rubbing it against a whetstone and examining the scratch?

 Carat has two meanings: first it is a unit for weighing precious stones. It is almost equal to 200 mg. This is based on the old European practice of weighing precious stones using the seed of a bean pod, somewhat similar ‘Manchadi’ (in Tamil).

Second, carat is a measure of purity of gold. Usually, a bit of copper is added to gold, while making ornaments, to give the hardness and to prevent distortion. 24 carat means 100 per cent gold.

Hence one carat represents 100/24 per cent. Thus 22 carat gold means 9l.67 per cent pure and 18 carat gold 75 per cent; the rest being copper.

Pasteurization is defined as the process of heating milk to a particular temperature and holding it at that temperature for a particular time till the pathogenic (disease causing) micro-organisms are destroyed causing minimum change in composition flavour and nutritive value of milk.

High temperature short time (HTST) pasteurization at 71.7 {+0} C for 15 seconds is the most common method of pasteurization. Pasteurization conditions are not sufficient to destroy thermo-resistant spores (reproductive part of microorganisms). Pasteurized milk has to be refrigerated.

Sterilization is a more severe thermal process where milk is subjected to temperature of about 135 {+0} C for few seconds followed by aseptic (free of micro-organisms) packaging. Milk processed in this manner is termed as ‘commercially sterile’, but it is not necessarily free of micro-organisms.

 These micro-organisms, which survive heat treatment, are unlikely to proliferate during storage and cause spoilage to the product.

 However, spores are destroyed during sterilization. Sterilized milk has longer shelf-life even at room temperature. Nutritional losses in sterilization are more compared to pasteurization. 

 Pasteurization is one of the methods of preservation of products such as milk, alcohol beverages etc. at higher temperatures. The process of heating of the product (milk or beverage) to a controlled temperature (usually below 100 {+0} C) to enhance the keeping quality and to destroy harmful microorganisms is known as pasteurization.

            There are two methods of pasteurization (of milk) in general use. One is low temperature holding (LTH) method in which milk is heated to 62.8 {+0} C (145 F) for 30 minutes in commercial pasteurizers (or) large closed vats which are heated by steam coils, hot water jackets etc.

The other method (i.e.) high temperature short-time (HTST) method in which the milk is heated to 71.7 {+0} C (161 F) for 15 seconds. The heating is accomplished by electricity (or) hot water and requires a heat exchange system, which preheats raw, cold milk and cools the hot pasteurized milk.

Pasteurization does not sterilize the products but kills those organisms that grow most readily at low temperatures. The surviving organisms must be kept from multiplying by constant refrigeration. 

Pasteurization is sterilization interpolated with a subtle nuance in the process conditions and hence in the final result as well.

Pasteurization is named after the great French chemist Louis Pasteur, is the process of partial sterilization, confined only with the killing of pathogens.

The subject is heated to temperature below its boiling point (usually less than 100 {+0} C) and held at that temperature for a particular time period, with the aim of killing only the pathogens.

The time-temperature combination is decided based on the heat resisting capability of the target micro-organism and the nature of the subject. For instance milk is heated to 73 {+0} C & held for 15 seconds, to destroy Coxciella burnetti and Mycobacterium tuberculosis, which are the target micro-organism.

Sterilization is extended pasteurization where the subject is heated above the boiling point and held for a particular time period to destroy all the microorganisms. The temperature is usually above 100 {+0} C.

Plastics are made of polymers which giant molecules are having long chains of repeating units derived from short molecules. These long chains are entangled with are one another and held together by weak inter-atomic forces such as Vander Waals force. These weak bonds can be easily broken up by sunlight and so long exposures to sunlight make them brittle. 

A soap molecule is made up of 2 parts – a long hydrocarbon part and a short ionic part containing – COONa+ group. The soap molecule is said to have a tadpole structure.

 The hydrocarbon part of the soap molecule is insoluble in water but soluble in oil and grease. The ionic portion of the soap molecule in hydrophilic. So the ionic portion of the soap molecule is soluble in water but insoluble in oil and grease. Now the clothes which contain dirt substances are soaked in water.

When soap gets dissolved in water it forms a colloidal suspension in which the soap molecules cluster together to form a micelle. The micelles remain suspended in water because negative charges at the end of each soap molecule repel each other.

In a micelle, the soap molecules are arranged in a radical manner with the hydrocarbon end directed towards the centre and the ionic end directed outwards. When greasy, oily clothes are immersed in soap solution, the soap micelle entraps the dirt particles by attaching the hydrocarbon part of the soap molecules to the greasy or oily particles.

Since the ionic part of the soap molecules remain attached to the water molecules, the dirt particles get dispersed in water and the cloth gets cleaned.

            Soap has been used as a detergent for more than 2000 years. Soap is made from oil or fat which are esters of fatty acids or glycerol. The fatty acids contain chains of 16 to 18 carbon atoms.

            When oil or fats are heated with a solution of sodium hydroxide, they breakdown to form sodium salt of the respective fatty acid and glycerol. The process of splitting the fat is called saponification. It produces soap which is separated from the solution by the addition of salt. A molecule of soap can be considered to be made up of two components. One part is a hydrocarbon and the other belongs to the COONa group.  Hydrocarbons are water repelling – hydrophobic and the other parts are water loving – hydrophilic. When water is dissolved in water it forms micelles. In a micelle, the soap molecules are arranged radically, with the hydrocarbon end towards the centre and the water loving end outwards. Dirt and grease present on a piece of cloth attach themselves to the hydrocarbon component of the soap molecule. The other component which is attached to the water molecules pulls the dirt away from the surface thereby making the cloth clean.

 Washing powder contains about 15-30 per cent detergents by weight. Sodium sulphate and sodium silicate are added to keep the washing powder dry. Sodium tripolyphosphate or sodium carbonate is added to maintain alkalinity which is helpful in removing dirt. Carboxyl methyl cellulose is added to keep the dirt suspended in water. A mild bleaching agent such as sodium per borate is also added to produce whiteness. 

Ordinary soap has little value as an antiseptic and it does not kill or inhibit many types of bacteria.

However, the important function of washing one’s hands with soap is the mechanical removal of bacteria through scrubbing.

 The skin normally contains dead cells, dried sweat, bacteria, oily secretions and dust. In addition, having been to the toilet one would probably have contaminated his hands with faecal bacteria.

 Soap emulsifies the mixture of all these and the water washes it away. Because soaps are good for physically removing bacteria from the skin it is important to wash the hands before eating and after going to the toilet.

This prevents the transfer of potentially harmful bacteria by the faecal to oral route.

Cosmetic soaps contain antimicrobial compounds that strongly inhibit gram-positive bacteria and these are used to decrease body odour by preventing microbial growth on body secretions. These are non-household soaps that do contain some type of antibacterial compound.

These compounds act as disinfectants and kill bacteria. Of course, during our everyday activities we do not need or want to completely disinfect or sterilize our skin because the normal population of bacteria on us acts as a barrier against infection by pathogens. Healthy skin is a bacterial battleground populated with friendly bugs. These eat our sweat and, defend our skin from less friendly bugs that would not only eat our sweat but us as well.

Staphylococcus aureus is a typical invader that causes pimples and boils (or worse) when it beats our defenses. When we wash, we not only remove dirt and invasive bacteria, we also release a tide of friendly bugs from pores to recoat our skin for protection. This is fine unless you work in food preparation, you will then be required to use soaps that contain broad spectrum bactericides that kill all types of bacteria indiscriminately. Effective hand washing depends on many factors – the soap or cleaning agent, running water, clean towel or air drier for drying and on good technique.

Normal hand soap can remove germs, if used properly, and left to drain and dry between uses, not left sitting in a puddle of soggy soap and stagnant water. Pump dispensers are generally better than soap bars. Water must be clean and taps must not be contaminated by dirty hands – that is why hospital sink taps have elbow levers or, more rarely, foot pedals.

 Effective rinsing and drying, to remove any contaminated water without adding further contamination from a damp towel, are vital components of proper hand washing technique. This is why disposable paper towels are usually used in hospitals. Studies on hand-washing techniques by nurses have shown that some areas of the hand are less well-cleaned than others – fingers and the web between thumb and first finger are commonly inadequately leaned.a