- Food preservation technique –
- Heat treatment – Cooked food and milk sterilized by UHT. Heat kills micro-organisms, alters protein structure, destroys enzyme activity of micro-organisms in food.
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Chilling and freezing at -20oC slow down microbial activities and chemical changes resulting in spoilage. Freezing promotes the retention of nutrients and does not destroy nutrients.
- In canning, food is cooked under pressure in sealed containers. Cooking destroys enzymes and micro-organisms and most canned foods keep well for more than one year.
- Sugaring and salting remove water essential for enzyme action and microbial growth.
- Pickling in vinegar reduces growth of micro-organisms.
- Chemicals such as nitrate and nitrite prohibit the growth of micro-organisms.
- Benzoate, sulphite and vitamin C retard chemical spoilage in fruit juice.
- Give an account of the uses of food additive.
Food additives include monosodium glutamate, benzoic acid, vitamins, butylated hydroxytoluene(BHT). Food additives prevent oxidation, add flavor, texture and color, kill bacteria, increase vitamin content, etc
The chemical preservatives that are used as food additives are of two kinds: antimicrobial and antioxidant.
Antimicrobial agents like the nitrates(III) and nitrates(V) of sodium and potassium have been added to cured meats like bacon, sausage, luncheon meat for a long time. Nitrates(III) are valuable in preventing the growth of Clostridium botulinum, the bacteria that produces deadly botulism poisoning in canned foods.
Sulphur dioxide and sulphates(IV) are another group of antimicrobial preservatives that have been added to alcoholic beverages and dried fruits for centuries. They have been included in fruit juices, jellies and jams. They prevent the growth of yeast and are useful as bleaches and antioxidants to prevent browning in alcoholic beverages dried fruits, fruit juice and vegetables.
Antioxidants are used to oppose the oxidative decay of food due to microbial activity. Atmospheric oxidation is the chief cause of destruction of fats and oils in food. Fats and oils deteriorate rapidly by air oxidation, producing carboxylic acids, aldehydes and ketones, which together give spoiled fats their characteristic rancid taste and odor. An antioxidant added to the food can hinder oxidation. BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene) are common antioxidants used in food to retard this development of oxidative rancidity in unsaturated fats and oils.
These phenols appear to work by donating the H-atom of the –OH group to the free hydroperoxide radical (ROO.) involved in the autoxidation of fats and oils, thereby stopping the chain reactions in oxidative spoilage:
AH + ROO. → ROOH + A. where AH represents the antioxidant,
and A.is a radical derived from the antioxidant, e.g.
An effective natural antioxidant is vitamin E, which, like BHT and BHA, is a phenol. Vitamin E, is more expensive than the synthetic antioxidants. Vitamin C (ascorbic acid) and sulphur dioxide. An acidic antioxidant like vitamin C can slow down the fast browning of a piece of apple exposed to air. Sulphur dioxide and sulphates(IV) also act as antioxidants (or reductants) which preferentially react with oxygen and prevent the oxidation of ethanol to ethanoic acid in wines.
Fructose is a sweetener found in many fruits and table sugar. mannitol and sorbitol are 2 monosaccharides that have structures strikingly similar to glucose. When mannitol is broken down in the body, it generates only half the calories of glucose. Mannitol is 200 times as sweet as sucrose. Mannitol is used as a sweetener in the so-called ‘sugarless’ gum. Saccharin and aspartame are two other artificial sweeteners. Aspartame is normally metabolized in the body as a dipeptide. It is used in soft drinks, chewing gum, powdered beverages and tabletop sweeteners.
Emulsifiers keep small droplets of a non-polar liquid (usually an oil) dispersed in a polar liquid (water). They are often used in dairy products and are present in ice-cream, mayonnaise, margarine and butter. Emulsifiers usually have a supply of hydroxyl groups as polar groups and the non-polar end of the emulsifier molecule dissolves in the oil, while the polar end dissolves in water. In this way, molecules of the two immiscible liquids are held together and do not separate. Emulsifiers include mono- and di-glycerides of stearic acid, lecithins (in milk chocolate and powdered milk). Polysaccharides have numerous hydroxyl groups, which form hydrogen bonds with water to prevent the segregation of water from the less polar fats in the food, and to provide a more even blend of the water and oils throughout the food.
Coloring agents are a mixture of dyes added to make the food more attractive. Caramel is a coloring agent added to oyster sauce. Yellow dyes are added to chicken feed to give chicken skin a yellow color.
Beta-carotene is often added to butter and margarine to give their yellow appearance. About half of the 30 coloring agents are extracted from natural products like carrots, beets and grape skins. Tartrazine is an example of synthetic dye used in some fruit squash.
Sulphur dioxide is used to bleach food products like cheese and bread where they act as preservatives. Sulphur dioxide dissolves in water and alkali to form the sulphite ion, responsible for the bleaching action. White bread is made from flour bleached with sulphur dioxide.
If the pH of fruit juice is too high, citric acid and lactic acid are added to give it a good flavor. Phosphoric acid, tartaric acid and maleic acids are also used. If the pH of fruit juice is low, buffer salts such as sodium citrate or tartrate are used. The acidic additives act as preservatives to prevent the growth of micro-organisms and as antioxidants to prevent rancidity and browning in dressings.
Nutrients include vitamin C (fruit juice) vitamin B (enriched flour), vitamin D (milk and margarine), ammonium ferric citrate (infant milk formulations and bread flour)
Possible menace of food additives include (1) allergies such as rashes and stomach upsets, (2) hyperactivity in children caused by food colorings, (3) long term illness.
MSG has been associated with the so-called ‘Chinese restaurant syndrome’, an unpleasant reaction characterized by burning sensation, thirst, headache, chest pain, vomiting and abdominal discomfort..
Anti-caking agents are added to hygroscopic foods – in amounts of 1% or less – to prevent caking in humid weather and to keep foods fast flowing. Magnesium acts as an anti-caking agent by incorporating water into its structure as water of hydration and it keeps the surface of sodium chloride crystal dry.
- Give an account of the properties of the s-block metals.
The s-block elements have two outermost s-electrons outside a complete electron shell. For instance, K has an electronic configuration of [Ar]4s1 where [Ar] stands for the configuration of Argon. The outermost s electron is easily lost to form a monovalent Group I cation with a stable octet structure and these s-block elements have low ionization potentials, making them very reactive reducing agents. Group 2 metals are generally less reactive. S-block metals react with water or steam to form an alkaline solution and hydrogen. Group I metal reactivity increases down the Group as the first ionization potential decreases. Despite its more positive first ionization potential, Lithium has a more negative standard electrode potential than sodium because the hydration energy of lithium is more negative (Li+(g) releases more energy than Na+(g) upon hydration.
Atoms of Group 1 metals are packed in a body-centered cubic way, resulting in a density less than 1 g cm-3. Atoms of Group 2 metals have a hexagonal close packing structure, making it denser than the Group I counterparts.
Metallic bonding is stronger in Group 2 elements due to the presence of more electrons available for delocalization and hence Group II elements have higher melting point. Group I metals have melting point as low as 70oC. Due to the increased atomic size down the Group, metallic bonding and hence melting point also decrease down the Group
Ionization potential decreases down Groups I and II due to an increase in atomic size.
Group I oxides are soluble and dissolve in water to form an alkaline solution. Group II oxides are insoluble because the ionic bonding is stronger than that in Group I oxides. X2O + H2O → 2XOH
Large Group I metals form stable peroxide and superoxide because these metal form cations of low polarizing power. 4KO2 + 4H2O → 4KOH + 3O2 + 2H2O; K2O2 + H2O → H2O2 + K2O
Small Li+ has strong polarizing power and the increased covalent character of the peroxide formed makes it thermally unstable. Li2O2 does not exist while K2O2 and KO2 exist. In Lithium compounds with large anions such as LiNO3 and Li2CO3 are thermally unstable due to the increased covalent character of these compounds.
2LiNO3 → 2LiNO2 + O2 and Li2CO3 → Li2O + CO2
For similar reason, Na2CO3 is not decomposed by heat while NaHCO3 breaks down on heating because Na+ polarizes the large HCO3-.
Hydrides of Group I metals are strongly ionic and dissolve in water vigorously to form hydrogen and a strong alkali.
MH + H2O → MOH + H2; MgH2 is less ionic and it reacts with water form hydrogen and an alkaline solution.
Increased covalent character of LiH makes it less ionic. Lithium and magnesium show diagonal relationship, which means that these two metals and their compounds have similar properties. Unlike other Group I hydrides, LiH reacts slowly with water.
Group I salts are all soluble. The solubility of Group II salts is lower. Solubility of Group II hydroxide increases down the Group because the lattice enthalpy decreases considerably down the group as the size of the cation increases. In Group II sulphate, the lattice enthalpy remains fairly constant as the cation size increases down the Group because the inter-ionic distance does not change much in the presence of a large anion, SO42-. So, Ba(OH)2 is more soluble than Mg(OH)2 while BaSO4 is much less soluble than MgSO4.
Lithium is useful in making the anode (+) of a lithium cell while barium salt is used for making X-ray exposure of the bowel. Group I metals are reactive towards air and should be stored in paraffin oil.
- Give an account of the study of reaction rate, detailing various methods of measurements.
To study the order of a reaction between A and B, there are various methods:
(1) Initial rate method – To measure the initial reaction rate, the time taken to reach a specific state of a reaction, t is measured. 1/t will be taken as the initial rate of the reaction. Keeping [A] constant and vary [B], then measure the initial rate of the reaction. The specific state to be reached must be close to the starting point of the reaction. For instance, a small amount of phenol is added to a bromine-forming reaction mixture to monitor the time t after which a certain amount of bromine is produced. 1/t gives the initial reaction rate. Also, In the thiosulphate-acid reaction, the time needed for the formation of a certain amount of sulphur t, is noted and 1/t is taken as the initial reaction rate. Keeping [B] constant and vary [A], then measure the initial rate of the reaction rate. From the initial rate data obtained, the order of reaction a, b in the rate law rate = k [A]a [B]b, can easily be deduced.
For an iodine-producing reaction, a small amount of thiosulphate is added to the reaction mixture and note the time t for the brown color of iodine to appear. 1/t is then taken as the initial rate.
For the iodine-propanone reaction, the order of reaction with respect to [propanone] can be found by mixing excess H+ and I2 and find the change in [propanone] indirectly by finding the [I2] at various times using a colorimeter. The order of reaction in [H+] and [i2] can be found in a similar way.
When doing the initial rate method, the total volume of reaction mixture is kept constant by adding a suitable volume of water in order to make the change in reactant concentration simple. That is, doubling the volume of reactant B used would double the [B]. Measuring the initial rate at doubled [A], constant [B], the order of reaction in A is found.
(2) Use excess reagent A, and measure the changes in [B] with time indirectly by finding the changes in concentration of some products. Titration and colorimetry are two common methods for finding concentration changes. For a iodine-producing or iodine-consuming reaction, the changes in [I2] is done by titrating the reaction mixture with standard thiosulphate solution and the reactant concentration can be calculated. The reaction mixture must first be quenched either by removing the H+ in the acidic medium or lowering abruptly the reaction temperature. For a reaction involving colored reactants or products, the progress of the reaction may be monitored with a colorimeter. The concentration of the colored species can be found by calibrating the colorimeter with a standard solution of the colored species. For example, a 0.01 M [I2] can be used to calibrate the colorimeter so that the exact [I2] in the reaction mixture would be calculated accurately. Colorimetry is a fast and effective technique and no quenching is required and any changes in concentration of a colored species are measured rapidly.
Once the changing concentration of [B] is calculated, the order of reaction with respect to B could be determined by plotting ln [B] versus time or 1/{B} versus time. A linear ln [B] vs t plot indicates that the reaction is first order in [B]. The slope of this plot is –k. A linear 1/[B] vs t plot indicates that the reaction is second order in [B] and the slope of the graph is +k. The approach is repeated using excess reagent B, and measure the changes in [A]. The order of reaction with respect to [A] can be done by similar plots.
