Physicochemical properties of food

Physicochemical properties of food

1. PHYSICOCHEMICAL PROPERTIES OF FOOD

2. INTRODUCTION The important groups of organic compounds present in different foods are:- Carbohydrates Proteins and amino acids Lipids Nucleic acids Enzymes Pigments Organic acids Polyphenols and tannins Flavouring principles Vitamins. Water

3. CARBOHYDRATES The different group of carbohydrates are: Sugars 4. cellulose & hemicellulose Starch 5. Pectin , gums and mucilage Glycogen

4. SUGARS : the important sugars occurring in foods are glucose,fructose,sucrose,maltose and lactose. Soluble in water, sweet in taste Glucose and fructose occur in fruits and honey Sucrose occurs in sugarcane, beetroot and honey. Lactose occurs in milk and maltose in malt. STARCH AND GLYCOGEN: Complex polysaccharides formed by combination of large number of glucose molecules Insoluble in cold water and form colloidal dispersion in hot water Found in cererals,nuts, legumes and oil seeds. Occurs in small amount in animal foods

5. CELLULOSE AND HEMICELLULOSE: occur in cell walls, husk and other supporting structure of plants foods. not digested by human beings. PECTINS,GUMS AND MUCILAGES: It occurs in many fruits and vegetables. Used in preparation of jams and jellies. Plant gums are used as thickeners in certain food products.

6. AMINO ACIDS AND PROTEINS They occur in free state in all foods. Proteins are complex compounds formed by combination of a large number of amino acids. Animal foods,nuts and legumes are rich in proteins while cereals are moderate sources, vegetables are fair sources and fruits are poor sources. Form colloidal dispersion in water. E.g.. Egg white, curd from milk and gelatine. LIPIDS The main compounds are fats and oils. They are compounds of glycerol and fatty acids. Several complex lipids such as phospholipids occur in many foods. Fats occur in the form of emulsion in milk and egg. Phospholipids help in the formation of emulsions.

7. OTHER COMPONENTS NUCELIC ACIDS : present in varying amount in different foods Enzymes : organic catalyst occurring both in animal and plant foods. Essential in many biochemical reactions that takes place in plant and animal tissues. PIGMENTS : plant food contain pigments such as chlorophyll, xanthophyll, anthocyanin's, carotenoids etc. Meat contains myoglobin while egg yolk contains xanthophyll and carotenoid pigments. ORGANIC ACIDS : citric acid, malic acids occur in fruits. Also found in vegetables and animal tissues. POLYPHENOLS AND TANNINS : occur in plant foods FLAVOURING PRINCIPLES : each food has a characteristic flavour for e.g. condiments and spices have distinct flavour because of essential oils and flavour principles.

8. VITAMINS: occur in varying amounts in different foods. A part of them is lost due to processing etc. they can be reduced or made up by fortifying the foods. WATER: present in large amount in fresh fruits, vegetables,meats,fish,egg and milk (70-90%). Cereal , pulses, and dries nuts contains only small amount of water (8-12%). It is present in two forms Free water Bound water When water is removed from vegetables it turns from soft to a different texture due to dehydration. The perishable nature is due to the presence of high water content.

9. Organic compounds present in foods Organic compounds present in foods are: Colloids i:e, compounds having large molecular weights and forming only dispersion in water. E.g., protein,starch,glycogen and agar-agar. Crystalloids i;e , compounds having small molecular weights and can form true solutions e.g., sugar and amino acids. Lipids exist in free form in oilseeds, nuts, meat and fish while they exist in form of emulsion in milk and egg yolk.

10. COLLOIDS Colloidal solution contains substances whose molecular aggregates possess a diameter greater the 1mµ and less than 100 mµ. CLASSIFICATION : Divided into: Lyophobic colloids( no affinity for water) Lyophilic colloids(greater affinity for water e.g., proteins, starch) PROPERTIES : large molecular weight possess electrical charge. can be separated by crystalloids by dialysis while mixture such as protein can be separated by ultracentrifuge.

11. OSMOTIC PRESSURE The process by which water is drawn into a solution through a semi permeable membrane is called osmosis. The pressure that must be exerted on the solution to prevent the flow of water into the porcelain vessel is equal to the osmotic pressure exerted by the solution. OSMOTIC PRESSURE OF NON ELECTROLYTES: 1g molecular wt. of different non electrolytes when dissolved in 1lt will exert an osmotic pressure of 22.4 atmospheres. Molecular wt. of glucose and cane sugar are 180 and 342 while that of serum albumin is 73,000. hence serum albumin will exert the same osmotic pressure as 342g of cane sugar or 180g of glucose when dissolved in the same volume of water.

12. HYPOTONIC,HYPERTONIC AND ISOTONIC SOLUTIONS: Water moves readily across cell membranes through special protein-lined channels, and if the total concentration of all dissolved solutes is not equal on both sides, there will be net movement of water molecules into or out of the cell. Whether there is net movement of water into or out of the cell and which direction it moves depends on whether the cell’s environment is isotonic, hypotonic, or hypertonic.

13. APPLICATIONS OF OSMOSIS TO FOOD PROCESSING: Osmosis plays an important role in food preservation and processing. It maintains the original size and shape of pickled whole fruit. Mangoes are usually pickled with salt, when salt is added it draws out water from mangoes and it shrinks .this is due to osmotic pressure exerted by strong salt solution.

14. FOOD DISPERSIONS It may be divided into five groups: Solids in liquids (e.g., gelatine dissolved in water) one liquid in another insoluble liquid ( water in oil emulsions) Gas in liquid (foam) Gas in solid (solid foam (foam candy)) Solid in gas (solid aerosol – smoking of meat and fish)

15. FOOD SOLS AND GELS: Colloidal dispersions of polysaccharides can be divided into: Sols – free flowing liquids at room temp. Gels- relatively firm, do not flow. FOOD SOLS: have a diameter of 1mµ to 100 mµ . E.g., solution of egg albumin in water and dilute solution of gelatine in water. Important properties of sols are Electrical double layer Rheology Optical property

16. ELECTRICAL DOUBLE LAYER: In sols possessing continuous aqueous phase, the particles have electrically charged surface. The ionized groups are the source of these charges In dissolved electrolyte, and electrical double layer may exist around each particle. RHEOLOGY : 1) The viscosities of hydrophilic sols are much greater than water. The increase in the concentration of the colloids increases the viscosity of sols.

17. FOOD GELS: Consist of continuous phase of interconnected macromolecules intermingled with a continuous liquid phase such as water. Common food gels include- jelly,gelatin gels and starch gels. Posses varying degree of rigidity and elasticity , depending on the type of gelling agent. A sol can be transformed into gel under certain conditions: Lowering of temp. Chemical alteration of the gelling agent Adjustment of pH or addition of salt Addition of water competitive compound e.g., sugar.

18. EMULSIONS Liquid/liquid systems of 2 immiscible substances are called emulsion. Substances or particle size = 10-100 microns. Examples: butter (w/o), margarine (w/o), mayonnaise (o/w), salad dressing (o/w), milk (o/w), cream (o/w), and chip-dip (o/w).

19. An emulsifying agent is made up of two parts. One is hydrophilic (water loving) and the other is hydrophobic (water hating). The emulsifier holds the disperse phase within the continuous phase. This results in the emulsion becoming stable.

20. A ) Mayonnaise is an example of a stable emulsion of oil and vinegar, when egg yolk (lecithin) may be used as an emulsifying agent. B) Stabilisers are often added to emulsions to increase the viscosity of the product. These help improve the stability of the emulsion, as over time the emulsion may separate. Stabilisers also increase shelf life, E461 methylcellulose, used in low fat spreads. PROPERTIES OF EMULSIONS: Droplet size distribution: Optical properties Rheology of emulsions

21. Droplet size distribution: Emulsions change their size distributions over time with the average droplet size shifting to larger values A sharply defined distribution containing a the maximum fraction of small-diameter droplets is usually more stable Rheology of emulsions: Continuous Phase : O/W emulsion can be partially controlled by clays and gums W/O emulsion by the addition of high-melting waxes and polyvalent metal soaps Internal Phase : No impact to final emulsion viscosity Droplet Size & Dist : The viscosity of emulsions having similar size distributions about a mean diameter is inversely proportional to the mean diameter

22. MECHANISM OF ACTION OF EMULSIFIER Emulsifiers ma be grouped into three heads depending on their mechanism of action in forming stable emulsions: Emulsifiers obtained at the oil water interface Finely divided particle absorbed at the interface Water dispersible hydro colloids which increase viscosity of the continuous phase. when all the three classes of emulsifiers are used in combination optimal emulsion stability may be achieved.

23. STABILITY OF EMULSIONS stability changes in food emulsions can occur through the process of Creaming Flocculation Coalescence The creaming phenomena involves the flotation or sedimentation of dispersed emulsified droplets into emulsions layer one richer and other other poor in fat. Flocculation is the agglomeration of droplets to form loose and irregular clusters.it is the first step before creaming takes place.

24. 3) Coalescence is the irreversible union of small droplets to form large droplets . 4) Emulsion can be stabilised against creaming, flocculation and coalescence by introducing a strong interfacial film around each droplet, by adding electric charges to the droplet surfaces and increasing the viscosity fo continuous phase.

25. FOAM It is a dispersion of gas bubbles in a liquid or semisolid phase. Bubbles are separated from each other by liquid or semisolid wall (lamellae) that are elastic in stable foams. Depending on thickness of bubble size, foam can be as dense as continuous liquid phase or almost as light as the dispersed gaseous phase. Characteristics of foam: Large amount of entrapped gas. Extensive surface area Rigid, semi rigid and elastic walls

26. FORMATION OF FOAM The formation of foam depends on the presence of a foaming agent in the continuous phase prior to dispersion of gas. The foaming agent is absorbed to reduce surface tension and resist the coalescence of bubbles. Typical foams include whipped cream, ice-cream, cake mix, meringue, and the froth on beer. Low density and a thin walled turbid structure are essential to yield a fluffy product.

27. Foaming agent may be classified as: Surface0active lipids (phospho-lipids) Glucosides and saponins Cellulose derivatives Proteins (albumins ) Foam can be formed either by dispersion or condensation. In dispersion, gas is injected into the solution through orifice. In condensation, gas under pressure is passed into the solution that is to be formed.

28. FOAM STABILITY Foam stability is related to the resistance of the foam wall to bursting stresses. Loss of liquids in wall is caused by : Gravitational force A suction effect at the periphery of the wall dur to high curvature. Deformation forces brought about by gas movement. Foam stability can be increased by increasing the viscosity of the solution and by producing particulate matter.

29. FOAM DESTRUCTION In products like fruit juices, coffee extract, excessive foaming may lead to waste of material. to avoid this wastage use of antifoam agents can be done. Water insoluble dimethyl poly- siloxanes (silicone oils) are used in food industry. The use of antifoam agents causes an immediate collapse of foam.

30. HYDROGEN ION CONCENTRATION( Ph ) The acidity and alkalinity is of great importance in food processing. Fruits contains organic acid and have an acid reaction while foods such as milk and eggs have neutral reaction. The term hydrogen ion concentration is used to express the degree of acidity or alkalinity of a food or a given solution.

31. STRONG ACIDS AND WEAK ACIDS A strong acid is one which is virtually 100% ionised in solution. The strength of the acid depends on its hydrogen ion concentration. The larger the degree of dissociation, stronger is the acid. In strong acid all the hydrogen ion exist as free ion. A weak acid is one which doesn't ionise fully when it is dissolved in water . In week acids only a small part exist as free ion while the rest exist as bound ion.

32. Titrable acidity and free H ion: It refers to the total H ions present in the acid. STRONG BASES AND WEAK BASES: A strong base like NaOH or KOH dissociates completely in dilute solutions while a weak base like ammonium hydroxide dissociates only to a small degree. The degree of alkalinity of a base depends on the concentration of OH ion.

33. IONSIATION OF WATER Water is neutral, distilled water contains small amount of hydrogen ions and hydroxyl ions. Since the concentration of H ion is equal to that of the OH ion in water is constant i;e , irrespective of weather acids or alkalis are added to water.

34. pH and pK scales pK is a measure of acid strength. It depends on the identity and chemical properties of the acid . pKa is defined as the negative logarithm to the base 10 of the Ka in g ions/ L or as logarithm to the base 10 of the reciprocal of Ka . The pH of a solution is defined as the negative logarithm to the base 10 of the hydrogen ion concentration. It is a measure of [H+] in a solution. pKa = - log Ka pH = - log H+ The relationship between pH and pK is given by Henderson - Hassel Bach equation. pH = pKa + log  [ A − ]/[ HA ]

35. pH scale The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic .

36. BUFFER SOLUTIONS A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it . Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range; an example of a buffer solution is blood.

37. acidic buffer solutions An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt. example would be a mixture of ethanoic acid and sodium ethanoate in solution. In this case, if the solution contained equal molar concentrations of both the acid and the salt, it would have a pH of 4.76. You can change the pH of the buffer solution by changing the ratio of acid to salt, or by choosing a different acid and one of its salts. Alkaline buffer solutions An alkaline buffer solution has a pH greater than 7. Alkaline buffer solutions are commonly made from a weak base and one of its salts. example is a mixture of ammonia solution and ammonium chloride solution. If these were mixed in equal molar proportions, the solution would have a pH of 9.25.

38. BOUND WATER IN FOODS Bound water is usually defined in terms of the ways it is measured; different methods of measurement give different values for bound water in a particular food . Some characteristics of bound water include: It is not free to act as a solvent for salts and sugars It can be frozen only at very low temperatures (below the freezing point of water). It exhibits essentially no vapor pressure Its density is greater than that of free water Bound water has more structural bonding than liquid or free water thus it is unable to act as a solvent . Eg . cacti or pine tree needles