Paper No. 01. Paper Title: Food Chemistry. Module -10: Physico-chemical properties of food lipids

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1 Paper No. 01 Paper Title: Food Chemistry Module -10: Physico-chemical properties of food lipids The physical and chemical properties of fats are found to depend mainly on their composition. A number of these properties are made use of in identification and ascertaining their purity. Physical properties of lipids The physical properties of edible fats and oils depend primarily on the molecular structure, interactions, and organization of the triacylglycerol molecules that they contain. In particular, the strength of the attractive interactions between the molecules and the effectiveness of their packing within a condensed phase largely determines their behavior, density, and rheological properties. Rheological properties Most liquid oils are Newtonian liquids with intermediate viscosities, typically between 30 & 60 mpa sec. at room temperature. The viscosity of liquid oils tends to decrease steeply with increasing temperature and can be conveniently described by a logarithmic relationship. Most solid fats actually consist of a mixture of fat crystals dispersed in liquid oil matrix. The rheological properties of these solid fats are highly dependent on the concentration, morphology, interaction, and organization of the fat crystals present in the system. Solid fats normally exhibit a type of rheological behavior known as plasticity. A plastic material behaves like a solid below a critical applied stress, known as the yield stress (τ) but behaves like a liquid above this stress. The rheological behavior of an ideal plastic material is known as a Bingham plastic. In practice, solid fats tend to exhibit nonideal plastic behavior. For example, above the yield stress the fat may not flow like an ideal liquid and may exhibit non-newtonian behavior (e.g., shear thinning). Below the yield stress, the fat might not behave as an ideal solid and exhibits some flow characteristics (e.g., viscoelasticity).

2 In addition, the yield stress may not occur at a well-defined value, but may occur over a range of applied stresses because there is a gradual break down of a fat crystal network structure. The yield stress of a fat tends to increase with increasing solid-fat content(sfc) and tends to be higher for crystal morphologies that are able to form three- dimensional networks that extend throughout the volume of the system more easily (i.e., small needle shaped crystals). Density The density of a liquid is defined as the mass of material required to occupy a given volume. The density of lipids is also important in certain food applications because it influences the overall properties of the system, for example, the creaming rate of oil droplets in oil-inwater(o/w) emulsions depends on the density difference between the oil and aqueous phases. The densities of liquid oils tend to be around kg/m³ at room temperature and tend to decrease with increasing temperature. The densities of completely solidified fats tend to be around kg/m³, and they too decrease with increasing temperature. The density of a particular lipid depends primarily on the efficiency of the packing of the triacylglycerol molecules within it: the more efficient the packing, the higher the density. Thus, triacylglycerols that contain linear saturated fatty acids are able to pack more efficiently than those that contain branched or unsaturated fatty acids, and so they tend to have higher densities. The reason that solid fats tend to have higher densities than liquid oils is also because the molecules tend to be packed more efficiently. Thermal properties The most important thermal properties of lipids from a practical standpoint are the specific heat capacity (Cp), thermal conductivity (k), melting point (Tmp), and enthalpy of fusion (ΔHf). These thermal characteristics determine the total amount of heat that must be supplied (or removed) from a lipid system to change its temperature from one value to another. The specific heat capacities of most liquid oils and solid fats are around 2J/g, and increase with increasing temperature. Lipids are relatively poor conductors of heat and tend to have appreciably lower thermal conductivities (~0.165w/m/s) than water (~0.595w/m/s). Melting point

3 Fats do not melt sharply but soften over a range of temperature, and it is therefore impossible to apply the melting point technique, used in the identification of pure organic compounds, to them. Polymorphism is important to the understanding of the melting point behavior of fats, fatty acids, and their esters. Furthermore, polymorphism plays a significant role in any operation where fats are solidified. The melting point and heat of fusion of a liquid depend on the packing of the triacylglycerol molecules within the crystals formed: the more effective the packing, the higher the melting and the enthalpy of fusion. Thus, the melting points and heats of fusions of pure triacylglycerols tend to increase with increasing chain length. They are higher (1) for saturated fatty acids than for unsaturated fatty acids; (2) for straight-chaind fatty acids than for branched fatty acids; (3) for triacylglycerols with a more symmetrical distribution of fatty acids on the glycerol molecule; (4) for trans than for cis unsaturated forms ; and (5) for more stable polymorphic forms. Solid fats are plastic over a fairly wide range of temperature. As a fat is warmed, the no. of crystals distributed through the liquid fat diminishes and the amount of liquid increases so that the fat softens. If a no. of crystals exceeds a critical amount, the fat will be hard and brittle and will lose plasticity. On the other hand, if the amount of liquid exceeds a critical level, the fat will flow. Glycerides also have a tendency to supercool. i.e., to remain as liquids at a temperature below their melting point as they are cooled down from a higher temperature. The consistency of a fat at say 30 C may be different if it is heated slowly from room temperature than if it has been quickly cooled from a higher temperature. After standing, it will again come to equilibrium and the effects of super cooling will disappear. The thermal stability of lipids can be characterized by their moke, flash, and fire points. Clear point - A small amount of fat is placed in a capillary tube and heated at a controlled rate. The temperature at which the fat completely melts and becomes transparent is called the "clear point". Slip point - A small amount of fat is placed in a capillary tube and heated at a controlled rate. The temperature at which the fat just starts to move downwards due to its weight is called the "slip point".

4 Smoke point - The smoke point is the temperature at which the sample begins to smoke when tested under specified conditions. Smoke point is the temperature at which lipids begins to break down to glycerol and free fatty acids, and produce bluish smoke. The glycerol is then further broken down to acrolein which cause the irritation to eyes and throat. Smoking point is necessary to be considering for deep frying operation. Refined oil has more smoke point temperature than unrefined oil. It is measured by a standard method in an open dish specified by the American Society for Testing Materials so that the evolution of smoke can be readily seen and reproduced. Cloud point - This gives a measure of the temperature at which crystallization begins in liquid oil. A fat sample is heated to a temperature where all the crystals are known to have melted (e.g., 130 o C). The sample is then cooled at a controlled rate and the temperature at which the liquid just goes cloudy is determined. This temperature is known as the cloud point, and is the temperature where crystals begin to form and scatter light. It is often of practical importance to have an oil which does not crystallize when stored at 0 o C for prolonged periods. A simple test to determine the ability of lipids to withstand cold temperatures without forming crystals is to ascertain whether or not a sample goes cloudy when stored for 5 hours at 0 o C. Flash point - The flash point is the temperature at which the volatile products generated by the liquid are being produced at a rate where they can be temporarily ignited by application of a flame, but cannot sustain combustion. Fire point - The fire point is the temperature at which the evolution of volatiles because of thermal decomposition occurs so quickly that continuous combustion can be sustained after application of a flame. Measurement of these temperature are particularly important when selecting lipids that are going to be used at high temperature (e.g., during baking or frying). The thermal stability of triacylglycerols is much better than that of free fatty acids; hence the propensity of lipids to breakdown during heating is largely determined by the amount of volatile organic material that they contain, such as free fatty acids. The temperature varies with the amount of volatile organic material that contain, such as free fatty acids. The temperature varies with the amount of free fatty acids. The smoke point of a fat used for deep fat frying decreases with use of the fat. Fats & oils with low molecular weight fatty acids have low smoke, flash & fire points. The no. of double bonds present has little effect on the temperature required. Smoke, flash, and fire points are particularly useful in connection with fats used for any kind of frying.

5 Optical properties The most important optical properties of lipids are their refractive index and absorption spectra. The index of refraction is the degree of deflection of a beam of light that occurs when it passes from one transparent medium to another. The refractive indices of fats & oils are often measured both because they can be rapidly and accurately determined and because they can be rapidly and accurately determined and because they are useful in identification of these substances and the testing of their purity. An Abbe refractometer with temperature control is used and the measurement is usually at 25 C. With high melting fats 40 C or even 60 C can be used, but temperature must be controlled and noted. The index of refraction decreases as the temperature rises; however, it increases with increase in the length of the carbon chains & also with the no. of double bonds present. The refractive index of particular oils is mainly determined by the molecular structure of the fatty acids that it contains. The refractive indices of liquid oils typically fall between 1.43 & 1.45 at room temperature. The absorption spectrum of oil can also have pronounced influences on the final appearance of a food product. Pure triacylglycerols have little inherent color because they do not contain groups that absorb light in the visible region of the electromagnetic spectrum. Neverthless, commercial oils tend to be colored because they contain appreciable amounts of pigments that do absorb light (e.g., carotenoids & chlorophyll). For this reason, edible oils often undergo a decolorization step during their refinement. In emulsified foods, lipids also contribute to the opacity of the product because of their ability to scatter light, which in a direct result of the difference in refractive index between the liquid and aqueous phases. Rancidity The development of off-flavours in lipids is known as rancidity. This results in loss of nutritional value and is not suitable for consumption. Two types of rancidity are as follow: a) Hydrolytic rancidity when fat is hydrolysed by lipase, free fatty acids are released. The formation of low molecular wt. fatty acids causes the deterioration of fats.

6 b) Oxidative rancidity The oxidation of unsaturated fatty acids by oxygen results in the formation of peroxides. The peroxides further degrades aldehydes and ketones having pronounced off-odour. Chemical properties A no. of chemical tests have been evolved during the years of study of oils and fats which are based on the partical determination of the chemical composition of the oil or fat. These tests serve both to identify the fat and to detect the presence of adulteration. Reichert -Meissl No. The R.M. value is defined as the no. of milliliters of 0.1N alkali (such as potassium hydroxide) required neutralizing the volatile water soluble fatty acids in a 5g sample of fat. The Reichert Meissl test determines the amount of butyric & caproic acids which are readily soluble in water and the caprylic and capric acids which are slightly soluble. The Reichert Meissl No. is particularly valuable in detecting adulteration in butter. Although the R.M. no. varies for butter with season, nutrition, & time in the lactational cycle of the cow, it is usually between 24 and 34, higher than other edible oils. Polenske number The polenske no. is the no. of milliliters of 0.1 N alkali necessary to neutralize the volatile water-insoluble fatty acids which are present in a g sample. The distillation procedure as set up in the Reichert Meissel & Polenske methods does not remove all of the volatile acids from the saponification mixture; but if the procedure is follows accurately and the size of the sample restricted to 5g, reproducible results and valuable information can be obtained. Saponification Number The saponification no. is defined as the no. of milligrams of potassium hydroxide required to saponify 1g of fat or oil. When potassium hydroxide reacts with a triglyceride, three moles of potassium hydroxide react with one mole of fat. If the triglyceride contains low molecular weight fatty acids, the no. of molecules present in a 1g sample of the fat will be

7 greater than if the fatty acids have long carbon chains and high molecular weights. The fat with the low molecular weight fatty acids will consequently have a high saponification number. The saponification no. of butter and coconut oil is and , respectively. [(ml N KOH) (ml N HCl)] 56.1 Saponification no.= g sample Iodine Number The iodine no. is the no. of grams of iodine or iodine compound absorbs by 100 g of fat. The double bonds present in the unsaturated fatty acids react readily with iodine or certain iodine compounds to form an addition compound even while the fatty acid is combined with glycerol in the fat. The Iodine no. is therefore a measure of the extent of unsaturation of the fatty acids present in a fat. Iodine no. of coconut oil, butter, palm oil, olive oil, sunflower oil and linseed oil is 7-10, 25-28, 45-55, 80-85, and , respectively. Acid no. The acid no. is the mg of KOH required to completely neutralize free fatty acids present in one gram of fat or oil. Prolonged storage of oils may cause the formation of free fatty acids. The oil having high acid no. is not suitable for consumption. Acetyl Value The acetyl value is a measure of the amount of hydroxyl fatty acids present in a fat. Castor oil is rich in ricinoleic acids, C17H32(OH)COOH, and the determination is of value for this & a few other oils. REFERENCES: Marangoni, A. and Narine, S. (2002). Physical Properties of Lipids. Marcel Dekker, Inc., New York, NY.

8 Satyanarayana, U. and Chakarapani, U. (2009) Lipids. In: Biochemistry, Books and Allied (P) Ltd., Kolkata, India. P Damodaran, S., Parkin, K. L. and Fennema, O. R. (2007) Lipids. In: Fennema s Food Chemistry (Fourth Edition), CRC Press, Taylor & Francis Group, Boca Raton Swaminathan, M. (2006) Lipids. In: Handbook of food and Nutrition, Bangalore Printing & Publishing Co. Ltd., Bangalore Walstra, P. (1987). Fat crystallization. In: Food Structure and Behaviour (Blanshard, J.M.V. and Lillford, P., eds.), Academic Press, London, chap. 5. Walstra, P. (2003). Physical Chemistry of Foods. Marcel Dekker, Inc., New York, NY. Gunstone, F.D. and Norris, F.A. (1983). Lipids in Foods: Chemistry, Biochemistry and Technology. Pergamon Press, Oxford. McClements, D.J. (2004). Food Emulsions: Principles, Practice and Techniques, 2 nd edn. CRC Press, Boca Raton, FL.