INTRODUCTION:

Edible oils are vital constituents of our daily diet which provide energy, essential fatty acids, and serve as carrier of fat-soluble vitamins. Edible oils have made an important contribution to the diet of people in many countries serving as good source of protein, lipids, fatty acids for human nutrition and enhance the foods we eat by providing texture, mouth feels, impartation of flavours and contribution to the feeling of satiety after eating. These of wide applications in foods where there are used in frying, salad dressing, baking, cooking, ice cream manufacture. Its application is increasing day by day for food purposes. Fats and oils are the energy source of the highest of three basic foods (carbohydrates, proteins, lipids) and serve for three purposes such as being an energy source, a structural component, and making powerful biological regulators.

They also play major role in metabolic reactions in human body. 10% or fewer calories consumed daily should be from saturated fat and 20-35% of total daily calories come from PUFA and MUFA. In the current era vegetable oils and fats are found to be about 80-85% of edible oils and fat consumed by the public. Now-a-days these are considered as important food for the world. They also serve as carriers for vitamin soluble oil, and may contain essential for health fatty acids that are not manufactured by the human body.

Due to high usage of edible oils at current era and wide range of applications in various industries (cosmetic, food, pharmaceutical, lubricants, medical characteristics, etc) the oils available are not up to standard mark to meet the consumer satisfaction in terms of their texture and stability of food products. The competition between various brands to meet the consumer standards have lead them to produce adulterated products. This article provides the information about the physicochemical characters to evaluate the stability of oils.

Fats and oils consist of a wide group of compounds that are soluble in organic solvents and insoluble in water. They have lower densities than water and at normal room temperature range in consistently from liquids to solids depending on their structure and composition. The words oils, fats, and lipids are all used to refer to fat, oils are usually used to refer to fats that are liquids at room temperature, while fats are usually used to refer to that are solid at normal temperature. Lipids are used to refer to both liquids and solids fats. Edible oils are referred as any oil from plant sources (either fruits or seeds). Reports have shown that approximately 75% of the world’s production of oils and fats come from plant sources. Oils can be categorised in different ways the type of plant it was extracted from, the level of refinement, the method of extraction etc. Generally oils are usually named by their biological source, however each oil has a range of physical, chemical, and compositional parameter by which it can be categorised.

Bio synthesis of Fatty acid and Triacylglycerols:

Normally plants produced fatty acids which may have zero to three double bonds. These commonly found usual fatty acids include palmiticacid (16:0), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2) and linolenic acid (18:3). In oil seed plants, these fatty acids are predominantly stored as triacylglycerols (TAG) which is the major storage form in seed.

The fatty acids are synthesised in plastids from acetyl-coA as a starting substrate and on acyl carrier free fatty acids move to cytosol where they are further incorporated into phosphatidylchoine (PC) pool, which undergo modifications such as desaturation or hydroxylation, epoxylationetc and their inclusion into TAG takes place. The later processes occur in endoplasmic reticulum. This fatty acid play important role in cellular metabolism as a way to store energy and also by providing energy than required.

Fatty acids are known to play important role in cell division and growth. They are an integral component of cell membranes, hormones, neurotransmitters etc. Intake of different fatty acids has a direct influence on human health. For example, increased intake of saturated fatty acids has been linked to cardiovascular diseases. Thus it is considered desirable to have diets low in saturated fatty acids. Besides these, some very long chain poly unsaturated fatty acids (VLC-PUFA, C20-C22) such as arachidonic acid (ARA, 20:4), Eicosapentanoic acid (EPA, 20:5) and Docasahexanoic acid (DHA, 22:6) which are usually derived from marine resources, have been shown to play an important role in human nutrition.

Fig-1. Metabolism of fatty acids

PUFA are of two types: omega-3 and omega-6 fatty acids. Linolenic acid (LA; 18:2) is a major omega-6 fatty acid with α-linolenic acid (ALA; 18:3) is a major omega-3 fatty acid. These fatty acids are not readily synthesized in human body. These should be synthesised regularly in our diet. The major sources of fatty acids is marine fishes. In human body, Linolenic acid and α-linolenic acid can be further metabolised to form longer chain fatty acids which play a crucial role in human growth and development.

Fig-2. Alpha-Linolenic acid

Fig-3. Linolenic acid

Fig-4. Structures of omega-3 and omega-6 fatty acids

PHYSICOCHEMICAL PARAMETERS:

The estimation of the physicochemical properties of edible oils is essential in the design process and important factor that determines the overall quality and stability of a food system. Its physical and chemical parameters were used to monitor the compositional quality of oils. It is necessary to monitor the quality of oil to avoid the use of abused oils. A good type of edible oils which can be used as cooking purpose must consider with a proved range of physicochemical parameters. By understanding the properties we can evaluate the oil for human use.

Several factors affect the edible oil quality such as

· Agronomic techniques

· Seasonal conditions

· Sanitary state of drupes

· Ripening state

· Harvesting

· Carriage systems

· Method and duration of storage

· Processing technology.

Major factors affecting edible oil quality are

Temperature:

By increasing temperature auto oxidation of oils and decomposition of hydro peroxides occurs. The formation of auto oxidation product is slow at low temperature.

High temperature also causes in changes in shear rate of oils thereby decreasing the viscosity of oils which causes degradation.

Oxygen:

Atmospheric oxygen reacts instantly with lipid and other organic compounds of oil to cause structural degradation in the oil which leads to loss of quality of food and is harmful to human health. The oxidative stability of edible oils not only depends on conditions of storage but also on the history of raw material and the refining process.

Oxidation processes play an important role in the deterioration of fats and oils with ‘Rancidity’ as the main effect. It also causes unpleasant taste, smell and changes in colour, viscosity, density, and solubility.

Further consequences include the loss of essential fatty acids, degradation of vitamins and pro-vitamins. These changes strongly influence the nutritional value and sensory quality of edible oils. The oxidative products can easily react and forms complex with amino acids and proteins. Therefore oxidation is very important in terms of palatability, toxicity, and nutritional value of edible oils.

Moisture:

Presence of moisture in oil enhances the rate of hydrolytic reactions.

Nutrients:

It is possible to determine by different analytical techniques how to assess the quality of edible oil and to avoid possible adulterations. Sometimes refining processes also modify the chemical properties of oils to the point of which could be detrimental to human health. The specifications are different for refined oils, crude oils, bleached oils.

Atmospheric oxygen reacts instantly with lipid and other organic compounds of the oil to cause structural degradation in the oil which leads to loss of quality of food and is harmful to human health.

These physicochemical parameters namely

· Density

· Moisture content

· Boiling point

· Refractive index

· Viscosity

· Specific gravity

· Acid value

· Peroxide value

· Iodine value

· Saponification value

These properties are used to assess the quality and functionality of oils.

Density Measurement:

Densities of oil samples were measured by a Relative Density (R.D) bottle with a capacity of 10 ml according to the following formula:

(Mass of the sample (M))

Density (ρ) = –––––––––––––––––––––––––– g/ml

(Volume of the bottle (V))

The uncertainty in density measurements was ±0.0001g/cm³.The density was determined for temperature ranging from 20°c to 50°c, with a degrees step increase. The density of vegetable oils is dependent on their fatty acid composition, minor components and temperature. The difference in the density of oils may be due to the refined and unrefined character of the studied oils.

The density of vegetable oils linearly decreases with increase in temperature. Oils with the density of lower values are highly appreciable to consumers. The low density is obtained by more rigid π bonding between c-c bonds and becomes more strenuous. The relative density of oil at any temperature compared to water at a specified temperature is known as the mean molecular weight diminishes and also as the degree of unsaturation increases.

Moisture Content:

Three crucibles are weighed and into each 10g of oil sample were added. The samples were dried to constant weights in an oven at 105℃, cooled in desiccators and weighed. The procedure was repeated thrice for each sample and the average value was determined.

The higher the value of the moisture content of the oil, the greater the value used for food texturing, baking, and frying and industrially in the manufacture of soaps, detergents, cosmetics, and oil paints.

Boiling Point Determination:

The boiling point of oil samples were determined by a thermometer ±1℃. The boiling point depends upon the degree of unsaturation of fatty acids.

Increase in the temperature leads to the decrease in the viscosity and leads to degradation.

Refractive Index:

Refractive index of the oil samples were measured using Abbe refractometer connected to a thermostatically controlled water bath that maintained the temperature of the refractometer at 40 ± 0.1℃. The determination of refractive indices was done following the procedures of cocks and van Rede.

Refractive index could be used in the preliminary identification of the oils and fats. Refractive index plays an important role in many branches of biology, chemistry. The refractive index of fats and oils is sensitive to their composition. In fats RI increases with increasing chain length of fatty acids in the triglycerides or with increasing unsaturation. This makes it an excellent spot test for uniformity of composition of oils and fats. It is generalized that the refractive indices of oils increases with increase in the number of double bonds. With increase in temperature, the refractive indices of oil decrease. The refractive indices can also be influenced by oxidative damage of oil. The refractive index decreases with the molecular weight of fatty acids.

Viscosity:

The viscosity of oils and their blends was determined by BROOLFIELD DVII + Pro viscometer at a constant shear rate at constant temperatures which were controlled by a microprocessor assisted water bath using spindle S51.

Viscosity of oil is a measure of the oil’s resistance to shear. High viscosity implies a high resistance to flow while a low viscosity indicates a low resistance to flow. Changes with temperature, decreasing temperature increases viscosity. The rise in temperature enhances movement of molecules and reduces intermolecular forces so the layer of liquid easily pass over one another and thus contribute to reduction in viscosity. Oil viscosity also depends upon molecular structure and decrease with the unsaturation of fatty acid. It may be due to double bonds that make bonding more rigid and rotation between c-c bonds becomes more strenuous.

Specific Gravity:

Specific gravity bottle was used in measuring the specific gravity. The specific gravity of oil is the ratio of weight in air of a given volume of the oil at a define temperature to that of the same volume of water at same temperature. Cleaned, dried bottle was weighed. It was filled with water maintained at 20°C and weighed again. The bottle was emptied, dried and filled with oil and weighed. The value was calculated using equation.

(Weight of oil)

Specific gravity= –––––––––––––––––––––––

(weight ofwater at 20°C)

Specific gravity is the heaviness of a substance to that of water, and it is expressed without units. The reductions in specific gravity are due to removal of some polar compounds from oil by alkali refining. It can be used in wide range of industries. It is particularly useful because it allows access to molecular information in a non-invasive way.

Acid Value:

The acid value (AV) is defines as the number of milligrams of potassium hydroxide required to neutralize the free acids present in one gram of oil. The acid value of oils was determined by titrimetric method according to AOAC (Association of official Analytical Chemists) method.

Weight out accurately about 5 gm of the oil under test into a 250 ml conical flask and add50ml of neutral alcohol. Heat the flask over a water bath for about 30 minutes. Cool the flask and the contents to room temperature and add few drops of phenolphthalein indicator. Titrate with the standard N/10 KOH solution until a faint permanent pink colour appears at the end point.

The acid value was calculated with the equation

(56.1×V×N)

Acid value= –––––––––––––––

AV= Acid value

V = Volume of standard KOH solution in ml

N = Normality of standard KOH solution

W = Weight of oil sample in grams

Acid value (AV) is an important indicator of vegetable oil quality. According to Demian, acid values are used to measure the extent to which triglycerides in the oil has been decomposed by lipase and other physical factors such as light and heat. Acid value of oils indicates the amount of free fatty acid present in the oil. It determines the purity of oils. The higher the values, the lower the possibility of the oils to be used as cooking purpose. Higher values indicates that triglycerides of oil are converted into fatty acids and glycerol which causes rancidity of the oil. Acid value increases with days of storage under ambient conditions. Acid value determination is often used as a general indication of the condition and edibility of the oil. This is because an increase in acid value is accompanied by development of objectionable flavours and odours.

Peroxide Value:

Peroxide value (PV) measures the milli equivalents of oxygen (hydroperoxides) per 1000 gram of oil. The peroxide value is a measure of the concentration of substances that oxidise potassium iodine to iodine. The oil sample taken into a glass stopper iodine flask, is dissolved in chloroform. The measured volume of glacial acetic acid reagent and potassium iodide is accurately added and after thorough mixing, is placed in the dark for exactly fifteen minutes. At the end of the specified time, the mixture is diluted with distilled water to prevent free iodine loss. The amount of free iodine is determined by titration with sodium thiosulfate, using starch as indicator. A corresponding blank reagent is simultaneously prepared.

Peroxide value = (S-B) ×W×N

S=Volume of sodium thiosulfate consumed by the sample oil

B=Volume of sodium thiosulfate used for blank

W=Weight of oil sample

N=The normality of sodium thiosulfate

Peroxide value is used as a measure of the extent to which rancidity reactions have occurred during storage. The quality and stability of fats and oils can be indicated by using the peroxide value. Peroxide value is the most common indicator of lipid oxidation. The unrefined vegetable oils are characterised by greater PV values, compared to refined oils.

The peroxide value was found to increase with the storage time, temperature and contact with air of oil samples. Oils exposed to both atmospheric oxygen and light showed a much larger increase in peroxide value during storage. High peroxide value indicates high degree of unsaturation which inturn responsible for oxidative rancidity.

Iodine value:

Iodine value (IV) indicates the degree of the unsaturation of the oil. It is defined as the number of grams of iodine absorbed by 100 grams of oil. The iodine value was determined using Hanus method. The oil sample taken into a glass stopper iodine flask, is dissolved in chloroform. The measured volume of Hanus reagent is accurately added and after thorough mixing, is placed in the dark for exactly one hour. A corresponding blank reagent is simultaneously prepared. At the end of the specified time, the reaction is stopped by adding potassium iodide and diluted with water to prevent loss of the free iodine. The amount of free iodine is determined by titration with sodium thiosulfate using starch as indicator.

The iodine value was calculated with the equation

((V1-V2)×0.01269×100)

Iodine value= –––––––––––––––––––––––

IV= Iodine value

V1=Volume of sodium thiosulfate solution used for blank reagent

V2=Volume of sodium thiosulfate used for the sample

M=Weight of the oil sample

The iodine value is the indicator of the degree of unsaturation. It determines the stability of oils to oxidation. The unsaturated character affects the stability of oils, and, as a result, leads to the appearance of degradation effects during storage. One of the application of the Iodine value is that it is used in the determination of the amount of iodine compounds. Iodine value can also be increased with time. To maintain and preserve oil quality, it is advisable to keep it in cool places in airtight, dark containers flushed with nitrogen and the container should be glass, which is better than PVC or simple container. The lower the values the greater the oxidative storage stability. The oxidative and chemical changes in oils during storage are characterised an increase in free fatty acids contents and a decrease in total unsaturation of oils.

Saponification value:

The saponification value (SV) is defined as the weight of potassium hydroxide in milligrams needed to saponify one gram of oil. It is determined by taking 1.0g of oil sample in a conical flask to which 25ml 0.5N alcoholic KOH is added and heated under a reserved condenser for 30-40 min to ensure that the sample was fully dissolved. After cooling the sample, Phenolphthalein indicator was added and titrated with 0.2 N HCl until a pink end pont was reached. A blank was determined with the same time conditions.

Saponification value= ((B-T)×N×56.1)

–––––––––––––––––

SV=Saponification value

B=ml of HCl required by blank

T=ml of HCl required by oil sample

N=Normality of HCl

W= Weight of oil in gm

Saponification value provides the information of the average chain length and hence the molecular weight of the fatty acid in the oil. It also indicates the deterioration of oils. The shorter the average chain length of the fatty acids, the higher the saponification value and the lower the average molecular weight of the fatty acids and vice-versa. The lower saponification values suggests that the mean molecular weight of fatty acids is lower or that the number of ester bonds is less. This might imply that the fat molecules did not interact with each other.

ADULTERATIONS IN EDIBLE OILS:

Due to the greater demand of the edible oils in national and international market adulteration in high price oil with low price oil is a major issue. Therefore there is an urgent need for authentication and prevention of adulteration for the sake of consumers. By assessing the adulterations we can help to reduce consumers health risks. The quality of edible oils is of utmost concern.

Generalised issues in edible oil adulterations:

Mixing cold pressed oil with refined one:

Refined oils are being used in the adulteration of cold press oil. During the process of refining, compounds such as trans fatty acids and steradines are formed which cannot be observed in cold pressed oils. Those fatty acids are not essential and are not good for health. They increases the risk of coronary heart diseases. Trans fatty acids from partially hydrogenated oils are more harmful than naturally occurring oils.

Mustard oil adulterated with argemone oil:

Argemone Mexicana is used in the adulteration of edible oils which causes epidemic dropsy. Toxic alkaloids present in the Argemone are determined to be harmful to health.

Loose edible oil adulterations:

From the past the loose edible oils have been heavily adulterated which causes high risk of cancer, paralysis, liver damage, cardiac arrest. In many cases it has been found that Mineral oil, karanja oil, castor oil, and artificial colours are heavily used in various reputed brand edible oils preparation.

Development of advance techniques for edible oil detection such as spectrophotometric methods, chromatographic methods have been developed for quantification which easily differentiates pure and adulterated oil.

Level of some selected metals in edible oils:

During quality control of edible oils, various parameters such as Iodine value, Saponification value, Peroxide value, Moisture content, Acid value are estimated because they determine the quality and hence their economic value. Edible oils have been estimated for various metals using spectroscopic methods in which Atomic absorption spectroscopy is widely used one. Trace elements and their chemical forms are naturally present in the vegetable oils as they are absorbed from the soil. There is possibility that entry of these elements can be done in extraction and refining process. Metals arrive in the plant through deposition as well as bioaccumulation from the soil via the natural metal sources and environmental pollution. In addition, agricultural habits play an important role in the metal contents of the oils. Examples of Zinc and Copper that are found potentially in oil samples caused by environmental contamination.

Flame atomic absorption spectrometer equipped with air/acetylene flame with deuterium lamp and hollow cathode lamp for individual metals as radiation source was used for determination of Zn, Cu, Fe and flame photometer was used for determination of Na and k. The following metals are analysed by the spectroscopic methods and are investigated for the presence of essential metals in their acceptable limit.

Effect of refining process in edible oils:

Oils can be categorised in various ways:

The type of plant it was extracted from, the level of refinement, the method of extraction. Specifications of oils can differ from refined oils to crude oils from those of bleached oils. These refining process modify the chemical properties and constituents of the oils to the point of which could be detrimental tp human health. Refining is the term applied to series of processes in the oil by introduction of an alkali and centrifugal separation of the heavy insoluble materials. It is also associated with the removal of phospholipids, colour bodies and other soluble and insoluble impurities. It helps in least possible damage to the triglycerides and maximum retention of the beneficial components. After refining pure oil is obtained with desirable properties and include basic processing steps such as degumming, neutralisation, bleaching, and deodorisation.

During refining process colour reduction of oil sample occurs due to bleaching and deodorisation. This can be attributed to the removal of pigments and other residues which could pose some health and hygienic risks. Chemical properties were largely affected in the course of refining process. This causes huge reduction in free fatty acids thus reducing obesity, insulin resistance, inflammation, hypertension, and vascular diseases. The rise in the unsaponifiable matter which is linked to some of the post refining process especially vitamin fortification or other additives like vegetal squalene re-introduced into the oil so as to increase the oxidative stability, effectively inhibit chemically induced colon, ling and skin tumourgenesis and several cosmetic applications.

Evidently refining process have more significant changes to chemical properties of the edible oils. The visible change is the general reduction in chemical properties. The refining effects of the unsaponifiable matter (tocols, tocopherols, squalene, vitamins) should be try to re-introduce in the system while limiting their refining effects due to their importance.

Anti oxidant properties of edible oils:

The spontaneous reaction of atmospheric oxygen with organic compounds leads to a number of degradative changes that reduce the lifetime of edible oils which causes further deterioration of lipids in foods. Many of the scientists have demonstrated the role of oxygen in the degradation of oils which lead to further investigation of its inhibition.

Free radicals of different forms are constantly generated for specific metabolic requirement and quenched by an efficient antioxidant network in the body. The generaton of free radical species exceeds the level of antioxidant mechanism, it leads to oxidative damage to the tissues and biomolecules, leading to disease conditions like degenerative diseases. Many coumarin derivatives have special ability to scavenge reactive oxygen species, free radicals such as hydroxyl radicals, superoxide radicals which influences the process involving the free radical injury. The coumarin derivatives are extremely variable in structure, due to various types of substitutions in their basic structure, which can influence their biological activity. For free radical scavenging activity various methods such as DPPH, Superoxide and nitric oxide radical scavenging can be used.

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Received on 26.05.2019 Modified on 21.06.2019

Res. J. Pharmacognosy and Phytochem. 2019; 11(3):179-185.

DOI: 10.5958/0975-4385.2019.00030.X