2011-35493, 2011-85007 Biochemistry 34.1, HEJ, Sir Avvin Pelovello
Lipids are biologically produced molecules that are relatively insoluble in water and soluble in polar and non-polar organic solvents. Fatty acids are amphipathic, containing both polar and non-polar groups. Oils are long straight -chain carboxylic acids composed of saturated and unsaturated fatty acids. In the experiment, the acid value of used vegetable, lauric, and coconut oil were determined. The obtained average acid values (mg KOH/g) for the four oil samples are 1.19, 2.18, 0.87, and 1.86. Based on the data, the rancidification of these used oils are apparent due to the higher level of acid value as compared to the acceptable one at 0.6 mg KOH/g. This showed the occurrence of rancidification, or the production of foul odor and taste in fats. There are three pathways for rancidity; oxidation of peroxides (oxidative rancidity); hydrolysis of unsaturated fatty acids and production of free fatty acids (hydrolytic rancidity); and microorganism activity (microbial rancidity). lipids, fatty acids, acid value of fats, oxidative rancidity, hydrolytic rancidity
Lipids are compounds that are defined bas ed on solubility. They have low solubility in water and high solubility in nonpolar solvents. The nonpolar nature of lipid is due to the large portion of hydrocarbons in the molecule. Lipids have diverse biological functions, including being highly concentrated energy sources, membrane components (structure), and molecular signals (Moore & Langley, 2008). Biological lipids are either hydrophobic, containing only nonpolar groups, or amphipathic, possessing both polar and nonpolar groups (Garrett & Grisham, 2010). Fatty acids are amphipathic lipids because they have polar carboxylic acid group and a long nonpolar hydrocarbon tail. They normally have even number of carbon atoms. Presence of carboncarbon double bonds in a fatty acid chain makes it unsaturated. While fatty acid with only single bonds is saturated. Unsaturated fatty acids are usually in cis configuration rather than trans (Campbell & Farrell, 2015). Derivatives of fatty acids include fats and oils which serve as stored forms of energy in living organisms (Nelson & Cox, 2013). Fats and oils are triglycerides or triacylglycerols. They are lipids that are formed through ester linkages between three hydroxyl groups (of glycerol) and fatty acids. The apparent distinction between fat and oil is that the former is a solid at room temperature making it saturated while the latter is liquid at room temperature making it unsaturated (Moore & Langley, 2008). Fatty acid composition in fats and oils has been explored due to its association with health related risks (Garrett & Grisham, 2010). The objective of the experiment is to determine the acid value of commercially available oils.
Primarily, 200 mL of fat solvent was prepared by mixing 100 mL of absolute ethanol and 100 mL of diethyl ether in a beaker. Covering the solution is required due to its volatile trait. Then, 1.0 mL of fat sample or used cooking oil was dissolved in 10 mL of fat solvent. Next, 2 drops of phenolphthalein indicator was added and it was mixed thoroughly. It was titrated against 0.01 M KOH until a pale pink end point is achieved. Four fat samples were analyzed and for each sample, three trials were executed. The volumes of the titrant consumed were noted. Finally, the acid value of the fat for each trial was calcul ated using the equation below.
=
× × () ℎ ()
Oils are naturally occurring esters long straight-chain carboxylic acids. They belong to the saponifiable group of lipids. Lipids are biologically produced. They are relatively insoluble in water and soluble in polar and non-polar organic solvents. Triacylglycerol molecules mostly constitute edible oils. Both saturated fatty acids, such as oleic acid and linoleic acid, and unsaturated fatty acids, such as palmitic acid and stearic acid, form these oils. A single fatty acid that could be esterified up to three times into glycerol backbone, or even different ones, lead to the formation of these oils. (Atinafu & Bedemo, 2011). Acid value (AV) is the amount of potassium hydroxide in milligrams required to neutralize the free acids presence in a gram of sample. It is generally a measure of the free fatty acids in oil. Fatty acids are normally found in the triglyceride form, but hydrolysis of these fatty acids frees them and accounts to t he the
decreased quality of oil. Acceptable levels for all oil
finally
samples should be below 0.6 mg KOH/g (Atinafu & Bedemo). From the experiment, the following values
microorganisms (Luck & Lipinski, 2002). Oxidative rancidity is due to the cleaving of unsaturated fatty acid double bonds that releases
were obtained from the titration of the used vegetable oil with potassium hydroxide:
volatile aldehydes and ketones. This type of rancidity commonly occurs with unsaturated fatty acids, both mono- and poly-, as the lipids undergo oxidation
Titration of used vegetable oil with KOH.
0.910
0.910
1.007
0.920
with
the
presence
and
activity
of
reaction (Thomas, 2005). Hydrolytic rancidity is due to the hydrolysis of triglycerides, releasing free fatty acids and lower the quality of the fat. This type of rancidity has catalytic requirements to develop with the presence of water.
2.0
1.23
2.1
1.29
1.7
1.05
3.6
2.21
3.5
2.16
3.5
2.16
2.0
1.11
1.4
0.78
1.3
0.72
3.1
1.89
hydroperoxides formed in the initial stages of lipid
3.0
1.83
oxidation are determined through titration with iodide ion. High values of peroxide are indicative of a rancid
The acid values were obtained by using the formula in the lab manual. Since the sample is oil, the density was used to convert it to grams of fat.
× × () = ℎ () 0.002 0.0020 0 × 0.01 0.01 × 56105.6 56105.6 = 0.910 × 1.00 1.00 = . / /
Short-chain fatty acids are commonly affected by this type of rancidification (Freeman, 2000). Microbial rancidity is due to the presence of microorganisms such as bacteria and molds. Enzymatic activity aid these microorganisms in metabolizing lipids and fats, producing the foul odor and taste (Allen & Hamilton, 1994). Other values that are t ested for the quality of fats. One example is the peroxide value. This is a test for the propensity of the fat to undergo oxidative rancidification. The measure of peroxides and
fat, but moderate and lower values may not be conclusive and correlated with rancidity (Grossi, et. al, 2015). Another value is the anisidine value. Hydroperoxides produce volatile aldehydes such as hexanal and leaves the non-volatile part to the glyceride molecule. High values of anisidine is also an indication of the fat undergoing oxidation (FoodLab, 2012). The aforementioned values are also tested with their respective tests, such as the peroxide value test for peroxides and p-anisidine value test for anisidine value.
The results above show the amount of free fatty acids present in the oils based on the titration with potassium hydroxide. The acid values of these oils are above the acceptable levels for consumption. Hydrolysis of the fatty acids present in these used oils predominate in the sample, giving a higher amount of free fatty acids and lower oil quality or rancidity. Rancidification is a general term for the process that causes a substance to become rancid or have unpleasant odor or taste. This is due to three pathways. One is through degradation by oxygen through a free radical process, another is through the hydrolysis of triglycerides into free fatty acids, and
Acid value refers to the amount of potassium hydroxide in milligrams that is required to neutralize a gram of fat sample. Fats undergo rancidification or production of foul odor and taste. There are three types: oxidative rancidity, hydrolytic rancidity, and microbial rancidity. Oxidative rancidity is due to the fats undergoing oxidation reaction and consequent elevation of peroxide values in the sample. Hydrolytic rancidity is due to the cleavage of double bonds in unsaturated fatty acids, producing volatile aldehydes and ketones,
as well as free fatty acids. Microbial rancidity is due to
Nelson, D., Cox, M. (2012). Leningher principles of
the presence of microorganisms and enzymatic activity that break down fat. This results to a lower quality of fats. The acceptable level for consumption
biochemistry (6th edition). edition). USA: W.H. Freeman and Company.
is at 0.6 mg KOH/g of acid value. Results showed that the oil samples used in the experiment have a higher acid value to that of the
Thomas, A. (2005). Ullmann’s encyclopedia of industrial chemistry, fats and fatty oils. WileyVCH, Weinheim.
acceptable level. This is due to the t ype of oil used. All oil samples were used in the past, proving the lower quality of these fats. Rancidification also occurs over time, and repeated use of the fats can easily promote processes for either of the three types of rancidity to occur. It is recommended to use fresh sample of oil to test for the acid values for consumption. This will ensure the quality of the commercially available edible oils. Other values to test the quality of fats may also be tested, such as peroxide value, anisidine value, TBA value, and other important quality check systems.
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G-W.,
(2002).
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encyclopedia of industrial chemistry, foods, food additives. Wiley-VCH, Weinheim.
