Olive oil Olive oil is a vegetable oil obtained by pressing whole olives (the fruit of ''Olea europaea'', a traditional Tree fruit, tree crop of the Mediterranean Basin) and extracting the oil. It is commonly used in cooking for frying foods, as a cond ...

contains small amounts of free

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

(meaning not attached to other fatty acids in the form of a

triglyceride A triglyceride (from '' tri-'' and '' glyceride''; also TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and other vertebrates ...

). Free acidity is an important parameter that defines the quality of olive oil. It is usually expressed as a percentage of

oleic acid Oleic acid is a fatty acid that occurs naturally in various animal and vegetable fats and oils. It is an odorless, colorless oil, although commercial samples may be yellowish due to the presence of impurities. In chemical terms, oleic acid is cl ...

(the main fatty acid present in olive oil) in the oil. As defined by the

European Commission The European Commission (EC) is the primary Executive (government), executive arm of the European Union (EU). It operates as a cabinet government, with a number of European Commissioner, members of the Commission (directorial system, informall ...

regulation No. 2568/91 and subsequent amendments, the highest quality olive oil (extra-virgin olive oil) must feature a free acidity lower than 0.8%. Virgin olive oil is characterized by acidity between 0.8% and 2%, while lampante olive oil (a low quality oil that is not edible) features a free acidity higher than 2%. The increase of free acidity in olive oil is due to free fatty acids that are released from triglycerides.

Free fatty acids formation

The presence of free

olive oil Olive oil is a vegetable oil obtained by pressing whole olives (the fruit of ''Olea europaea'', a traditional Tree fruit, tree crop of the Mediterranean Basin) and extracting the oil. It is commonly used in cooking for frying foods, as a cond ...

is caused by a reaction (

lipolysis Lipolysis is the metabolic pathway through which lipid triglycerides are hydrolysis, hydrolyzed into a glycerol and free fatty acids. It is used to mobilize stored energy during fasting or exercise, and usually occurs in Adipose tissue, fat adip ...

) started when lipolytic enzymes (that are normally present in the pulp and seed cells of the

olive The olive, botanical name ''Olea europaea'' ("European olive"), is a species of Subtropics, subtropical evergreen tree in the Family (biology), family Oleaceae. Originating in Anatolia, Asia Minor, it is abundant throughout the Mediterranean ...

) come in contact with the oil (that is contained in particular

vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...

s) due to loss of integrity of the olive. High values of free acidity in

can be due to different factors such as: production from unhealthy olives (due to

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s and

mould A mold () or mould () is one of the structures that certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi ...

s contamination or attacked by flies and

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s), bruised olives, delayed

harvesting Harvesting is the process of collecting plants, animals, or fish (as well as fungi) as food, especially the process of gathering mature crops, and "the harvest" also refers to the collected crops. Reaping is the cutting of grain or pulses fo ...

and storage before processing. The

reaction is greatly enhanced by the presence of an aqueous phase, so when oil is separated from

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

during processing,

slows down and stops.

Measurement of free acidity

Free acidity is a defect of

that is tasteless and odorless, thus can not be detected by

sensory analysis Sensory may refer to: Biology * Sensory ecology, how organisms obtain information about their environment * Sensory neuron, nerve cell responsible for transmitting information about external stimuli * Sensory perception, the process of acquir ...

. Since

vegetable oils Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of edible plants. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed ...

are not aqueous fluids, a

pH-meter A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electro ...

can not be used for this measure. Various approaches exist that can measure oil acidity with good

accuracy Accuracy and precision are two measures of ''observational error''. ''Accuracy'' is how close a given set of measurements (observations or readings) are to their ''true value''. ''Precision'' is how close the measurements are to each other. The ...

Manual titration

The official technique to measure free acidity in

(as defined by the

regulation No. 2568/91) is a manual

titration Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of Quantitative research, quantitative Analytical chemistry, chemical analysis to determine the concentration of an identified analyte (a substance to be ...

procedure: a known volume of the oil to be tested is added to a mix of

ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R� ...

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

and

phenolphthalein Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...

. Known volumes of 0.1 M

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utili ...

(KOH) (the titrant) are added until there is a change in the color of the solution. The total volume of added titrant is then used to estimate the free acidity. The official technique for acidity measure in

is accurate and reliable, but is essentially a laboratory method that must be carried out by trained personnel (mainly because of the toxic compounds used). Hence it is not suitable for ''in situ'' measurements in small

oil mill An oil mill is a grinding mill designed to crush or bruise oil-bearing seeds, such as linseed or peanuts, or other oil-rich vegetable material, such as olives or the fruit of the oil palm, which can then be pressed to extract vegetable oils, whi ...

Near-infrared spectroscopy

One of the most promising methods is based on optical

near-infrared spectroscopy Near-infrared spectroscopy (NIRS) is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum (from 780 nm to 2500 nm). Typical applications include medical and physiological diagnostics and research inc ...

(NIR) where the optical

absorbance Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...

, i.e. the fraction of intensity of the incident light that is absorbed by the oil sample, is used to estimate the oil acidity. An oil sample is placed in a

cuvette In laboratories, a cuvette () is a small tube-like container with straight sides and a circular or square cross-section. It is sealed at one end, and made of a clear, transparent material such as plastic, glass, or fused quartz. Cuvettes are des ...

and analyzed by a spectrophotometer on a wide range of

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s. The results (i.e. the absorbance data for every tested wavelength) are thus processed by a statistical algorithm, such as

Principal Component Analysis Principal component analysis (PCA) is a linear dimensionality reduction technique with applications in exploratory data analysis, visualization and data preprocessing. The data is linearly transformed onto a new coordinate system such that th ...

(PCA) or

Partial Least Squares regression Partial least squares (PLS) regression is a statistical method that bears some relation to principal components regression and is a reduced rank regression; instead of finding hyperplanes of maximum variance between the response and independent ...

(PLS), to estimate the oil acidity. The feasibility to measure

free acidity and

peroxide value Detection of peroxide gives the initial evidence of rancidity in unsaturated fats and oils. Other methods are available, but peroxide value is the most widely used. It gives a measure of the extent to which an oil sample has undergone primary oxid ...

by NIR spectroscopy in the wavenumber range 4,541 to 11,726 cm⁻¹ was reported. Many commercial spectrophotometers exist that can be used for analysis of different quality parameters in olive oil. The main advantage of NIR

spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

is the possibility to carry out the analysis on raw olive oil samples, without any chemical pretreatment. The main drawbacks are the high cost of commercial spectrophotometer and the need of

calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...

for different types of oil (produced by olives of different varieties, different geographical origin, etc.).

Electrochemical impedance spectroscopy

Another approach is based on

electrochemical impedance spectroscopy Dielectric spectroscopy (which falls in a subcategory of the impedance spectroscopy) measures the dielectric properties of a medium as a function of frequency.Kremer F., Schonhals A., Luck W. Broadband Dielectric Spectroscopy. – Springer-Verlag ...

(EIS). EIS is a powerful technique that has been widely used to characterize different

food Food is any substance consumed by an organism for Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...

products such as the analysis of

milk Milk is a white liquid food produced by the mammary glands of lactating mammals. It is the primary source of nutrition for young mammals (including breastfeeding, breastfed human infants) before they are able to digestion, digest solid food. ...

composition, the characterization and the determination of the freezing end-point of

ice-cream Ice cream is a frozen dessert typically made from milk or cream that has been flavoured with a sweetener, either sugar or an alternative, and a spice, such as cocoa or vanilla, or with fruit, such as strawberries or peaches. Food colouring ...

mixes, the measure of

meat Meat is animal Tissue (biology), tissue, often muscle, that is eaten as food. Humans have hunted and farmed other animals for meat since prehistory. The Neolithic Revolution allowed the domestication of vertebrates, including chickens, sheep, ...

ageing, and the investigation of ripeness and quality in

fruit In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

References

{{reflist, 2 Olive oil Lipids Food analysis