The composition, thermal stability and phase behavior of tamarind (
Tamarind, (
In Mexico, the tamarind does not reach an important industrial level or a full and integral use yet, as is also the case in many of the poorest producing regions of this fruit located in Latin America, Asia or sub-Saharan Africa. This tropical plant has been introduced and naturalized in more than 50 countries around the world (Rao
The flowers, leaves and pulp of the fruit are edible and have important nutrients. The latter has a characteristic sweet-acidic flavor due to the combination of high contents of tartaric acid and reducing sugars. In fact, the tamarind is the largest natural source of this acid widely used in Asian gastronomy (De Caluwé
In several countries, the tamarind pulp, aside from being consumed fresh, is used for flavoring and applied as a component of food, for example in jams, sweet sauces and hot sauces, and also as an important ingredient in juices and some soft drinks (El-Siddig
The tamarind seed, which includes an external cover or testa (20–30%) and the nucleus or endosperm (70–75%), contains important compounds such as antioxidants and dyes (Bhadoriya
The presence in seeds of tannins and some other colorant materials in the test portion, makes its direct consumption inadequate for food (Kumar and Bhattacharya,
Tamarind seed is the raw material for the elaboration of the polysaccharide jellose, some adhesive products, and for obtaining tannins. Jellose with gelling properties is commercially available as a food additive and is recommended to be used as a stabilizer in ice cream, mayonnaise and cheese, and as an ingredient or agent in pharmaceutical products (El-Siddig
Concerning the tamarind seed oil (TSO), studies have been reported about some of its properties, such as its fatty acid composition and other basic physicochemical characteristics. Several authors have reported different extraction yields of tamarind seed oil from different producing regions, some of them with relatively high values such as 16.25% (Morad
The fatty acid composition of tamarind seed oil has been addressed by several authors such as Pitke
The knowledge concerning the effects of heating fats and oils is basic and can be used to determine some of their physical and chemical properties which are important for the evaluation of their potential applications. The complexity and diversity of the thermal profiles and phase behavior of fats and oils are due to the variety of both the triacylglycerides (TAG) and their main fatty acid constituents, FA (Tan and Che Man,
As it is known, information on the thermal stability of vegetable fats and oils is essential for the identification of their potential uses, and many of the most important properties of fats and oils are related to their phase behaviour, mainly in the solid/liquid and liquid/solid transitions. This behavior is dependent on, among other variables, the composition of fatty acid and triacylglycerides, as well as the polymorphic properties assumed in the solid phase (O’Brien,
In order to contribute to tamarind seed oil knowledge and to visualize alternatives uses within the perspective of an integral use of tamarind, the objective of this paper was to study the seed of the tamarind and characterize its extracted oil by infrared spectroscopy and fatty acid composition, as well as analyzing the thermal stability and phase behavior of the tamarind seed oil by thermogravimetry and differential scanning calorimetry.
Samples of tamarind fruits (
The official analysis techniques were used to determine the moisture, ash, crude fiber and crude fat (Horwitz,
The oil was extracted from the dehydrated kernels of the tamarind seed (with approximately 10% moisture) using hexane as a solvent in a Soxhlet apparatus and the resulting micelle was processed in a rotary evaporator for oil recovery. The raw TSO was purified by a modification of the Wesson method described in Solís-Fuentes
A Perkin-Elmer Spectrum Two FT-IR spectrophotometer was used. The data were processed with the help of the Spectrum software. In the case of the liquid sample, its IR analysis did not require any previous preparation.
Fatty acid methyl esters were prepared by saponification of purified tamarind seed oil and an acid-catalyzed methylation, using an alcoholic solution of 2.5% KOH and a methanol solution to 2% H2SO4. (Anu and Rao,
For the analysis of the fatty acids constituents of the acylglycerols of TSO, a gas chromatograph Shimadzu was used, equipped with a column of polydimethylsiloxane with 5% phenyl, ZB-5MSi, of 30 m x 0.25 mm x 0.25 μm, coupled to a Shimadzu mass spectrometer provided with a full spectrum scan detector (SCAN) of 50 to 500 -m/z. A mixture of fatty acid standards (FAME mix 37 SUPELCO) was used to support the identification of the methyl esters in the sample. The operating conditions in the chromatograph were: an initial temperature of 100 °C for 5 min, with a heating rate of 4 °C/min up to 250 °C for 15 min; the carrier gas was helium at 0.8 mL / min, the injector temperature was 250 °C and, an ionization source of electronic impact was used.
The thermal stability of the TSO was analyzed by thermogravimetry in atmospheres of nitrogen and air in a TGA Q5000 IR (TA Instruments), using a sample holder of alumina. The changes in mass were monitored in a working interval of 25 to 1000 °C to obtain the TG and DTG data and graphs. The capture of the data in an ASCII code made analysis possible and the results were graphically edited in the Origin software (OriginLab,
TSO samples were analyzed in a TA Instruments DSC Q2000 differential scanning calorimeter equipped with a TA Universal Analysis data analysis station. The purge gas was nitrogen at a flow of 50 mL/min. The instrument was calibrated with Indium (melting point 156.6 °C; DHf, 28.45 J/g). Samples between 2 and 12 mg were weighed in a thermobalance (TA instruments) in aluminum SFI capsules with a precision of ±0.1 mg and then hermetically sealed. An empty and sealed capsule was used as reference. Each of the samples was heated up to 90 °C with an isotherm of 5 min with the aim of destroying its thermal history. To analyze the thermal behavior of the oil, the following conditions were used: Crystallization profile: the sample was heated up to 90 °C for 5 min and cooled to 5 °C/min to -80 °C to record the crystallization profile, crystallization enthalpies, and the onset and offset temperatures of the phase changes.
Fusion profile: the sample was heated from −80 to 5 °C/min up to 90 °C, and the melting profile, the enthalpy and the onset and offset temperatures of the phase changes were recorded.
The amount of solids in the oil samples as a function of temperature were calculated based on the experiments in the DSC, according to the methodology outlined by Lambelet and Raemy (
The samples of the physiologically mature tamarind fruit were pods with brown skin rind and rust colored pulp with an average mass of 16.37 g per fruit. The mass of each anatomical constituent part of the pods was on average 21.61% husk, 58.0% pulp and 20.34% seeds. The fruit had between 3 to 4 seeds for each pod unit.
The tamarind seeds had an average mass of 0.87 g/seed with a bright, dark color. They were constituted by an external cover or testa and a nucleus or endosperm; the latter represented, on average, 77.73 ± 3.56% of the total seed mass.
Physical and chemical characteristics of the fruit and oil extracted from the tamarind seed
Characteristic | Value±SDa |
---|---|
Average mass, g | 16.37±4.31 |
Husk, % | 21.61±4.90 |
Pulp, % | 58.00±2.10 |
Seeds, % | 20.34±2.50 |
Seeds, average number | 3.81±1.60 |
Moisture, % | 11.61±0.33 |
Nucleus of the seed, % | 77.73±3.56 |
Ash, % db | 3.71±0.27 |
Crude fiber, % db | 2.09±0.51 |
Crude fat, % db | 3.76±0.20 |
Consistency, 25 ° C | Liquid |
Color: | Amber |
Odor: | Characteristic |
Taste: | No taste |
Refractive index | 1.465 |
Saponification value, mg KOH/g | 174.80±9.87 |
Free fatty acids, % | 3.12±0.24 |
Acidity index, mg oleic acid/g | 6.22±0.47 |
:Average values of two determinations ± standard deviation
The refined oil of the tamarind seed had a liquid consistency under environmental conditions (around 25 °C), a light amber color, a characteristic smell, and an undefined flavor.
TSO had a refractive index of 1.465, saponification and acidity indices of 174.80 ± 9.87 mg KOH/g and 6.215 ± 0.47 mg oleic acid/g, respectively; and a content of 3.12 ± 0.24% of free fatty acids. These values are close to those reported by Pitke
FTIR spectra of (a) refined tamarind seed oil; (b) main signals from 2000-900 cm−1of purified tamarind seed oil; (c) main signals from 2000-900 cm−1of methyl esters of fatty acids from tamarind seed oil.
At the center of the spectrum near 1753 cm−1 there is a strong signal corresponding to the carbonyl groups due to the ester bonds of fatty acids and glycerol of the triacylglycerols of the oil, including free acid carbonyl bonds from the free fatty acids present in the sample analyzed. Finally, the region of low frequency between 1500–800 cm−1, is considered the area where a group of signals appears due to the absorption of characteristic groups of lipid compounds, including double bonds, conjugated dienes, etc. (Shahidi and Wanasundara,
The FTIR analysis was also used together with the thin layer chromatography, as a complement to corroborate the progress and result of the methylation process of the tamarind seed oil, prior to the determination of its composition.
Fatty acid composition of tamarind seed oil
Fatty acid | Delta notation | Retention time | % Area |
---|---|---|---|
Caprylic | 8:0 | 8.174 | 0.23 |
Capric | 10:0 | 14.801 | 0.08 |
8-oxooctanoic | 8-O=8:0 | 15.090 | 0.47 |
9-oxo nonanoic | 9-O=9:0 | 18.583 | 1.18 |
Azelaic | 8-COOH-8:0 | 22.246 | 0.55 |
Lauric | 12:0 | 21.476 | 0.44 |
Tridecylic | 13:0 | 24.588 | 0.22 |
Cis-7-Hexadecenoic | 16:1-delta-7c | 26.795 | 0.41 |
Myristic | 14:0 | 27.544 | 1.44 |
Pentadecylic | 15:0 | 30.335 | 0.85 |
Palmitoleic | 16:1-delta-9c | 32.410 | 1.95 |
Palmitic | 16:0 | 33.222 | 11.91 |
cis-10-Heptadecenoic | 17:1-delta-10c | 34.961 | 0.26 |
Margaric | 17:0 | 35.542 | 0.95 |
Linoleic | 18:2-delta-9c, 12c | 37.567 | 4.94 |
Oleic | 18:1-delta-9c | 37.718 | 18.99 |
Stearic | 18:0 | 38.102 | 7.63 |
Oxiraneoctanoic, 3-octyl | 9,10-O-18:0 | 41.549 | 1.78 |
Paulinic | 20:1-delta-13c | 42.036 | 4.22 |
Arachidic | 20:0 | 42.604 | 5.33 |
6-Hexadecenoic acid, 7-methyl | 7-Me-16:2-delta-6t | 43.197 | 1.62 |
Octadecanoic, 9,10-dihydroxy | R9,R10-di-OH-18:0 | 44.604 | 2.43 |
Behenic | 22:0 | 47.809 | 7.57 |
Tricosylic | 23:0 | 51.049 | 0.74 |
Lignoceric | 24:0 | 55.933 | 20.15 |
Non identified | 3.65 | ||
Total saturated fatty acids | 63.95 | ||
MCSFA | 2.95 | ||
LCSFA | 32.32 | ||
VLCSFA | 28.46 | ||
Total unsaturated fatty acids | 32.39 |
MCSFA: medium-chain saturated FA; LCSFA: long-chain saturated FA; VLCSFA: very-long-chain saturated FA
TSO fatty acids are 32.39% unsaturated, with the majority being oleic acid (18.99%), linoleic (4.94%), and paulinic acid (4.22%). Also, 9-cis-hexadecenoic acid (palmitoleic 1.95%) and 7-cis-hexadecenoic acid (0.45%), were present. Some other complex fatty acids were also identified: Epoxy acids, oxo acid, and hydroxylated and branched fatty acids, whose content as a whole was 7.48%. 3.65% of the total fatty acids could not be identified.
Different reports found on the fatty acid composition of tamarind seed oil differ greatly from one another, both considering the presence of different fatty acids in the oil, as well as in the percentages assigned to each of them. The results obtained in the present work are closely related, but with some differences, to those reported by Pitke
According to the results of this work, the TSO does not contain short-chain fatty acids (< 6C) and the percentages of saturated fatty acids classified according to chain length were medium-chain fatty acids (MCSFA, between 6C and 12C): 2.95%, long-chain saturated FA (LCSFA, between 14C and 20C): 30.73% and very-long-chain saturated FA (VLCSFA,> 20C): 28.46%. The unsaturated fatty acids in the oil were all of long-chain.
An interesting feature of the composition of the TSO is its high content of LCSFA and mainly VLCSFA. The saturated fatty acids with aliphatic-chain > 20C, that is arachidic, behenic, tricosylic, and lignoceric acids, were 33.79% in total.
The role of long- and very-long-chain saturated fatty acids in nutrition and human health is not fully established yet, but it is known that very-long-chain saturated fatty acids (VLSFA) play an important role in the structure, cellular and organic functioning of the body. The VLSFA are major components of ceramides and sphingomyelins, lipids formed by a main chain of sphingosine with an acylated FA, often saturated (Quehenberger
Lemaitre
On the other hand, as ceramides are the base molecules of sphingolipids, they are very abundant in the lipid bi-layer of cell membranes and their exocytosis to the intercellular space allows them to function as the cementing substance due to the amphiphilic chemical structure in the stratum corneum, one of the four different sublayers of skin epidermis (Sahle
Lignoceric acid is found in trace amounts in almost all edible vegetable oils, with some exceptions, such as peanut oil, whose content is close to 1%. It is contained in higher amounts in sources with no commercial production. Some wild plant seeds have been reported with high content of long- and very-long-chain saturated fatty acids, in which lignoceric acid is present in high percentages (Spitzer
The tamarind seed, being a residue of the consumption and industrialization of this fruit with a wide acceptance in the world, acquires an interesting role as a potential source of oil with long- and very-long-chain fatty acids and especially as a source of lignoceric acid, identified, as noted earlier, with important functional properties which are protective of the health of the human organism in regards to the cardiovascular, endocrine and skin systems.
When vegetable fat and oils are exposed to high temperatures, a process of deterioration occurs. However, these oils sometimes have natural antioxidants that tend to increase the stability of the products made with them. The stability of the oil to heat treatment is an aspect of great importance as is the degree and speed of decomposition of the constituents of the oil. It depends on many factors that include both the treatment conditions (time, temperature level, presence of oxygen, water and other constituents, etc.) and the fatty acid and acylglyceride compositions of the oil.
TG and DTG curves of tamarind seed oil in nitrogen atmosphere.
The second stage corresponded to the decomposition of the constituent acylglycerols of the TSO, which, as has been identified by other researchers, occurs in an inert atmosphere, around an interval that was from 220 to 420 °C (Melzer
The main data of the thermogravimetric analysis of TSO are presented in
Main data of thermogravimetric analysis of tamarind seed oil
Stage | Nitrogen atmosphere |
Air atmosphere |
||||||
---|---|---|---|---|---|---|---|---|
∆Tdecom °C | Tpeak °C | -∆mass % | Rdecom mg/°C | ∆Tdecom °C | Tpeak °C | -∆mass % | Rdecom mg/°C | |
1 | 108.88–223.16 | 166.80 | 6.94 | −0.014 | 93.88–217.36 | 145.41 | 6.58 | −0.008 |
2 | 224.10–509.07 | 418.42 | 90.48 | −0.163 | 218.00–361.25 | 322.27 | 22.79 | −0.019 |
3 | 362.10–477.91 | 417.63 | 50.78 | −0.079 | ||||
4 | 478.50–600.16 | 544.02 | 15.07 | −0.017 | ||||
Total | 108.88–509.07 | 97.42 | 93.88–600.16 | 95.22 | ||||
Residue | 2.58% | 4.78 |
Value of temperature (T) and rate (R) in the Table represents the means of two determinations, with SD < 1.0 °C and SD < 0.001 mg/°C, respectively, from
TG and DTG curves of tamarind seed oil in air atmosphere.
The phase behavior of TSO was analyzed by DSC.
Crystallization and fusion curves of tamarind seed oil.
Solid/liquid relationships of tamarind seed oil in the characteristic refrigeration, environmental, and body temperature ranges.
In relation to body temperature, for example, for edible fats and oils products, these data on oil phase behavior can give valuable indications about their palatability, gumminess, workability, and general behavior. The same happens with refrigeration, environmental, or higher temperatures.
The most important factor in the consistency of fats and oils is the proportion of the solid and liquid phases; they become firmer as the solid content increases. The solid-fat profile shows the temperature range in which the oil changes its consistency and therefore, transits from the solid phase to the liquid one (or vice versa), including the temperature region in which it has the ability to be molded, called the plastic range (between 15 and 25% of solids). There is an interval where the fat or oil has the best consistency to be worked on. The solid profile also shows the lowest temperature at which the oil is totally liquid, and if the profile of solids is flat (with a wide plastic range) or if the profile has a steep slope. In this sense, the TSO has a melting range of 50.8 °C (from −22.2 to 28.6 °C). At temperatures higher than 28.6, the oil is completely liquid, with a total absence of crystalline nuclei. Said range mainly includes the temperatures considered as environmental. Those that correspond from the refrigeration conditions to close to the body temperature. From 5 °C and lower, the percentage of solids in TSO is greater than 78%.
Within the range of temperatures that are considered environmental, around 5 to 25 °C, the percentage of solid fat in the liquid oil fluctuates from 78 to 5.15%. Thus, TSO in this environmental temperature range is perceived as a semi-solid to liquid consistency as the temperature increases. In the range of body temperatures the oil shows a gradual decrease in the solid content until it reaches zero at 28.6 °C.
As shown in
Some of the most critical factors in the behavior of products made with fats and oils refer to these two properties; for example, butter, margarine and spreads depend on the consistency of the fat portion and its ability to be spread over a slice of bread. A wide plastic range and a soft consistency are required for the fillers and margarines used for thin rolling at refrigerater temperatures. This same plasticity is necessary for the handling of shortenings in ice cream and aerated milkshakes (O’Brien,
According to
According to the results obtained in this work, the oil extracted from the tamarind seed has a refractive index of 1.465, and a saponification value of 174.8 mg KOH/g. The major FA of the TSO are: Lignoceric, oleic, palmitic, stearic, and behenic acids, with the presence of arachidic, linoleic, and paulinic acids, among others in smaller quantities.
The high content of saturated fatty acids of long- and very-long-chain acids is notable, and whose concentration encompasses about 60% of the total, of which lignoceric acid represented more than a third.
The TSO showed an initial decomposition temperature of its triacylglycerol components of 224.1 °C in inert atmosphere and 218 °C in air atmosphere. The analysis of the phase behavior of the oil showed that the transition to the solid phase starts at 20.16 °C and the transition to the liquid phase ends at 28.6 °C.
The S/L ratios showed that at temperatures considered environmental, the oil has a semi-solid and liquid consistency within that temperature range.
The composition, thermal stability and phase behavior presented by the tamarind seed oil make it potentially usable in applications in the food industry, Pharmacology, and cosmetology.
The authors thank the Institute of Materials Research of the UNAM, Mexico for its support in carrying out the calorimetric analysis included in this work. They would also like to thank Prof. Siegfried Erich Haid for revising and correcting the manuscript.
This work was carried out with the normal financial support of Universidad Veracruzana and Universidad Nacional Autónoma de México.