Role of minor constituents in the photooxidation of virgin olive oil

Virgin olive oil was photooxidized at 2 and 40°C and at fluorescent light intensities of 0, 620, 2710, and 5340 lux. As expected, higher fluorescent light intensities induced higher peroxide formation in the oil. The thiobarbituric acid reactive substances (TBARS) were found to be good indicators of photooxidation during the early stage of the reaction. Pheophytin A and β-carotene were light- and temperature-sensitive, whereas α-tocopherol and total polyphenols were mostly affected by light. Pheophytin A functioned as a photosensitizer, resulting in rapid oxidation of the oil. β-Carotene was a strong natural inhibitor of photooxidation for all light intensities at 2°C, suggesting quenching properties for singlet oxygen. However, β-carotene antioxidant activity was reduced at 40°C because of its rapid destruction.     

Studies on olive oil standardization

Summary Experiments were conducted in the Research Experiment Station for Agricultural Technology, Athens, Greece, to study the effect of climatic conditions, state of maturity, infection by olive fly, and the effect of various fertilizers upon the quality of olive oil. Olive oil produced in localities where the temperature is low during the maturity period has more unsaturated fatty acids than oil produced in localities with higher temperatures. With the advance of maturity the proportion of unsaturated fatty acids increased. Infection by olive fly had no appreciable effect upon the relative proportion of saturated fatty acids. For studying of the effect of fertilizers the injection method was used. Nitrogen fertilizers injected into the branches gave considerable increase in the volume of fruit, its protein content, and the total amount of oil produced per acre.     

Color changes in olive oil

The following is a summary of the conclusions and facts brought out in this investigation:

1. That yellow olive oil packed in tin may turn green, and that this reaction is brought about by reduction of the yellow color through the action of the free fatty acids of the oil on the tinplate.
2. That this reaction will not take place at normal temperatures in the presence of light.
3. That the green color of olive oil, so formed, reverts to yellow by the action of light.
4. That the formation of green color in olive oil is dependent on the acid strength of the oil and depth of the original yellow color and increases proportionately with these.
5. That, in general, most olive oils on the market packed in tin, are of a greenish color, while the bottled product is yellow. This, of course, does not apply to fresh oil, but only that which is sufficiently old to have permitted any color change to have taken place.
6. That it is not possible to pack olive oil in cans without development of greenish color unless the original oil is rendered neutral or bleached. As it is not practical to render the oil absolutely neutral or to bleach it, the formation of green color in tin cannot be entirely prevented in practice but may be controlled to a certain extent.
7. That the development of excessive green color, which might be objectionable, can be prevented either by packing in cans only oil of low acidity (below 0.25%) or oil, the original color of which is light yellow. In the latter case the acidity is of little consequence as far as color is concerned.
8. That green-colored oil in cans can be reconditioned by transferring to bottles and exposing to light, whereby the original yellow color is restored.

     

Extracted California olive oil

Summary Extracted olive oil is not used exclusively for soapmaking but in fact is used to the utmost extent in edible oils. General processing procedures are outlined for the production of edible olive oil from high acid extracted olive oil.     

New possibilities in California olive oil

Summary and Conclusions I. There is a field for increasing the return on oil olives through feeding the pomace to livestock, and through increasing the oil yield by the use of more powerful presses, or exhausting the oil content of the pomace by solvent extraction. II. Neutralization of free fatty acids and decolorization are refining methods that can be utilized at any plant, but deodorizing is a problem that will probably require special equipment. III. Work leading to the establishment of olive oil standards is suggested. IV. In the event of the establishment of standards, a central refining and blending plant is suggested.     

Color-pigment correlation in virgin olive oil

The chlorophyll and carotenoid content of virgin olive oils from five varieties harvested at varying degrees of ripeness were determined. Colors were evaluated from the chromatic ordinates L*, a*, b* of the absorption spectrum. Oil color changes for different varieties or stages of ripeness are directly related to pigment content and a* and b* values. The statistical study made on both series of parameters proves that there is a good correlation between them. The carotenoid content and b* have one of the best correlation coefficients (r) and is easily measured. This methodology evaluates chlorophyll and carotenoid content, an additional attribute for evaluation of virgin olive oil quality.     

Pigments present in virgin olive oil

The qualitative and quantitative control of pigments in ripe olives and in extracted virgin olive oil has increased our knowledge of the influence on these compounds in the areas of ripening of the fruit, storage time in the factory and the oil extraction process. As the harvesting time of the fruits increases, pigment content decreases. During storage, the presence of lipoxygenase has been detected, as well as a considerable decrease in chlorophylls and a small decrease in carotenoids. During the extraction process, the chlorophyllic fraction is destroyed in the greater part, and although the carotenoid fraction is also affected, its concentration increases in the oil with respect to that in the fresh fruit. In the pigment degradation, in addition to the acid-catalyzed reaction, the presence of lipoxygenase suggests a role for this enzyme.     

Characterization of a 13-lipoxygenase from virgin olive oil and oil bodies of olive endosperms

Olive oil is one of the oldest known vegetable oils, and it is almost unique in that it can be consumed without any refining treatment. One of its most important quality problems is oxidative rancidity due to the oxygenation of polyenoic fatty acids and formation of compounds that derive from these fatty acid hydroperoxides. Beside autoxidation, lipoxygenases (LOXs) were suggested to be involved in this process. Here we show, that approximately 1.6% of all linoleic acid (LA) molecules within olive oil samples had been converted into LOX-derived (13S,9Z,11E)-13-hydroperoxy-9,11-octadecadienoic acid (13-HPODE) as determined by ¹H NMR- and HPLC analysis. LOX activity tests indicated the occurrence of an active 13-LOX exhibiting a pH optimum between pH 5.5 and 6.0. Furthermore, this enzyme preferentially metabolized free fatty acids. In order to elucidate the origin of this LOX, we analyzed olive endosperms for LOX forms. Chromatography of total protein extracts of the tissue showed LOX activity almost exclusively associated with a high molecular mass fraction. Light microscopic inspection, as well as the calculated phosphate, neutral lipid, and protein content of this fraction, suggested that this fraction may contain oil bodies and that LOX activity was associated with their membrane. This LOX activity had a pH optimum of 6.0. Activity assays at various temperatures indicated a significant catalytic efficiency of the enzyme up to 55°C. HPLC analysis of LA oxygenation products within the lipid fraction and of activity tests of isolated oil bodies showed that the LOX present in mature olive endosperm oil bodies was, as the enzyme from olive oil, a linoleate 13-LOX preferentially active on free LA. We suggest, that this oil body LOX from olive endosperm, is the one detected originally in olive oil and may survive at least in part olive oil production.     

Quality characteristics of olive leaf‐olive oil preparations

Olive leaf‐olive oil preparations were obtained by vigorous mixing at various levels of addition (5, 10, 15%w/w) of new or mature leaves. After removal of the plant material via centrifugation, quality and sensory characteristics of the preparations were determined. Oxidative stability (120°C, 20 L/h) and DPPH radical scavenging were increased ～2–7 fold depending on the level of leaves used due to enrichment with polar phenols, mainly oleuropein, and a‐tocopherol. The extraction process affected the chlorophyll content and organoleptic traits as indicated by acceptability and preference tests (n = 50). Forty‐four % of the panelists identified a strong pungency in preparations with 15% w/w new leaves. Fifty‐four % of them identified a bitter taste in those with 15% w/w mature leaves, which was attributed to high levels of oleuropein (～200 mg/kg oil). Olive leaf‐olive oil preparations had interesting properties regarding antioxidants present that can attract the interest of a functional product market. Practical applications: The wider use of olive oil and derived products is highly desirable. In this sense, the current study presents data that support introduction to the market of a new specialty olive oil based solely on olive tree products (olive oil and leaves). Thus, in addition to olive oil and olive paste, a new product, that is an olive oil enriched with olive leaf antioxidants, especially oleuropein produced via a “green” technique (mechanical means instead of extraction with organic solvent) can be made available for consumers.