Pure Linalool Research Articles

Assessment of the Anti-Amyloidogenic Properties of Essential Oils and Their Constituents in Cells Using a Whole-Cell Recombinant Biosensor.

Essential oils exhibit numerous medicinal properties, including antimicrobial, anti-inflammatory and antioxidant effects. Recent studies also indicate that certain essential oils demonstrate anti-amyloidogenic activity against β-amyloid, the protein implicated in Alzheimer's disease. To investigate whether the anti-aggregating properties of essential oils extend to α-synuclein, the protein involved in Parkinson's disease, we constructed and employed a whole-cell biosensor based on the split-luciferase complementation assay. We validated our biosensor by using baicalein, a known inhibitor of α-synuclein aggregation, and subsequently we tested eight essential oils commonly used in food and the hygienic industry. Two of them, citron and sage, along with their primary components, pure linalool (the main constituent in citron essential oil) and pure eucalyptol (1,8-cineole, the main constituent in sage essential oil), were able to reduce α-syn aggregation. These findings suggest that both essential oils and their main constituents could be regarded as potential components in functional foods or incorporated into complementary Parkinson's disease therapies.

Response surface methodology for optimization of enzymatic acylation of (R)‐(‒)‐linalool application to essential oils

AbstractThe essential oils of Lavender (Lavandula angustifolia) and Tropical Basil (Ocimum basilicum) are characteristically high in both linalool and linalyl acetate esters, key constituents determining essential oil (EO) quality and activity. The enrichment of these products in linalyl acetate has a great interest in the industry. Herein, we report the production of linalyl acetate using the free lipase PPL as a catalyst in a solvent‐free medium. Parametric studies referring response surface methodology (RSM) were applied to optimize the conversion. The optimal experimental condition for (R)‐linalool acylation (temperature at 60°C, catalyst amount at 150 mg, volume of acetic anhydride at 1 mL, and reaction time at 5 days) was obtained from an experimental design analysis. Under these conditions, the enzymatic acylation was carried out for both pure linalool, Lavandula angustifolia and O. basilicum essential oils as substrates. 24.94% conversion of pure linalool to linalyl acetate was obtained; however, essential oils gave lower conversions (conv = 13%).

Thermal Analysis of the Essential Oil of Aniba rosaeodora Ducke by TGA and DSC

Thermal analysis comprises a group of techniques to measure a physical property of a sample subjected to controlled temperature programming, including thermogravimetry (TGA), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). The species Aniba duckei Kostermans, synonymous with Aniba rosaeodora Ducke, occurs in the Amazon and is also known as rosewood. Essential oils are complex oily aromatic liquids obtained by extraction from highly volatile aromatic plant materials as secondary metabolites. This research aimed to chemically and thermally characterize the essential oil of Aniba duckei. Gas chromatography was used for the quantification, by the external standard method, of the main component. The thermal study of the oil and the linalool standard was carried out using DSC and TG-DTG. The essential oil yield was 1.93% and the analysis by GC-MS confirmed the majority presence of linalool, with a concentration of 89.34% in the oil. The technical analysis revealed some similarity between the curves of pure linalool and essential oil, which is attributed to the high content of linalool in the oil. The differences between the boiling points and enthalpies of the linalool and oil pattern are justified by the presence of minor components, their concentrations in the oil and because they are substances of different polarities, molecular masses and intermolecular forces. The techniques proved to be efficient to characterize the oil. The results of the thermal analysis of the essential oil of the Aniba rosaeodora plant species are unprecedented, enabling the determination of linalool in by DSC.

Molecular Determinants of Carbocation Cyclisation in Bacterial Monoterpene Synthases.

Monoterpene synthases are often promiscuous enzymes, yielding product mixtures rather than pure compounds due to the nature of the branched reaction mechanism involving reactive carbocations. Two previously identified bacterial monoterpene synthases, a linalool synthase (bLinS) and a cineole synthase (bCinS), produce nearly pure linalool and cineole from geranyl diphosphate, respectively. We used a combined experimental and computational approach to identify critical residues involved in bacterial monoterpenoid synthesis. Phe77 is essential for bCinS activity, guiding the linear carbocation intermediate towards the formation of the cyclic α‐terpinyl intermediate; removal of the aromatic ring results in variants that produce acyclic products only. Computational chemistry confirmed the importance of Phe77 in carbocation stabilisation. Phe74, Phe78 and Phe179 are involved in maintaining the active site shape in bCinS without a specific role for the aromatic ring. Phe295 in bLinS, and the equivalent Ala301 in bCinS, are essential for linalool and cineole formation, respectively. Where Phe295 places steric constraints on the carbocation intermediates, Ala301 is essential for bCinS initial cyclisation and activity. Our multidisciplinary approach gives unique insights into how carefully placed amino acid residues in the active site can direct carbocations down specific paths, by placing steric constraints or offering stabilisation via cation‐π interactions.

The Preservation and Enantiomeric Selection of Linalool by Nanoencapsulation Using Cyclodextrins

Linalool, a volatile terpene alcohol, is responsible for a characteristic aroma in food, beverages, and cosmetics. However, linalool’s low aqueous solubility and high volatility limit the applications and shelf life of linalool-containing products. Nanoencapsulation using beta-cyclodextrin (BCD), methyl-beta-cyclodextrin (MBCD) and hydroxypropyl-beta-cyclodextrin (HPBCD) was studied to improve the aqueous solubility and stability of linalool. Linalool has two enantiomers with distinct flavors and odors which affect product quality. The enantiomeric selectivity of the cyclodextrins (CDs) toward racemic linalool standard was evaluated. A computational simulation was performed to predict the conformations and interactions of the inclusion complexes. The 1:1 host-guest ratio from the computer simulation was implemented in the experimental study. Phase solubility study shows an improvement in linalool aqueous solubility after being encapsulated by CDs. The encapsulation efficiencies of linalool/BCD, linalool/MBCD, and linalool/HPBCD inclusion complexes are 66.30%, 51.38% and 32.31%, respectively. Nanoencapsulation by CDs can preserve linalool in the form of inclusion complexes compared to its free form. The amount of remaining linalool in linalool/BCD, linalool/MBCD, and linalool/HPBCD inclusion complexes are 89.57%, 87.07%, and 74.86%, respectively which are considerably larger than that of pure linalool (42.30%). CDs also show the enantiomeric selectivity toward (R)-linalool as evident from (R)-linalool percentage of 54.53% in the inclusion complex.

Mechanistic Insights on Skin Sensitization to Linalool Hydroperoxides: EPR Evidence on Radical Intermediates Formation in Reconstructed Human Epidermis and 13C NMR Reactivity Studies with Thiol Residues

Linalool is one of the most commonly used fragrance terpenes in consumer products. While pure linalool is considered as non-allergenic because it has a very low skin sensitization potential, its autoxidation on air leads to allylic hydroperoxides that have been shown to be major skin sensitizers. These hydroperoxides have the potential to form antigens via radical mechanisms. In order to obtain in-depth insights of such reactivity, we first investigated the formation of free radicals derived from linalool hydroperoxides in situ in a model of human reconstructed epidermis by electron paramagnetic resonance combined with spin trapping. The formation of carbon- and oxygen-centered radical species derived from the hydroperoxides was especially evidenced in an epidermis model, mimicking human skin and thus closer to what may happen in vivo. To further investigate these results, we synthesized linalool hydroperoxides containing a 13C-substitution at positions precursor of carbon radicals to elucidate if one of these positions could react with cysteine, its thiol chemical function being one of the most labile groups prone to react through radical mechanisms. Reactions were followed by mono- and bidimensional 13C NMR. We validated that carbon radicals derived from allylic hydrogen abstraction by the initially formed alkoxyl radical and/or from its β-scission can alter directly the lateral chain of cysteine forming adducts via radical processes. Such results provide an original vision on the mechanisms likely involved in the reaction with thiol groups that might be present in the skin environment. Consequently, the present findings are a step ahead toward the understanding of protein binding processes to allergenic allylic hydroperoxides of linalool through the involvement of free radical species and thus of their sensitizing potential.

Michelia Essential Oil Inhalation Increases Fast Alpha Wave Activity

Essential oils are volatile fragrance liquids extracted from plants, and their compound annual growth rate is expected to expand to 8.6% from 2019 to 2025, according to Grand View Research. Essential oils have several domains of application, such as in the food and beverage industry, in cosmetics, as well as for medicinal use. In this study, Michelia alba essential oil was extracted from leaves and was rich in linalool components as found in lavender and jasmine oil. The effects of inhaling michelia oil on human brainwaves still need to be elucidated. Ten male and ten female subjects were recruited. Thirty-two-channel electroencephalography was recorded. The raw data were spectrally analyzed for slow alpha, fast alpha, low beta, mid beta, and high beta activities. The results demonstrate that michelia leaf oil could reduce the alertness level observed by beta wave decrease and fast alpha wave activity increase. The inhalation of pure linalool showed virtually identical responses as the michelia oil inhalation. In conclusion, the sedative effects of michelia oil inhalation might originate from its major linalool component.

Antibacterial and antibiofilm activities of linalool nanoemulsions against Salmonella Typhimurium

Linalool is the active constituent of essential oils of several plant species like coriander, rosewood and cinnamon, which is known to have strong antibacterial activity. However, its application in food systems is limited because of low water solubility, high volatility and low stability. The present study was undertaken to overcome such problems by encapsulating the linalool into nanoemulsions using Tween 80 and water using ultrasonic assisted emulsification. The stable nanoemulsions with a mean droplet diameter of 10.9 ± 0.1 nm was obtained at a 1:3 (v/v) ratio of linalool and Tween 80. The antibacterial and antibiofilm activities of the stable nanoemulsions were further investigated. The linalool nanoemulsions showed twofold enhanced antibacterial activity against Salmonella Typhimurium through their increased ability to disrupt cell membrane integrity. Moreover, compared to the pure linalool, the linalool nanoemulsions showed an 11.5% higher antibiofilm activity. Nanoemulsions also effectively reduced the S. Typhimurium biofilm on the surface of cut pineapple. Sensory analysis showed acceptance of fresh cut pineapple with linalool nanoemulsions treatment. Therefore, results of the present study indicated the potential use of linalool nanoemulsions as a natural antibacterial and antibiofilm agent against S. Typhimurium, which could find applications in the food industry.

Intraspecific variability of wild thyme (Thymus serpyllum L.) occurring in Poland

The purpose of the study was to assess the variability between 16 wild thyme populations introduced into ex situ conditions, in terms of morphological traits and chemical profiles. The observations were carried out on 2-year-old plants. Total content of essential oil and phenolic compounds (flavonoids, phenolic acids and tannins) was determined according to Polish Pharmacopoeia 6th. The composition of essential oil was carried out by GC/MS and GC/FID. Populations differed in respect of examined traits, i.e.: plant’s height (from 50 to 90 mm), plant diameter (from 21.3 to 53.0 cm), length of stems (from 8.1 to 20.5 cm). Fresh mass of herb per plant ranged from 36.0 to 307.0 g, while dry mass from 15.63 to 73.11 g (CV 55.0 and 47.5%, respectively). Total content of essential oil varied from 0.23 to 0.97%. Three chemotypes were distinguished within investigated populations: geranyl acetate + β-terpineol + β-myrcene, geranyl acetate + β-terpineol + borneol and pure linalool type. The total content of phenolic compounds ranged, as following: flavonoids (0.17–0.26%), tannins (0.57–1.32%), phenolic acids (0.28–0.61%). To sum up, investigated populations differed significantly in respect of developmental features as well as content and composition of identified compounds. Obtained results may be used in future works on selection and breeding of wild thyme, providing a high-quality raw material characterized by specific chemical composition and pharmacological activity.

The effect of background plant odours on the behavioural responses of Frankliniella occidentalis to attractive or repellent compounds in a Y‐tube olfactometer

AbstractThe western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), causes major losses in agricultural and horticultural crops worldwide. The volatile compounds methyl isonicotinate, p‐anisaldehyde, eugenol, and linalool are known as olfactory attractants, and salicylaldehyde is known as a repellent for F. occidentalis under clean‐air conditions in laboratory experiments. In the present study we assessed the responses of F. occidentalis to these compounds when presented alone, in combination, and in the presence of background odours emanating from cucumber (Cucumis sativus L., Cucurbitaceae), capsicum (Capiscum anuum L., Solanaceae), chrysanthemum (Chrysanthemum morifolium Ramat., Asteraceae), clove basil (Ocimum gratissimum L.), and lavender (Lavandula angustifolia L., both Lamiaceae) plants in a Y‐tube olfactometer. In the presence of any background plant odour, the attractiveness of pure methyl isonicotinate to F. occidentalis proved to be consistently significant. Compared to clean‐air conditions, a slightly lower percentage of thrips chose the Y‐tube arm loaded with 10% p‐anisaldehyde in the presence of cucumber leaf odour. With non‐flowering clove basil plants in the background, F. occidentalis responses to 1% eugenol, a constituent of clove basil essential oil, were neutral, and the same applied to responses to pure linalool, a constituent of lavender essential oil, in the presence of flowering lavender plants. Also, thrips responses to pure or diluted salicylaldehyde were clearly influenced by plant background odours. We simulated a push‐pull situation and found a trend indicating that the percentage of F. occidentalis choosing the airflow loaded with the attractant methyl isonicotinate was higher when the airflow in the other arm of the Y‐tube was loaded with the repellent salicylaldehyde compared to clean air, and vice versa. We showed interactions between attractive or repellent volatile compounds and the environmental odours in the chemical ecology of F. occidentalis and the potential of a combined use of these compounds in thrips pest management.

Chrysanthemum expressing a linalool synthase gene ‘smells good’, but ‘tastes bad’ to western flower thrips

Herbivore-induced plant volatiles are often involved in direct and indirect plant defence against herbivores. Linalool is a common floral scent and found to be released from leaves by many plants after herbivore attack. In this study, a linalool/nerolidol synthase, FaNES1, was overexpressed in the plastids of chrysanthemum plants (Chrysanthemum morifolium). The volatiles of FaNES1 chrysanthemum leaves were strongly dominated by linalool, but they also emitted small amount of the C11-homoterpene, (3E)-4,8-dimethyl-1,3,7-nonatriene, a derivative of nerolidol. Four nonvolatile linalool glycosides in methanolic extracts were found to be significantly increased in the leaves of FaNES1 plants compared to wild-type plants. They were putatively identified by LC-MS-MS as two linalool-malonyl-hexoses, a linalool-pentose-hexose and a glycoside of hydroxy-linalool. A leaf-disc dual-choice assay with western flower thrips (WFT, Frankliniella occidentalis) showed, initially during the first 15 min of WFT release, that FaNES1 plants were significantly preferred. This gradually reversed into significant preference for the control, however, at 20-28 h after WFT release. The initial preference was shown to be based on the linalool odour of FaNES1 plants by olfactory dual-choice assays using paper discs emitting pure linalool at similar rates as leaf discs. The reversal of preference into deterrence could be explained by the initial nonvolatile composition of the FaNES1 plants, as methanolic extracts were less preferred by WFT. Considering the common occurrence of linalool and its glycosides in plant tissues, it suggests that plants may balance attractive fragrance with 'poor taste' using the same precursor compound.

Antimicrobial activity of the essential oils from the leaves of two morphotypes of Croton cajucara Benth

Croton cajucara Benth. (Euphorbiaceae) is a shrub native to Amazon region and locally known as sacaca. Two morphotypes are known, namely white ‘sacaca’ and red ‘sacaca’. The essential oils from the leaves of these morphotypes are rich in linalool and 7-hydroxycalamenene, respectively. The effectiveness of the oils from forty individuals from a germplasm bank regarding the antimicrobial activity against Staphylococcus aureus and Candida albicans was investigated by drop diffusion (or agar diffusion) method. Expressive inhibition zones for both microorganisms tested were observed. Essential oils rich in 7-hydroxycalamenene were more effective against S. aureus while linalool-rich essential oils acted inhibiting C. albicans. The minimal inhibitory (MIC) and microbicidal concentrations of the main constituents of the oils were also determined, using broth dilution assay. It was observed that isolated 7-hydroxycalamenene presented high bactericidal activity, with a MIC of 0.3 μg/mL, while for fungicidal activity a MIC of 45 μg/mL was recorded. For S. aureus, the isolated 7-hydroxycalamenene was more active than the whole oil, whereas the linalool-rich essential oil presented higher bactericide activity than pure linalool. For C. albicans, however, pure linalool was more active than the whole essential oil.

Linalool – a significant contact sensitizer after air exposure

Linalool is a widely used fragrance terpene. Pure linalool is not allergenic or a very weak allergen, but autoxidizes on air exposure and the oxidation products can cause contact allergy. Oxidized (ox.) linalool has previously been patch tested at a concentration of 2.0% in petrolatum (pet.) in 1511 patients, and 1.3% positive patch test reactions were observed. To investigate the optimal patch test concentration for detection of contact allergy to ox. linalool. Four concentrations of ox. linalool (2.0%, 4.0%, 6.0%, 11.0% pet.) were tested in 3418 consecutive dermatitis patients. Ox. linalool 2.0%, 4.0%, 6.0%, and 11.0% pet. detected positive patch test reactions in 0.83%, 3.2%, 5.3%, and 7.2% of the tested patients, respectively. The doubtful reactions increased with rising concentrations but relatively less, giving 5.1%, 6.4%, and 7.3% doubtful reactions, respectively, for ox. linalool 4.0%, 6.0%, and 11.0% pet. Few irritative reactions were seen. Raising the patch test concentration for ox. linalool gave a better detection of contact allergy, as many as 5-7% positive patch test reactions were detected. We suggest a patch test concentration of ox. linalool 6.0% pet. for future patch testing, giving a dose per unit area of 2.4 mg/cm(2) when 20 mg test substance is tested in small Finn Chambers.

Enzymatic production of linalool esters in organic and solvent-free system

This work investigated the influence of temperature, enzyme concentration, substrates molar ratio, in the absence and presence of organic solvent, at two molar ratios of the substrates on the enzymatic production of linalil esters using the immobilized lipase Novozym 435 as catalyst, different acids and linalool and Ho-Sho essential oil as substrates. The best reaction conversion was obtained at the highest temperature (70 degrees C), for both solvent free (3.81%) and with solvent addition (2.25%), for a solvent to substrates molar ratio of 2:1, enzyme concentration of 5 wt% and acid to alcohol molar ratio of 1:1. The reaction kinetics revealed that Ho-Sho essential oil afforded the greatest conversions when compared with pure linalool. Higher linalil esters production were achieved after 10 h reaction (5.58%) in 2:1 solvent to substrates molar ratio, with enzyme concentration of 5 wt%, at 70 degrees C and anhydride to alcohol molar ratio of 1:1 using Ho-Sho essential oil as substrate.

Composition and antimicrobial activity of essential oils of Cinnamosma fragrans

Essential oil samples of Cinnamosma fragrans from two regions in Madagascar, Tsaramandroso (38 samples) and Mariarano (30 samples), were analysed by GC/MS. Fifty-seven components were identified, accounting from 88.3% to 99.4% of the oils’ composition. The major components were linalool (72.5 ± 23.3%) in Tsaramandroso and 1,8-cineole (47.3 ± 10.2%) in Mariarano. Samples B8 (95.8% linalool) from Tsaramandroso and B143 (71.6% 1,8-cineole) from Mariarano containing the highest proportions of the two main components identified, were selected to determine antimicrobial activities against 10 microbial strains. Bacillus subtilis and Staphylococcus aureus were the most sensitive strains to both oils. Minimum inhibitory concentration (MIC) values were lower for B143 against all tested Gram-negative strains than pure 1,8-cineole. B8 showed higher MIC values than pure linalool against Salmonella typhimurium and Vibrio alginolyticus, and similar MIC values to linalool towards the other Gram-negative strains. Both essential oils exhibited higher MIC values towards Fusarium oxysporum than their respective pure major component. These results suggested the occurrence of synergism or antagonism effects between the different oil constituents.

Contact Allergens Formed on Air Exposure of Linalool. Identification and Quantification of Primary and Secondary Oxidation Products and the Effect on Skin Sensitization

Linalool (3,7-dimethyl-1,6-octadien-3-ol) is an important fragrance chemical, frequently used in scented products because of its fresh, flowery odor. Linalool is an unsaturated hydrocarbon and is therefore susceptible to oxidation in the presence of air. The primary oxidation products, that is, hydroperoxides, formed in the autoxidation process, are reactive compounds that can be suspected to act as sensitizers. In the present investigation, we studied the autoxidation of linalool with emphasis on the formation of hydroperoxides. The oxidation products were isolated using flash chromatography and preparative HPLC and were identified with NMR and GC/MS, using synthesized reference compounds. Two hydroperoxides and several different secondary oxidation products were identified, among which some contain structural features that make them potential allergens. The amounts of linalool and the major oxidation products were quantified over time, using GC and an HPLC-method, suitable for the analysis of thermolabile primary oxidation products. The hydroperoxide 7-hydroperoxy-3,7-dimethylocta-1,5-diene-3-ol was found to be present in 15% in an oxidized sample. The local lymph node assay (LLNA) was used to investigate the sensitizing potential of pure linalool, two samples of air-exposed linalool, and oxidation products of linalool (an alpha,beta-unsaturated aldehyde, a mixture of two hydroperoxides, and an alcohol). Pure linalool showed no sensitizing potential. The air-exposed samples of linalool produced clearly positive responses, and the hydroperoxides were the strongest allergens of the tested oxidation products. The study demonstrates the importance of autoxidation on the sensitizing potential of linalool. We also conclude that the sensitizing potential differs with the composition of the oxidation mixture and thus with the air exposure time.

FS06.2 Autoxidation of linalool and impact on the sensitizing capacity and allergenicity

Linalool is one of the most frequently used fragrance chemicals in scented products and a large population is exposed to it. It is therefore important to study the allergenic properties of linalool, and the effect of autoxidation.Objectives: To study the autoxidation of linalool and identify formed oxidation products, to investigate the impact of autoxidation on the sensitizing capacity and to study the frequency of contact allergy to oxidized linalool among consecutive dermatitis patients.Methods: Linalool was air‐exposed and the degradation followed with GC. Oxidation products were identified with GC‐MS and NMR. Pure linalool, 2 different states of oxidized linalool and 3 oxidation products were tested for their sensitizing capacity in the local lymph node assay (LLNA). Consecutive dermatitis patients were patch‐tested with oxidized linalool and a fraction of oxidized linalool.Results: Linalool started to decompose immediately when air‐exposed. Several oxidation products were identified among which 3 (2 hydroperoxides and an alpha, beta‐unsaturated aldehyde) contain structural features that make them potential allergens. LLNA showed that the 3 oxidation products were moderate allergens, and that the sensitizing potential of linalool increased with longer air‐exposure times. In the patch‐test study positive reactions were observed to oxidized linalool in 1.65% of the patients.Conclusions: The extensively used fragrance chemical linalool is not allergenic in itself. The autoxidation process that takes place at air‐exposure leads to the formation of sensitizing oxidation products. The frequency of consecutive patients reacting to oxidized linalool shows that the experimental findings are clinically important.

Antimicrobial activity of essential oils isolated from Portuguese endemic species of Thymus.

Thymus species are wild species mostly found in the arid lands of Portugal. Possible antimicrobial properties of Thymus essential oils have been investigated. The chemical composition of the essential oils and the antimicrobial activity of Thymus mastichina (L) L. subsp. mastichina, T. camphoratus and T. lotocephalus from different regions of Portugal were analysed. Hydrodistillation was used to isolate the essential oils and the chemical analyses were performed by gas chromatography (GC) and GC coupled to mass spectrometry. The antimicrobial activity was tested by the disc agar diffusion technique against Candida albicans, Escherichia coli, Listeria monocytogenes, Proteus mirabilis, Salmonella spp. and Staphylococcus aureus. Pure linalool, 1,8-cineole and a mixture (1 : 1) of these compounds were included. Linalool, 1,8-cineole or linalool/1,8-cineole and linalool/1,8-cineole/linalyl acetate were the major components of the essential oils, depending on the species or sampling place. The essential oils isolated from the Thymus species studied demonstrated antimicrobial activity but the micro-organisms tested had significantly different sensitivities. The antimicrobial activity of essential oils may be related to more than one component. Portuguese endemic species of Thymus can be used for essential oil production for food spoilage control, cosmetics and pharmaceutical use. Further studies will be required to elucidate the cell targets of the essential oil components.

Asymmetrical generalisation between pheromonal and floral odours in appetitive olfactory conditioning of the honey bee (Apis mellifera L.).

The capacity to generalise between similar but not identical olfactory stimuli is crucial for honey bees, allowing them to find rewarding food sources with varying volatile emissions. We studied bees' generalisation behaviour with odours having different biological values: typical floral odours or alarm compounds. Bees' behavioural and peripheral electrophysiological responses were investigated using a combined proboscis extension response conditioning-electroantennogram assay. Bees were conditioned to pure linalool (floral) or to pure isoamyl acetate (alarm) and were tested with different concentrations of both compounds. Electrophysiological responses were not influenced by conditioning, suggesting that the learning of individual compounds does not rely on modulations of peripheral sensitivity. Behaviourally, generalisation responses of bees conditioned to the alarm compound were much higher than those of bees conditioned to the floral odour. We further demonstrated such asymmetrical generalisation between alarm and floral odours by using differential conditioning procedures. Conditioning to alarm compounds (isoamyl acetate or 2-heptanone) consistently induced more generalisation than conditioning to floral compounds (linalool or phenylacetaldehyde). Interestingly, generalisation between the two alarm compounds, which are otherwise chemically different, was extremely high. These results are discussed in relation to the neural representation of compounds with different biological significance for bees.

The synergistic preservative effects of the essential oils of sweet basil (Ocimum basilicum L.) against acid-tolerant food microflora.

Essential oils extracted by hydrodistillation from five different varieties of Ocimum basilicum L. plants (Anise, Bush, Cinnamon, Dark Opal and a commercial sample of dried basil) were examined for antimicrobial activity against a wide range of foodborne Gram-positive and -negative bacteria, yeasts and moulds by an agar well diffusion method. All five essential oils of basil showed antimicrobial activity against most of the organisms tested with the exception of Flavimonas oryzihabitans and Pseudomonas species. The inhibitory effect of Anise oil, in comparison with mixtures of the predominant components of pure linalool and methyl chavicol, against the acid-tolerant organisms, Lactobacillus curvatus and Saccharomyces cerevisiae, was examined in broth by an indirect impedance method. Synergistic effects between Anise oil, low pH (pH 4.2) and salt (5% NaCl) were determined. The antimicrobial effect of Anise oil was also assessed in a tomato juice medium by direct viable count, showing that the growth of Lact. curvatus and S. cerevisiae was completely inhibited by 0.1% and 1% Anise oil, respectively. The results of the current study indicate the need for further investigations to understand the antimicrobial effects of basil oils in the presence of other food ingredients and preservation parameters.