Citrus paradisi - Rutaceae - grapefruit, Grapefrucht, Pampelmuse

Evergreen tree, usually 5-6(-15)m high; hybrid of Citrus × sinensis and Citrus maxima; widely cultivated in tropics and subtropics; leaves dark green, elliptic-ovate, midrib and veins glabrous, up to 15cm long; flowers fragrant; fruit 10-15 cm in diameter, peel greenish or yellow-orange; pulp mildly bitter.

Maybe created by spontaneous crossing of pamplemousse and orange in Barbados in the mid-18th century.
[Friedrich Zeller: Herkunft, Diversität und Züchtung der Banane und kultivierter Zitrusarten, Januar 2005.] http://www.uni-kassel.de/upress/online/frei/978-3-89958-116-4.volltext.frei.pdf

Naringin is the major flavonoid in grapefruit and gives the grapefruit juice its bitter taste. It is metabolized to the cytochrome P450 inhibiting flavanone naringenin in the human body. https://en.wikipedia.org/wiki/Naringin

„By application of the aroma extract dilution analysis on an extract prepared from fresh grapefruit juice, 37 odor-active compounds were detected in the flavor dilution (FD) factor range of 4-256 and subsequently identified. Among them the highest odor activities (FD factors) were determined for ethyl butanoate, 1-p-menthene-8-thiol, (Z)-3-hexenal, 4,5-epoxy-(E)-2-decenal, 4-mercapto-4-methylpentane-2-one, 1-heptene-3-one, and wine lactone. Besides the 5 last mentioned compounds, a total of 13 further odorants were identified for the first time as flavor constituents of grapefruit. The data confirmed results of the literature on the significant contribution of 1-p-menthene-8-thiol in grapefruit aroma but clearly showed that a certain number of further odorants are necessary to elicit the typical grapefruit flavor.“
[Characterization of the Most Odor-Active Volatiles in Fresh, Hand-Squeezed Juice of Grapefruit (Citrus paradisi Macfayden), Andrea Buettner and Peter Schieberle, J. Agric. Food Chem., 47 (12), 1999, 5189-5193]

Of thirty-eight aroma-active compounds detected in commercial cold-pressed grapefruit oil using GC-O and GC-MS (DB-Wax, DB-1), compounds with the greatest aroma intensities included 1,8-cineole, octanal, dodecanal, trans-4,5-epoxy-(E)-2-decenal, β-sinensal and nootkatone. „The aroma activities of sulphur compounds such as 4-mercapto-4-methyl-2-pentanol and methional were surprisingly low and neither of the two putative grapefruit aroma impact compounds, 4-mercapto-4-methyl-2-pentanone and 1-p-menthene-8-thiol, was detected.“
[Characterization of aroma‐impact compounds in cold‐pressed grapefruit oil using time–intensity GC-olfactometry and GC-MS., Lin, J., Rouseff, R.L., Flavour and Fragrance Journal, 16(6), 2001, 457-463]

„Twenty-five odor-active compounds were quantified in the fresh, hand-squeezed juice of White Marsh seedless grapefruits using stable isotope dilution assays. By calculation of the odor activity values of the odorants (ratio of their concentrations in the juice to their odor thresholds in water) it was shown that the fruity esters ethyl 2-methylpropanoate, ethyl butanoate, and (S)-ethyl 2-methylbutanoate, and the fruity, sweet winelactone, as well as the grassy smelling (Z)-hex-3-enal, and trans-4,5-epoxy-(E)-dec-2-enal with metallic odor, were among the most potent odorants of the fresh grapefruit juice. The typical sulfurous, grapefruit-like odor quality was mainly due to the catty, blackcurrant-like 4-mercapto-4-methylpentan-2-one [4MMP; 0.8 mg/kg fruit; OAV 8000] and the grapefruit-like smelling 1-p-menthene-8-thiol. These findings were confirmed by reconstitution experiments to simulate the aroma of the fresh grapefruit juice.“
4MMP produced a more significant difference to the panelists when omitted from the grapefruit recombinant and provided a more authentic, true-to-fruit aroma compared to 1-p-menthene-8-thiol.
[Evaluation of key aroma compounds in hand-squeezed grapefruit juice (Citrus paradisi Macfayden) by quantitation and flavor reconstitution experiments., Buettner, A., Schieberle, P., Journal of Agricultural and Food Chemistry, 49(3), 2001, 1358-1363]

„Forty-one aroma-active compounds were observed in unpasteurized single strength juice, whereas 27 components were found in the unflavored reconstituted concentrate. Aroma-active compounds were classified into grapefruit/sulfury, sweet/fruity, fresh/citrusy, green/fatty/metallic, and cooked/meaty groups. Five of six components in the sweet/fruity and 14 of 18 green/fatty/metallic components survived thermal concentration. However, only 4-mercapto-4-methyl-2-pentanone in the grapefruit/sulfury group, and linalool and nootkatone from the fresh/citrusy group, were found in the reconstituted concentrate. Methional was the only aroma compound in the cooked/meaty category found in both juice types. β-Damascenone and 1-p-menthen-8-thiol were found only in the reconstituted concentrate. 4-Mercapto-4-methyl-2-pentanol was found for the first time in grapefruit juice.“
[Lin, Jianming, et al. „Aroma composition changes in early season grapefruit juice produced from thermal concentration.“ Journal of Agricultural and Food Chemistry 50.4 (2002): 813-819]

„It is important to note in the grapefruit example that the naturally occuring (+)-nootkatone has a threshold of 1ppb, whreas the (-)-enantiomer has a threshold of 1ppm. Hence, any addition of racemic nootkatone can drastically reduce the odour character of a grapefruit oil.“
[Swift, Karl AD, ed. Advances in flavours and fragrances: From the sensation to the synthesis. Royal Society of Chemistry, 2007, 80]

„The chemical composition of the essential oil (EO) obtained by solvent-free microwave extraction (SFME) and hydrodistillation (HD) from the peel of grapefruit (Citrus Paradisi. L) was analysed by gas chromatography/mass spectrometry (GC/MS). Totally, twenty-five components were identified in the EO. Limonene was observed as dominant (91.5-88.6%) for two extraction methods, SFME and HD, respectively. β-Pinene (0.8-1.2%), linalool (1.1-0.7%), α-terpinene (0.7-1.0%) and the other minor components were also detected. Disc diffusion method was applied to determine the antibacterial properties of the EO. The results showed that the EO of grapefruit peel had a wide spectrum of antimicrobial activities against Staphylococcus aureus, Enterococcus faecalis, Staphylococcus epidermidis, Escherichia coli, Salmonella typhimurium, Serratia marcescens and Proteus vulgaris, with their inhibition zones ranging from 11 to 53 mm.“
[Essential oil composition and antibacterial activity of the grapefruit (Citrus Paradisi. L) peel essential oils obtained by solvent‐free microwave extraction: comparison with hydrodistillation. Uysal, B., Sozmen, F., Aktas, O., Oksal, B. S., Kose, E. O., International Journal of Food Science & Technology, Vol.46(7), 2011, 1455-1461]

„The sesquiterpene ketone (+)-nootkatone, which possesses a citrus-like character and a bitter taste, also occurs as a typical constituent in almost all peel oils [of citrus fruits, like lemon peel 0.06%, grapefruit peel 0.5-1.8%, but grapefruit juice oil 3.3% (!)]. Due to its low odor treshold of 800μg/l water, this bicyclic sesquiterpene contributes much to the overall character of an essential oil even at concentrations below 0.001% (10.000ppb). Its enantiomer (-)-nootkatone lacks both the citrus character and the low treshold… The characteristic fresh fruity top-note of grapefruit juice is mainly due to p-menth-1-ene-8-thiol, which is present only in concentrations below 1ppb. [This] Compound has an odor treshold of 0.1ng/l water, … one of the lowest odor tresholds of all naturall aroma chemicals.“
[Scent and Chemistry, Günther Ohloff, Wilhelm Pickenhagen, Philip Kraft, Wiley-VCH, 2012, 222ff]

„Aroma extract dilution analyses of the aromas of peels and juices of white and pink grapefruits revealed that rotundone, responsible for peppery, spicy, and woody odors, was detected for the first time at high flavor dilution factors of 256-1024. In both juices, rotundone was detected at the highest flavor dilution factor of 1024. Rotundone in grapefruits was quantitated by a stable isotope dilution assay with a newly synthesized deuterium-labeled internal standard, rotundone-d2,3: its levels were 2180 and 1920 ng/kg in white and pink grapefruit peels and 29.6 and 49.8 ng/kg in white and pink grapefruit juices, respectively. On the basis of these results, sensory analysis was performed to assess the effects of rotundone on a white grapefruit juice aroma reconstitute. This sensory analysis revealed that rotundone does not impart a woody odor or affect any of the existing attributes, but increases various attributes, thus confirming that rotundone is indispensable for the aroma of grapefruit juice.“
[Nakanishi, Akira, et al. „Quantitation of rotundone in grapefruit (Citrus paradisi) peel and juice by stable isotope dilution assay.“ Journal of agricultural and food chemistry 65.24 (2017): 5026-5033]

Citrus paradisi photo by Jeff Vanuga, USDA Natural Resources Conservation Service, PD (CC0)
https://commons.wikimedia.org/wiki/File:NRCSAZ02039_-_Arizona_(361)(NRCS_Photo_Gallery).tif