Something such as prosaic as the smell of freshly squeezed orange juice turns out to be more complicated than most can imagine. In grapefruits, a single compound called 1-p-methene-8-thiol is highly characteristic of the juice’s smell. In lemons, two versions of citral (geranial and citral), differing only in the position of groups across an untwistable double bond, are key. Geranial has a stronger citrus odor than neral, but the latter is sweeter. But when it comes to orange juice, no single volatile compound in orange juice can take credit for being its character-impact compound, and that’s just the beginning of a complicated but beautiful story revealing why freshly squeezed orange juice is unique.
What we know about the smells of fruits and other foods comes in part from a technique known as Gas-Chromatography-Olfactometry, which combines a classic lab instrument, that separates and identifies constituents, with a biological detector known as the human nose and perception, which tries to assess the relative intensity, character and smell-threshold of successively diluted components. Out of a total of 300 volatile compounds, perhaps two dozen compounds contribute to the aroma of freshly squeezed orange juice. Before we analyze the volatiles by chemical class to appreciate the aroma of freshly squeezed orange juice, there are three startling things to note about the concentration of these volatile compounds: (1) only three occur at a concentration exceeding 1 ppm ( or 1 microgram per milliliter), with that of limonene being the highest at about 70 ppm, (2) The 2nd most concentrated volatile is the poisonous methanol. But at 60 ppm, it is only 2/3 to 1/5th of the concentration found in wine, amounts that are still too small to harm us. (3) There is no consensus on what combination and concentration of key volatiles is needed to reproduce the smell of freshly squeezed orange juice . Some attempts have been made to combine key ingredients in various ratios, but all prepared mixtures fail to smell like the real McCoy.
The first variable that influences smell surprisingly comes from the way an orange is squeezed. How intensely it’s done by hand or if a blender is used will affect the smell. That’s because many of the odorous compounds originate from the oils in the fruit’s peel. Mechanical blending of the entire orange extracts more of those volatiles, from 3 to 10 times as many as from hand- squeezing.
Pulp is also a source of important smells. But it’s often removed by commercial producers to extend shelf life or to please finicky consumers. One group of compounds known as esters are made by plants from acids and alcohols. As the fruit ripens, esters are increasingly synthesized in the presence of the enzyme, Acyl-CoA. But, when orange cells are mashed during juicing, we lose esters through the action of hydrolase enzymes, one of the reasons the freshness of orange juice is ephemeral. Examples of orange esters include the fruity ethyl acetate and the intense and important pineapple-like ethyl butanoate (a.k.a. ethyl butyrate).
The other reason fresh orange juice is unrivalled is that microorganisms reproduce more quickly once an orange is crushed. To preserve juice, companies use pasteurization in which juices are flash-heated and then rapidly cooled to inactivate degradative enzymes and to slow the rate of other chemical reactions. The problem is that heat affects a sensitive group of compounds that are at least as important as esters in producing the characteristic smell of orange juice. That group is known as the aldehydes.
Aldehydes consist of a carbonyl group (C=O) sandwiched between a hydrocarbon group and a hydrogen. This places the molecule in a low oxidation state. Which specific aldehydes make a desirable contribution to the smell of orange juice and which concentration-range is optimal are not clearly understood. Orange juice has four different kinds of aldehydes that we can smell: saturated aliphatic aldehydes, unsaturated aliphatic ones, terpenic ones, and phenolic aldehydes. The 1st two are almost all from oils in the peel, adding and citrusy and green notes; while unsaturated aliphatic aldehydes contribute green, fatty and metallic aromas. The fourth group consists of the lemon’s pair of citral versions described earlier. Terpenic aldehydes also include the woody smell of β-Pinene.
In addition to esters and aldehydes, we have the alcohol-components of orange juice. An alcohol is an organic compound sandwiching an oxygen between a hydrogen and a hydrocarbon group. Chemically, an alcohol is more oxidized than an aldehyde but not as oxidized as an organic acid like vinegar. The most potent of the smells among alcohols in orange juices is linalool, which has a sweet floral note. Commercial orange juice, which includes the entire orange peel, ends up with an elevated concentration of this alcohol. But the commercial product, which also includes extraneous compounds formed in reactions with packaging material, has less of the the most important aliphatic alcohols in freshly squeezed orange juice—woody notes of (E)-hexen-2-ol and (Z)-hexen-3-ol, and less of the mushroom odor-compound 1-octen-3-ol.
Finally we will look at an essential compound from the class of terpenes, specifically d-limonene. Originating from the rind, when isolated d-limonene still smells like orange peel. But its mirror-image molecule and smaller peel-component, l-limonene, smells like turpentine. d-limonene has long been an additive in medicines and food, but what’s more interesting is that it’s being increasingly used as a green solvent. It’s not as strong as a cleaning fluid as dichloromethane or toluene(see KB values in table), but it’s better than hexane and unlike its “conventional” competitors, it’s non-toxic, non-flammable and far better-smelling. When mixed with surfactants, limonene can be conveniently formulated with surfactants to clean concrete, boats, inks and glues. A limonene-based cleaning agent ( PES-51) was used to decontaminate equipment following the Deepwater Horizon oil spill disaster in the Gulf of Mexico in 2011.
All we have learned gives us strong motivation not to waste orange peels. They not only contribute to the the aroma of juice, but they are one more step towards a green economy.
Rosaria Ciriminna. Limonene: a versatile chemical of the bioeconomy. Chemical Communications. Volume 50. December 18, 2014
Pilar Ruiz Perez-Cacho & Russell L. Rouseff (2008) Fresh Squeezed Orange
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