Part of the common lore of the jasmine flower is that it exudes a much stronger fragrance at night. While for centuries this difference in aroma from morning to night has been largely anecdotal, recent scientific studies have been conducted that show a true, measurable difference in the chemical composition of jasmine oil extracted from flowers harvested early in the morning, compared to early evening.
A study by Ahmad, Malek, Gan, Abdullah, and Rahman in 1998 discovered that jasmine concrete extracted from flower petals (Jasminum multiflorum L.) at 8:00am had higher levels of the aromatic compounds indole, cis-jasmone, benzyl alcohol, linalool, and benzyl acetate according to gas chromatography/mass spectroscopy. Interestingly, the evening harvested petals (at 6:00pm) produced a concrete with more prominent levels of eugenol, benzyl benzoate, farnesol, methyl palmitate, and methyl salicylate. The authors note that while both concretes had disparate chemical compositions, petals from both time periods created the same quantity of concrete. Based on these differences, the authors clearly suggest that the morning-picked jasmine petals produced a superior jasmine concrete, as the latter was lacking the prominent indole fragrance so often associated with the jasmine flower. (source)
Thus, the delicate jasmine petals are typically plucked from their vine or shrub in the late evening to early morning, depending on the species, before the flower bud has had a chance to open completely. For example, flower blossoms from Jasminum grandiflorum open in the early dawn and must be harvested early in the morning in order for the full potential of the fragrance to be realized. (source)
The movements of the harvester are deliberate so as not to damage or bruise the delicate flower petals, which changes the aroma profile and introduces off-notes into the eventual jasmine oil. In an effort to preserve the flower bud, harvesters typically use both of their hands and pick flowers one by one to ensure none of the thin petals fall off. Even with these constraints, an experienced harvester of jasmine blossoms can accrue an average of 10,000-15,000 flower petals per morning/night. Jasmine harvesters are paid by weight, so every flower picked counts!
It is imperative that the jasmine flowers be processed as quickly as possible into the concrete/pomade stage in order to preserve the fragrance of the flower. (Arctander, S. (1960) Perfume and Flavor Materials of Natural Origin. Artander) Harvests are typically brought to the processing plant hourly from the plantations, often by carts pulled by donkeys, mules, or horses. However, if the petals will not be used immediately for the processing of jasmine oil, the flower buds are stored in a cool place until the evening when they open and begin to release their incredible fragrance once again.
Depending on the region, harvesting of jasmine flowers typically occurs between March and November, with highest peaks taking place between May-August. One hectare tends to yield 10 tonnes of jasmine flowers. (Sachan, S. & Paarakh, P. (2009). Jasminum grandiflorum Linn (Chameli): Ethnobotany, Phytochemistry and
Pharmacology – A review. Pharmacologyonline, 2, 585-595)