Valaris

The plant

Bread flower is the common name of Vallaris glabra, an evergreen, woody climber of the family Apocynaceae. It grows up to about 2 to 3 m tall. Its flowers are borne in clusters near the tips of the branches and are white and cup-shaped. They open in the morning, but their fragrance is strongest in the evening. The plant is popular for landscaping in Thailand and other countries in Southeast Asia. It can be trained to drape over a trellis or pergola, or pruned into a bush. It attracts butterflies as a nectar plant.

Range and cultivation

The species is native to Sumatra, Java and the Lesser Sunda Islands. In Singapore it is a non-native plant. Each flower is about 1.0 to 1.5 cm across and has five pointed, slightly overlapping petals. The fruit splits into two dry follicles. The plant flowers best in full sun, since shade inhibits flowering, and it can be propagated by grafting, air-layering or stem cuttings.

Aroma chemistry

2-Acetyl-1-pyrroline gives freshly baked bread, jasmine rice, basmati rice, the herb pandan and bread flowers their customary smell. It is a substituted pyrroline, a cyclic imine and a ketone. Its odour threshold is low, 0.2 ng/L, so little of it is needed for the scent to be detectable.

2-Acetyl-1-pyrroline is the most significant aroma compound in the crust of wheat bread. A higher yeast content in bread has been shown to lead to higher levels of 2-acetyl-1-pyrroline.

The compound can form through Maillard reactions when food is heated, for example when bread dough is baked. In wheat bread it is thought to arise from the reaction of 1-pyrroline, a Strecker degradation product of proline, with methylglyoxal, a reactive carbonyl species that comes from reducing sugars.