Unique volatile compounds may help generate sweetness in Borneo oranges.

The Pontianak orange--Citrus nobilis var. microcarpa--is the sweet-flavored orange originally cultivated on the Borneo island of Indonesia. This cultivar is well known by Indonesians for its distinct flavor and sweetness.

Scientists at the National University of Singapore characterized the volatile compounds of Pontianak oranges. The flavor compounds of freshly-squeezed Pontianak orange juice were extracted using headspace solid phase micro-extraction (SPME) and solvent extraction methods with diethyl ether. This was done prior to separation and identification with gas chromatograph-mass spectrometry (GC-MS). The volatile compounds of hand-squeezed Pontianak orange peel oil were also identified after direct injection with GC-MS.

The researchers detected and identified around 70 flavor compounds in Pontianak orange juices and peel oils. These include terpenes, carbonyls, alcohols and esters as well as hydrocarbons. The peel oil displayed a distinct profile of volatile compounds from that of the juice. This was illustrated by the detection of some volatile compounds only in the juice.

As in other citrus fruits, the researchers found limonene to be the major compound in the Pontianak orange. Other characteristic compounds of citrus fruits, such as myrcene, alpha-pinene, linalool, perilla aldehyde, citral, ethyl butyrate and alpha-terpineol, were also detected in Pontianak oranges at various concentrations.

However, some compounds not commonly found in citrus fruits were identified in Pontianak orange juice. These include 4-ethenyl cyclohexene, diethyl carbonate, dibutyl sulfide and 2,6-dimethyl naphthalene. These may possibly be the compounds that help create the sweetness and distinct flavor of Pontianak oranges.

Further information. Jorry Dharmawan, Food Science and Technology Program, National University of Singapore, Block S3, Level 6, Science Drive 4, Singapore 117543, Singapore; phone: +65 6516 6666; email: jorry@nus.edu.sg.