Abstract
Stored shredded onions showed severe browning when they were steamed or boiled during further processing. The effects of storage temperatures and bacterial infection on the browning of heated shredded onion were investigated.
1. Freshly shredded onions did not discolor immediately after being steamed or boiled, whereas samples stored at 20°, 8°, 1°Crequired 2, 5 to 7, and 14 to 19 days, respectively to turn equally brown after heat-treatment.
2. The evolution of characteristic aroma of onions, dipropyl-disulphide, decreased quickly within 12 days at all storage temperatures, whereas hydrogen sulfide increased sharply when shredded onions showed the heat browning.
3. Phenolic compounds increased at the beginning of storage at all temperatures. Concentrations of free amino acids decreased as the discoloration intensified during storage at 8° and 20°C; they did not change at 1°C. Glucose, fructose, and sucrose contents increased slightly during the initial storage period, but they decreased as the tissues turned brown. Carbonyl compounds reacted with 2, 4-dinitrophenylhydrazine increased in the raw shredded onions during storage at 20 °C. More the compounds reacted in steamed onion than in the raw ones.
4. The bacterial count on the surface of shredded onions reached 107 per gram, concurrent with browning at all storage temperatures. The growth of microbial colonies and browning after heat treatment on shredded onions pretreated 70% ethanol were retarded 2 to 3 days compared to those on non-treated onion samples kept at 20 °C.
The juice extracted from highly infected shredded onions became highly discolored, whereas extracts from bacterial culture media turned just slightly brown. These observations suggest that browning of steamed or boiled shredded onions is initiated by bacterial infection during storage, and also suggest the browning pigments do not come from bacteria but from infected shredded onion.
