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Title: Volatile compounds and Some properties of soy yoghurt fermented by lactic acid bacteria
Author: Thabloga, Wannapa
ISNI:       0000 0001 3516 4716
Current Institution: University of Reading
Date of Award: 2005
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Soybean milk is a nutritional food consisting high protein content, unsaturated fatty acids and no cholesterol. However, soymilk and soy products are commonly characterised by a beany off-flavour leading to low-consumer acceptability in Western cultures. The aims of the cUJ:rent study are to reduce the main off-flavour, such as hexanal, by preparing soymilk freshly with control processing steps and investigating lactic acid bacteria to explor~the reduction of hexanal and enhance desirable volatile compounds in soymilk. Soybeans soaked in NaHC03 had an appearance and colour of soymilk better than the unsoaked soybeans. In addition, soymilk from soybeans soaked in purified water, dehulled soybeans, and boiled in NaHC03for 10 min, produced good qualities in total solids, and whiter than soymilk boiled for 30 min. The chosen procedure for the production of the soymilk, a blended (for 2 min) soybean milk was mixed by mixer emulsifier at 8000 rev min-I for 5 min, homogenized at 200 kg sq cm-I and finally separated the milk with centrifugation at 169 x g for 5 min. This soymilk was rich in total solids content (77.8 g kg-I). Soy yogl:mrt has a weak viscoelastic behaviour. The optimised ratio of soybeans: water (1 : 5) was used to prepare soy yoghurt. The aroma profiles ofsoymilk were identified by GC-MS analysis. The main volatile compound in soymilk was hexanal. Other volatiles present in high amounts were 1-octen-3-01, 1-hexanol, pentanal, 2-pentyl furan and 1-pentanol. Selected Strep. thermophilus and Lact. casei mixture produced acetaldehyde, diacetyl andacetoin and also reduced hexanal. The survival in soy yoghurt ofLact. casei was better than that ofLact. delbrueckii ssp. bulgaricus during 48h of incubation. Strep. thermophilus preferred to utIlIze sucrose to glucose whIle Lact. casei utilized glucose. Strep. thermophilus and Lact. casei did not reduce stachyose and raffinose. Strep. thermophilus and Lact. casei in single or mixture culture reduced hexanal to hexanol, and the hexanol was further reduced during the 24 h of the fermentation. The reduction ofhexanal to hexanol in soy yoghurt is desirable because hexanol has a weaker flavour than hexanal. Soy yoghurt fermented by mixed cultures ofStrep. thermophilus and Lact. casei had characteristics of yoghurt with a good curd and a pH of4.5-5.6. Strep. f' thermophilus grew rapidly during the first 6 h, and then grew slowly during the next 18 h of incubation. Lact. casei grew continually during 24h of incubation. Soy yoghurt fennented by mixed cultures contained desirable aroma compounds, including acetaldehyde, diacetyl, 2,3-pentanedione and acetoin. The soy yoghurts from soymilks with added glucose or sucrose had increased levels ofpyruvate, 2,3- pentanedione and diacetyl, suggesting that sugars contribute to the production of these products. The amino acids, methionine and threonine, did not contribute to the production of acetaldehyde in yoghurt but promoted ethanol fonnation. The microstructure of soy yoghurt was studied by scanning electron microscope. Soy yoghurt contained a gel network with branched chains and Strep. thermophilus and Lact. casei were found in the spaces or embedded in the netWork.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available