Whole milks were adjusted to pH 5.8, 6.2, or 6. 7 with HCl and batch pasteurized at 63°C for 30 min. Each was concentrated 5:1 (40% total solids) through a single tube polysulfone membrane Abcor ultrafiltration unit. Lactose (L), casein hydrolysate (CH), and one of two brands of yeast extract (YE1, YE2) were added into cooled retentates at 0.1, 0.3, 0.5, 0. 7 or 0.9% and equilibrated overnight at 4°C. Five percent proteinase positive (Prt+) Streptococcus cremoris UC 310+ (v/w) milk based culture was added. Unfortified retentate was also inoculated with 0.1, 0.3, 0.5, 0. 7 or 0.9% starter and pH readings were taken on all samples for 24 h during incubation at 23°C. Similar substrates were inoculated with proteinase negative (Prt-) S. cremoris UC 310-.
Lactose had no significant effect on acid production. Casein hydrolysate had a slight positive effect. Yeast extract had a significant effect at all preacidification levels and a significant difference was also noticed between the brands. Mean times required for the proteinase positive culture to reach pH 5.1 in 5x retentate from milk acidified to pH 5.8 were 24, 12, 10, 10, and 24 h for L, CH, YE1, YE2, and the control respectively. Proteinase negative variants of this strain had mean times of >24 h, 14 h, 11 h, 11 h, and >24 h respectively. These time differences were significantly different between Prt+ and Prt- variants. A minimum concentration of 0.2% yeast extract produced the most stimulation while greater quantities provided no additional benefit. Taste panelists were unable to detect yeast extract in retentates fermented by either culture variant.
| Identifer | oai:union.ndltd.org:UTAHS/oai:digitalcommons.usu.edu:etd-6407 |
| Date | 01 May 1987 |
| Creators | Pope, Brent Karl |
| Publisher | DigitalCommons@USU |
| Source Sets | Utah State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | All Graduate Theses and Dissertations |
| Rights | Copyright for this work is held by the author. Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user. For more information contact digitalcommons@usu.edu. |
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