Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

Genevieve L. Christen

Committee Members

Marjorie P. Penfield, David A. Golden, John R. Mount

Abstract

Research was conducted in four phases: In Phase I, activity of four lipases at four incubation temperatures over a 60-min incubation time was evaluated; In Phase II, the same four lipases were evaluated for 60 min at an apparent optimal incubation temperature as determined in Phase I; In Phase III, milk, prepared from skim milk and butter oil hydrolyzed with the lipases, was evaluated for lipolyzed flavor by a dairy flavor expert; In Phase IV, the lipase identified in Phase III that gave the most consistent and characteristic lipolyzed flavor to butter oil was used to make recombined milk. This milk was then compared by a sensory evaluation panel to lipolyzed milk prepared using two other methods.

In Phases I and II, butter oil was hydrolyzed with four lipases at four incubation temperatures and analyzed using two methods: extraction-titration and gas chromatography (GC). The temperature needed to achieve a specific level of free fatty acids (FFA) was evaluated for the four lipases. After evaluating all lipases at 35, 40, 45 and 50°C, one apparently optimal incubation temperature was chosen for each lipase. These were 45 °C for porcine pancreas, 35 °C for P. roqueforti, 45 °C for A. niger and 50°C for R. miehei. A general trend was evident for all lipases: as time increased, cumulative FFA released in butter oil by the lipases increased. The amount of activity varied with lipase and temperature. Pancreatic lipase activity was found to be reproducible at 45 and 50 °C in one phase, but not in the other; R. miehei activity was found reproducible at 50°C only in both phases; and activity for P. roqueforti and A. niger was not reproducible at any temperature. In GC analysis, pancreatic lipase and P. roqueforti were found to release more short-chain fatty acids (C4 - C12) (P < 0.05), while A. niger and R. miehei released more long-chain fatty acids (C14 - C18:3).

In Phases III and IV, recombined, 2% milk was prepared by combining skim milk and butter oil that had been hydrolyzed with lipases from porcine pancreas, P. roqueforti, A. niger and R. miehei. These milk samples were evaluated by a dairy flavor expert for characteristic lipolyzed flavor. Chemical analyses (extraction-titration and GC) were conducted on the milk samples. Pancreatic lipase was found to give the most consistent and characteristic lipolyzed flavor of all the lipases evaluated.

Lipolyzed milk, prepared by the three methods, was then evaluated by an experienced sensory panel using a paired comparison test. The methods included the current standard method (Bodyfelt et al., 1988), a modified version of the standard method (combining raw and recombined milk), and an experimental method (recombined milk consisting of skim and hydrolyzed butter oil). Panelists found a significant difference (P < 0.05) between samples from the standard method and the other two methods, however no significant difference (P > 0.05) was found between the samples from the methods using recombined milk. The differences the panelists detected were most likely off-flavors other than lipolyzed that were inherent to the recombined milk. When those inherent flavor differences were eliminated, panelists found that the standard method and experimental method had similar lipolyzed flavor. It is recommended that a substrate other than butter oil be used for hydrolysis to improve the repeatability of the experimental method for preparing lipolyzed milk reference samples.

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