Date of Award

Fall 12-20-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Food Science and Human Nutrition


Jason Bolton

Second Committee Member

Jennifer Perry

Third Committee Member

Brian Perkins


Craft beer production in the United States has been increasing significantly in recent years. As production and participation in the industry increases, so have consumer demands for more beer options. Breweries have met this call with experimentation of ingredients in their beer and also the styles they brew. One such beer, the sour beer, is an example of this experimentation and new product development. Sour beers are a style that differ from most other beers by how they are manufactured. Instead of being fermented with only yeast, both a lactic acid-producing bacteria and yeast are used. With increased competition amongst breweries and demand for consistent beer, new methods have been and are being developed to aid in quality control. Recent research has focused on the monitoring of yeast during fermentation enabled by fast and accurate assays, but there is a need for this same level of monitoring for bacteria in fermentation.

In consideration of this, a method was developed to monitor lactobacilli concentration in a kettle sour environment, which may improve the production efficiency and consistency of kettle sour beers. This study developed counting methods using the fluorescent stains Syto BC and Syto 9 with image-based cytometry using three lactobacilli strains: L. plantarum, L. bulgaricus, and L. brevis. A method for viability determination was also evaluated in this study.

These Lactobacilli species were grown and concentrations were compared using image cytometry with fluorescent stain and cultural enumeration. This procedure was completed in vitro at different dilutions and in a kettle sour environment at different time points. Results indicated that the method developed using fluorescence-based image cytometry to evaluate Lactobacillus spp. concentration is comparable to the standard method of plating in both a controlled setting and in a kettle sour fermentation.

The objective for validation of a viability determination method using fluorescence-based image cytometry was not met in this study and requires future work. While the Cellometer X2 was able to provide total and dead bacteria counts, the data was inconsistent and deviated from plate counts significantly in the lower viability ranges. Determination of why the data did not line up with expected viabilities is an opportunity for further work.

The use of the method described herein offer brewers a much faster means for analyzing lactobacilli concentration in soured wort. This not only will provide an opportunity to control the growth in kettle sour beer production, but also allow for the production of a standardized product due to the availability of near instantaneous information for quality control, greatly reducing potential for batch-to-batch variability.