Thomas H. Shellhammer

Pressure Death and Tailing Behavior of Listeria monocytogenes Strains Having Different Barotolerances

The objectives of this study were to investigate the variability among Listeria monocytogenes strains in response to high-pressure processing, identify the most resistant strain as a potential target of pressure processing, and compare the inactivation kinetics of pressure-resistant and pressure-sensitive strains under a wide range (350 to 800 MPa) of pressure treatments. The pressure resistance of Listeria innocua and nine strains of L. monocytogenes was compared at 400 or 500 MPa and 30 degrees C. Significant variability among strains was observed. The decrease in log CFU/ml during the pressure treatment was from 1.4 to 4.3 at 400 MPa and from 3.9 to >8 at 500 MPa. L. monocytogenes OSY-8578 exhibited the greatest pressure resistance, Scott A showed the greatest pressure sensitivity, and L. innocua had intermediate resistance. On the basis of these findings, L. monocytogenes OSY-8578 is a potential target strain for high-pressure processing efficacy studies. The death kinetics of L. monocytogenes Scott A and OSY-8578 were investigated at 350 and 800 MPa. Survivors at 350 MPa were enumerated by direct plating, and survivors at 800 MPa were enumerated by the most-probable-number technique. Both pressure-resistant and pressure-sensitive strains exhibited non-first-order death behavior, and excessive pressure treatment did not eliminate the tailing phenomenon.

Application of a Short Intracellular pH Method to Flow Cytometry for Determining Saccharomyces cerevisiae Vitality

ABSTRACT The measurement of yeasts intracellular pH (ICP) is a proven method for determining yeast vitality. Vitality describes the condition or health of viable cells as opposed to viability, which defines living versus dead cells. In contrast to fluorescence photometric measurements, which show only average ICP values of a population, flow cytometry allows the presentation of an ICP distribution. By examining six repeated propagations with three separate growth phases (lag, exponential, and stationary), the ICP method previously established for photometry was transferred successfully to flow cytometry by using the pH-dependent fluorescent probe 5,6-carboxyfluorescein. The correlation between the two methods was good (r2 = 0.898, n = 18). With both methods it is possible to track the course of growth phases. Although photometry did not yield significant differences between exponentially and stationary phases (P = 0.433), ICP via flow cytometry did (P = 0.012). Yeast in an exponential phase has a unimodal ICP distribution, reflective of a homogeneous population; however, yeast in a stationary phase displays a broader ICP distribution, and subpopulations could be defined by using the flow cytometry method. In conclusion, flow cytometry yielded specific evidence of the heterogeneity in vitality of a yeast population as measured via ICP. In contrast to photometry, flow cytometry increases information about the yeast populations vitality via a short measurement, which is suitable for routine analysis.

Effect of Harvest Maturity on the Chemical Composition of Cascade and Willamette Hops

Considerable expertise is required to grow high-quality hops, and brewers and hop growers alike have a common goal of obtaining the highest quality hops possible. Changes in the chemical composition of hops during plant maturation is a dynamic process requiring a comprehensive chemical and sensory analysis in order to maximize the characteristics of interest to brewers. The effect of harvest date, location, and cultivar on key chemical components of Willamette and Cascade hops was investigated for the 2010 and 2011 growing seasons. Hops were harvested at three time points (early, typical, and late) within a 3 week interval from two different farms in the Willamette Valley, Oregon. A split-plot experimental design for each cultivar was used; each farm represented a main plot and harvest years were designated as subplots. American Society of Brewing Chemists standard methods of analysis were used to measure moisture content, hop acids and their homologs, hop storage index, total essential oil content, and volatile profile by GC-FID. Additionally, difference testing, descriptive analysis, and consumer acceptance testing was conducted using beers brewed with either typical or late harvested Cascade hops from the 2010 harvest year. The response of analytes was dependent on the cultivar being examined, its location within the Willamette Valley, as well as timing of harvest. Hop acids did not change appreciably during plant maturation for the period examined, while hop oil content increased. Increases in oil quantity were strongly correlated (r > 0.90) with increases in α-pinene, β-pinene, myrcene, limonene, methyl heptanoate, and linalool concentrations. Clear sensory differences were found between beers brewed with typical and late harvested Cascade hops using triangle testing, consumer preference testing, and descriptive analysis.

Aroma Extract Dilution Analysis of Beers Dry-Hopped with Cascade, Chinook, and Centennial

Abstract Cascade, Chinook, and Centennial hops are used extensively throughout the brewing industry either individually or in various combinations to add hoppy aroma to beer. This high use of hops, particularly via late- or dry-hopping, creates a need to better understand the chemical contribution of these hop varieties during dry-hopping beer in order to predict brewing performance. Solvent-Assisted Flavor Evaporation (SAFE) and Aroma Extract Dilution Analysis (AEDA) was performed on unhopped beer that was dry-hopped individually with each of these varieties as well as the unhopped base. This technique was used to determine the aroma compounds that were the greatest contributors to the dry-hop character of these hops. The analysis of beer prepared with Cascade, Chinook, and Centennial identified 9, 10, and 11 character impact compounds, respectively. Commonalities were observed among the three varieties regarding 2-furanmethanol, linalool, geraniol, cis-geranic acid methyl ester, and n-decanoic acid in dry-hopped beer. Variation between the hop volatiles found to be important for Centennial and Chinook dry-hop aroma was a function of only a few character impact compounds, whereas Cascade was slightly different, anchored heavily by benzenacetaldeyde. The relative similarities and differences that these three hop cultivars attribute to beer during dry-hopping were revealed by comparing which compounds were important for the characteristic aroma profiles of these cultivars in single dry-hop beers.. This knowledge is important for brewers wishing to introduce potential replacement hops and/or reductions for these hop cultivars in the future and guide the direction of future blending studies.

Sensory Directed Mixture Study of Beers Dry-Hopped with Cascade, Centennial, and Chinook

Abstract American craft beer style and flavor is often driven by the unique qualities of American hops. Cascade, Chinook, and Centennial hops are used prominently for dry-hopping singly and/or in blends to impart an intense hoppy aroma to beer. A sensory directed dry-hopping mixture study was performed to understand the contribution that each of these hops make to beer aroma. Utilizing a 4th degree simplex-lattice mixture-design, sixteen beers were prepared (including an “unhopped” control) by dry-hopping a common “unhopped” base beer with different blends of ground whole cone hops made from the three hop cultivars. The treatments were evaluated by trained panelists using descriptive analysis, where the response variables used by the panel encompassed the sensory attributes that described the unique aromatic features of these three hops, (i.e., citrus, tropical/fruity, tropical/catty, and herbal). Using these outputs, the sensory contributions of each individual cultivar, as well as mixtures of the cultivars, were examined on a per attribute basis. These results can be used to select combinations or blends of the three hops for use during dry-hopping that provide similar or dissimilar overall aroma intensity and quality in dry-hopped beer.

GIANT SEQUOIA (SEQUOIADENDRON GIGANTEUM [TAXODIACEAE]) SEEDLING SURVIVAL AND GROWTH IN THE FIRST FOUR DECADES FOLLOWING MANAGED FIRES

ABSTRACT Giant sequoia (Sequoiadendron giganteum) seedling survival was nearly seven (6.78) times greater in heavily charred soils than in non-burned soils 34 and 35 yr after the first experimental burns in Kings Canyon National Park, California, and the first such management burns in any western national park. Tree height, especially trees growing in intensely burned areas, was found to be highly correlated with sunlight and less so with moisture. Moisture and light were important to establishment of giant sequoias but continued growth of these trees in the first four decades of life appears to be more dependent on high levels of sunlight. Surveyed vegetation and downed wood indicated that 35 yr after management burns stand structure has developed to the point where the management areas are susceptible to destructive crown fires.

Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops (Humulus lupulus)

Dry-hopping, the addition of hops to beer during or after fermentation, is a common practice in brewing to impart hoppy flavor to beer. Previously assumed to be inert ingredients, recent evidence suggests that hops contain biologically active compounds that may also extract into beer and complicate the brewing process by altering the final composition of beer. Experiments described herein provide evidence of microbial and/or plant-derived enzymes associated with hops ( Humulus lupulus) which can impact beer quality by influencing the composition of fermentable and nonfermentable carbohydrates in dry-hopped beer. Fully attenuated and packaged commercial lager beer was dry-hopped at a rate of 10 g hops/L beer with pelletized Cascade hops, dosed with 106 cells/mL of ale yeast, and incubated at 20 °C. Real extract of the treated beer declined significantly within several days with a reduction of 1 °P (% w/w) after 5 days and then slowly to a total reduction of approximately 2 °P after 40 days. When fully fermented, this was equivalent to the production of an additional 4.75% (v/v) of CO2 and an additional 1.3% (v/v) of alcohol. The refermentation of beer driven by dry-hopping was attributed to the low but persistent activities of several starch degrading enzymes present in Cascade hops including amyloglucosidase, α-amylase, β-amylase, and limit dextrinase. The effect of hop-derived enzymes on beer was time, temperature, and dose-dependent. Characterizing bioactive enzymes in hops will help hop suppliers and brewers to address the unexpected quality and safety issues surrounding hopping practices in beer.