The effect of mixed culture fermentation of Saccharomyces cerevisiae and Saccharomyces cerevisiae var. diastaticus on fermentation parameters and flavor profile.

Belgian Saisons and Lambics are two well-known examples in the brewing industry of mixed fermentations, combination of two or more yeast and/or bacteria strains. The purpose of this study was to determine the impact different pitch rates of Saccharomyces cerevisiae (traditional brewing yeast) and S. cerevisiae var. diastaticus (a variant associated with Belgian styles) had on the fermentation kinetics and concentration of the volatile compounds in the finished beers. A series of brews were performed utilizing ratios of S. cerevisiae and diastaticus. The fermentations were heavily monitored, and a model was used to fit fermentation variables. It was found that mixed fermentations produced behaviors that were predictable and proportional to the mixture ratios. As expected, the pure cultural fermentations of diastaticus had a slower fermentation midpoint (M) at 45.45 h versus 28.28 h for S. cerevisiae with the mixed ones falling in between the two. Flavor and aroma play a key role in the acceptability of beer. The mixed fermentations showed a combination of the two different yeast strains aromatic profiles. When combined, there was a strong linearity between alcohols (R2  = 0.94), esters (R2  = 0.89), and the overall total (R2  = 0.91) volatile compounds. PRACTICAL APPLICATION: Modeling is a widely utilized tool in several different fields. The purpose of this research is to apply modeling techniques to describe the fermentation speed and flavor profile of a mixed fermentation between S. cerevisiae and diastaticus. The equations from this data can be used by brewers for product development purposes to make beers with certain flavor profiles within a desired timeframe.

Comparing steady-state to unsteady-state respiration rate measurement methods for design of modified atmosphere packaging of grape tomatoes and blueberries with microperforations.

Designing modified atmosphere packages (MAPs) for fresh produce requires respiration rate (RR) data. A steady-state (SS) approach is widely used but is expensive, tedious, and time-consuming. Unsteady-state (USS) methods mitigate shortcomings of the SS approach, but comparisons between the two approaches have not been done to verify the design outcomes of MAPs, especially those with microperforations. RR measurement methods for grape tomatoes and blueberries were compared. Data were then used to design microperforated MAP packages to compare predicted design specifications created from RR data with observed shelf life. Results show that the USS method provides similar magnitudes of RR and predicts similar numbers of perforations as the SS method. Observations of packages produced using 100 µm perforations, using measured respiration data, suggest that both methods underestimated what might have been deemed correct by about one microperforation. PRACTICAL APPLICATION: Designing packaging for fresh produce requires the knowledge of produce respiration. Steady-state methods are conceptually simple, but time-consuming. Unsteady-state methods are rapid. This work compares methods on design of packages.

Permeable Gas Cavity at Elevated Pressure Enhances Modified Atmosphere Packaging of Fresh Produce.

Modified atmosphere packaging (MAP) of fresh produce involves exploiting package properties to satisfy respiration activity of produce. While effective, package material properties are not infinitely adjustable to match needs of all products. Additional ways of providing favorable in-package gaseous environments are needed. This work explores the use of permeable inserts filled with gas at elevated pressures as a means to achieve in-package gaseous atmospheres that may not be possible by the package alone. Mathematical models were developed to predict transient package atmospheres for packages containing respiring produce and pressurized permeable inserts. The model was validated for semirigid tray packages containing grape tomatoes and Granny Smith apples. With inserts initially pressurized with oxygen at approximately 200 to 300 kPa (about 30 to 45 psi), about 2 weeks additional shelf life was observed relative to controls for both tomatoes and apples in test packages. Additionally, simulations provide design guidance for pressurized inserts for the case of very high respiration rate produce such as spinach. PRACTICAL APPLICATION: This work promises to expand application of modified atmosphere packaging (MAP). Currently, applications are limited by gas transfer material properties of existing packaging films. However, packaging offers other important functions that may not be well served by materials that satisfy critical gas permeation requirements. This work demonstrates an approach that disconnects packaging material specifications from MAP design.