Evelien Syryn

Hopping technology in relation to beer bitterness consistency and flavor stability

The fate of α-acids, iso-α-acids, and their chemically modified variants was monitored in pilot brews as a function of hopping regime and beer aging. HPLC analysis indicates that α-acids, iso-α-acids, and dihydroiso-α-acids in beer are not stable during forced aging. This is reflected further in the sensory performance of these beers. Beer exclusively bittered with tetrahydroiso-α-acids was completely stable, in terms of hop components, under the experimental conditions employed. In addition, overall flavor stability was significantly improved. These results provide further evidence that hop-derived bitter acids, including the light-stable dihydroiso-α-acids, could play an important role in beer flavor deterioration during storage.

Overexpression of L-Arabinose Isomerase for Production of the Low-Calorie Bulk Sweetener D-Tagatose

High-Cell-Density Cultivations show a huge potential to produce recombinant proteins to amounts greatly exceeding the availability in natural resources. An interesting example of a recombinant protein is an L-arabinose isomerase, which is able to convert D-galactose to the low-caloric and low-glycaemic bulk sweetener D-tagatose. Within this study, the L-arabinose isomerase from Geobacillus stearothermophilus was expressed intracellularly in Escherichia coli. The cultivation medium contained glucose, yeast extract and various macro- and micronutrients. The effect of air flow rate on E. coli growth and expression of L-arabinose isomerase was studied. After 52 hours, an Optical Density and Dry Cell Weight of 154 ± 4 and 54.8 ± 1.3 g L-1 were reached, respectively by regulating the air flow rate between 0.2 and 30 L min-1. A corresponding L-arabinose isomerase activity of 6.99 ± 0.46 U mL-1 was reached. A drawback of High-Cell-Density Cultivation is the production of the by-product acetic acid which may inhibit growth. However, the acetic acid concentration was maintained as low as possible during fermentation to avoid inhibitory effects inherent to this compound. With the L-arabinose isomerase produced, a conversion percentage of 37.1 ± 1.5% was achieved, corresponding to 94.9 ± 3.7 g L-1 D-tagatose. Thus, the implementation of a High-Cell- Density Cultivation led to an efficient expression of the L-arabinose isomerase enzyme and D-tagatose production. Also the storage stability of the cells was investigated during several months at 4°C. A stable L-arabinose isomerase enzyme was noticed during at least 8 months storage at 4°C.