Net effect of wort osmotic pressure on fermentation course, yeast vitality, beer flavor, and haze.

The net effect of increased wort osmolarity on fermentation time, bottom yeast vitality and sedimentation, beer flavor compounds, and haze was determined in fermentations with 12 degrees all-malt wort supplemented with sorbitol to reach osmolarity equal to 16 degrees and 20 degrees. Three pitchings were performed in 12 degrees/12 degrees/12 degrees, 16 degrees/16 degrees/12 degrees, and 20 degrees/20 degrees/12 degrees worts. Fermentations in 16 degrees and 20 degrees worts decreased yeast vitality measured as acidification power (AP) by a maximum of 10%, lowered yeast proliferation, and increased fermentation time. Repitching aggravated these effects. The 3rd "back to normal" pitching into 12 degrees wort restored the yeast AP and reproductive abilities while the extended fermentation time remained. Yeast sedimentation in 16 degrees and 20 degrees worts was delayed but increased about two times at fermentation end relative to that in 12 degrees wort. Third "back-to-normal" pitching abolished the delay in sedimentation and reduced its extent, which became nearly equal in all variants. Beer brewed at increased osmolarity was characterized by increased levels of diacetyl and pentanedione and lower levels of dimethylsulfide and acetaldehyde. Esters and higher alcohols displayed small variations irrespective of wort osmolarity or repitching. Increased wort osmolarity had no appreciable effect on the haze of green beer and accelerated beer clarification during maturation. In all variants, chill haze increased with repitching.

A new method of optical detection of yeast acidification power.

We describe here a newly developed method for a contact-free optical pH measurement in yeast suspensions supplemented with glucose, and containing the pH sensitive triphenylmethane dye bromocresol green. It is suitable for performing the acidification power test (based on measuring the rate of pH drop of yeast suspension caused by active extrusion of acidity from cells after glucose addition) used for assessing yeast vitality in fermentation industries. Using this methodology we monitored the pH in yeast suspensions in the course of acidification in the pH range of 3.5-5.3. Optical pH measurement allows simultaneous testing of several samples, minimizes the sample volume, simplifies sample handling and reduces the hands-on time in sample processing.

Factors affecting the outcome of the acidification power test of yeast quality: critical reappraisal.

Brewery bottom yeast strain 95 from the Pilsner Urquell propagation unit was used to reappraise the efficiency of the acidification power (AP) test consisting in determining the spontaneous (oxygen-induced) and glucose-induced medium acidification caused by yeast and lactic acid bacteria under standard conditions, and used widely for assessing and predicting the vitality of industrial strains. AP was evaluated in yeast stored for different periods of time (0-28 d) at 4 degrees C, at different temperatures before and during the test (0-55 degrees C), and at different concentrations of cells and glucose and different cells-to-glucose ratios. All these factors had a strong effect on acidification kinetics and the AP value. By contrast, the duration of the lag period between yeast collection and the test (0-6 h) had no perceptible effect on the AP value. The best results were achieved at saturation concentrations of cells (> 10 g pressed yeast or approximately 14 g yeast slurry per 100 mL) and glucose (approximately 3 %) and at 25 degrees C. Since an exact evaluation of acidification characteristics depends strongly on the kinetics of the process, the AP test should include monitoring the time course of the acidification.

Calibration of silicon photodiodes by photoacoustics.

We present a new approach to the calibration of silicon photodiodes that are used for light (radiometric) power measurements. We describe the photoacoustic method for the determination of internal quantum efficiency and absolute spectral response of silicon photodiodes in the photovoltaic short-circuit mode. Measurements were taken for the different structures of silicon photodiodes.