# Effect of Ultrasound on Fermentation of Thick Molasses Worts by Distiller's Yeast
###### Abstract
In certain instances, the rarefaction phase can be so pronounced that it results in a significant separation between the molecules of the liquid, leading to the formation of voids known as cavitation bubbles. During the rarefaction phase, these bubbles undergo growth. Subsequently, when the compression phase ensues, the bubbles undergo a size reduction. Occasionally, the bubbles can diminish to such an extent that they reach a critical point, resulting in their subsequent collapse. The collapse of these bubbles can generate high pressure and temperature. The principal physical effects of this phenomenon are shockwave-induced damage and microjet impacts. Other mechanical effects include agitation, turbulence, vibration, pressure, shear forces, and acoustic streaming. In addition to the physical effects of cavitation, chemical effects occur. One of the most significant is the sonolysis of water (H\({}_{2}\)O \(\rightarrow\) H\({}^{+}\) + OH\({}^{-}\)), which may produce free radicals. These radicals can combine to form other molecules, potentially affecting microorganisms or enzymes. Recent studies on the application of ultrasound in fermentation processes indicate that it enhances mixing and facilitates the release of CO\({}_{2}\) from the medium, thereby optimising the kinetics of cellular CO\({}_{2}\) expulsion [34; 35; 36; 37; 38; 39]. Additionally, ultrasound has been reported to improve the transport of sugars into cells and the efflux of ethanol and carbon dioxide from cells. Elevated ethanol and carbon dioxide concentrations near the cell membrane have been observed to alter membrane integrity and potentially hinder fermentation rates. Therefore, sonification can eliminate this negative impact [40,41]. One of the trends in current ethanol production technologies is to reduce water consumption, among other things, before increasing the sugar content (and thus density) in fermentation wort. This is also aimed at reducing the amount of liquid subjected to distillation (and therefore the energy expenditure necessary to heat this wort to the process temperature) [42, 43, 44, 45, 46]. A significant problem with the fermentation of thick wort is forcing the yeast cells to work under increased osmotic pressure, translating on a macroscopic scale into a lengthening of fermentation and decreased ethanol yield. There are reports in the literature [30,47, 48, 49] suggesting that the efficiency and dynamics of fermentation can be improved by using ultrasound, so we thought it would be worth conducting a study in this area. The strains Thermosacc Dry (Lallemand Inc., Milwaukee, WI, USA) and Ethanol Red (Leaf by Lesaffre, France), which we have selected as biological models, are high-performance distiller yeast strains of the species _Saccharomyces cerevisiae_ from two of the world's leading yeast producers. These strains are widely used to produce fuel ethanol and ethanol for the food, chemical, and pharmaceutical industries. These strains show high resistance to unfavourable fermentation conditions such as high osmotic pressure or the presence of fermentation inhibitors [50, 51, 52, 53, 54, 55]. The main objective of our research was to determine whether the use of ultrasonic treatment can positively influence the fermentation of molasses wort, especially with a density higher than 18 \({}^{\circ}\)Blg, with a limited fermentation time of 72 h. The main positive effect should be an acceleration of fermentation and an increase in the yield of ethanol produced. An additional objective was to determine whether changing the sonication intensity by varying the ultrasound amplitude would cause changes in the course and efficiency of fermentation.