**3.5 Future outlook**

*New Advances on Fermentation Processes*

efficiency from sugar to acetic acid increased to 362%.

and higher oxygen transfer rates (OTR) and oxygen uptake rates (OUR); therefore, aerobic and anaerobic environmental conditions are created simultaneously over a short period of time. Therefore, increasing agitation speed, up to a certain level, can lead to the production of larger amounts of acetic acid over shorter periods of time. In addition, the high dissolved oxygen content caused by the increased agitation speed in this system does not appear to cause oxidative stress or damage to proteins in cells, which could inhibit *A. aceti* growth [33]. As a whole, under conditions using an optimal inoculum ratio and an optimal agitation speed, the conversion

**3.4 The dynamics of changes in the sugar, alcohol, and acetic acid contents and** 

The dynamics of changes in the sugar, alcohol, and acetic acid contents and in the pH values during the fermentation of acetic acid from apple juice under optimal conditions can be observed in **Figure 6**. In the beginning, when the sugar level is high, *S. cerevisiae* works to produce alcohol, increasing the alcohol contents. In conjunction with the production of alcohol, *A. aceti* began to produce acetic acid, causing the acetic acid level to increase. As *S. cerevisiae* produces alcohol, *A. aceti* simultaneously grows until the alcohol contents produced by *S. cerevisiae* are sufficient for *A. aceti* to produce acetic acid. In the mixed culture fermentation, *A. aceti* which is an obligate aerobic microbe uses dissolved oxygen for growth and for the oxidation of alcohol into acetic acid. However, the medium also undergoes an anaerobic state due to a lack of oxygen, allowing *S. cerevisiae* to convert sugar into alcohol. Another advantage of the use of mixed cultures is that the continuous consumption of oxygen by *A. aceti* appears to cause *S. cerevisiae* to grow without the multiplication of cell mass. Thus, the sugar present in the substrate can maximally be converted into alcohol by *S. cerevisiae*, and the alcohol can subsequently maximally be converted into acetic acid by *A. aceti*. However, at the end of the process, a decrease in the resulting acetic acid levels was observed. This decrease may be due to the unfavorable pH of the medium, which could inhibit the microbes from metabolizing substrates and producing acetic acid, or may be due to acetic acid overoxidation due to the limited

**in the pH value during fermentation under optimal conditions**

*The dynamics of the changes in glucose, alcohol, and acetic acid contents and pH values under optimal* 

**198**

**Figure 6.**

*fermentation conditions.*

Under the right conditions, the production of acetic acid can be maximized by using a simple system, such as submerged batch fermentation using a mixed culture that acts synchronously. Optimization can be performed by considering the character and needs of all microbes involved, which are the nutritional adequacy of the medium, the microbial proportions in the inoculum, and the agitation speed. The use of a mixed culture could shorten the fermentation time, reduce fermentation losses, and increase the acetic acid yields [16]. Some other advantages of this system compared with a gradual system are the relatively simple operation and easy handling of this system, which no particular control is required during the fermentation process, and the low risk of contamination. Thus, the application of this system for industrial purposes can be considered. However, the future scaling up of this process should consider other factors, including automation systems and the use of cutting-edge technologies in both the production and monitoring processes, to further improve the productivity and product quality without increasing production costs.
