Vinegar Production from Corinthian Currants Finishing Side-Stream: Development and Comparison of Methods Based on Immobilized Acetic Acid Bacteria  
[PERSON]  
1 Department of Chemistry, University of Patras, 26504 Patras, Greece; [EMAIL_ADDRESS] (I.P. ); [EMAIL_ADDRESS] (A.A.K)  
[PERSON]  
1 Department of Chemistry, University of Patras, 26504 Patras, Greece; [EMAIL_ADDRESS] (I.P. ); [EMAIL_ADDRESS] (A.A.K)  
[PERSON]  
2 Department of Agricultural Development, Agri-Food, and Natural Resources Management, School of Agricultural Development, Nutrition & Sustainability, National and Kapodistrian University of Athens, 34400 Athens, Greece; [EMAIL_ADDRESS] (A.A.K.) [PERSON]  
3 Department of Food Science and Human Nutrition, Agricultural University of Athens, 75 Iera Odos, 11855 Athens, Greece; [EMAIL_ADDRESS] (A.A.K.) [PERSON]  
of ~2.25 billion US$ in 2020, expecting to reach ~2.55 billion US$ by 2026, considering the uncertainties caused by the COVID-19 pandemic [3]. Vinegar is produced by the action of acetic acid bacteria (AAB), which are commonly found in the raw materials used for vinegar production. The currently recognized AAB genera belong to the family of _Acetobacteraceae_, with most species found in vinegar belonging to _Acetobacter_ and _Gluconacetobacter_[2]. In industrial vinegar production, three main methods can be distinguished: the slow, traditional _Orleans_ (or _French_) method (slow, surface acidification carried out in wooden barrels), the fast _Generator_ method (actification by forced aeration in the presence of wood shavings or other inert material), and the rapid _Submerged_ method (batch fermentation with forced aeration). Despite the vast scientific knowledge gained on the production of vinegar, the industry mainly uses submerged fermentation (SmF) techniques, in which the main vessel is the _acetator_ (actification reactor). Acetification can be spontaneous, using culture from a previous batch, or by adding selected cultures to secure large-scale vinegar production, in terms of yield, safety, process stability, short fermentation time, product losses, and undesirable characteristics caused by uncontrolled fermentation. However, the use of selected AAB starters is still a long way from being a common practice [4]. On the other hand, molecular methods have helped identify AAB strains with appropriate traits. According to [4], _Acetobacter_ spp. predominate at low acetic acid concentrations. When acetic acid exceeds 5%, species such as _Komagataiebacter europaeus_ or _Gluconacetobacter intermedus_ are predominant. Generally, a process that involves a mixed culture of a \"quick starter\" (e.g., _Acetobacter pasteurianus_ or _Acetobacter aceti_) and one with high resistance to acetic acid (e.g., _K. europaeus_), is considered the best practice to ensure good performance and evolution of the fermentation. The final quality of the product also depends on the suitable culture selection [4]. Among other innovative methods that have been proposed to improve fermentation efficiency, cell immobilization techniques have been successfully applied at both research and industrial level for food production including vinegar [5; 6; 7; 8; 9]. However, most modern vinegar factories produce vinegar by SmF processes that last 4 to 5 weeks. Therefore, there is still a need for developing appropriate technologies for fast and cost effective production of vinegar, but not at the expense of quality. Cell immobilization is particularly interesting as it can improve the fermentation process by increasing the culture density in the reactors (and consequently the fermentation rates and yields), allowing culture reuse and facilitating continuous operation and recovery of the products. Materials that are cheap, easily available, and inert, such as wood shavings, cellulose, hydrocolloid gels, etc., can be used as culture immobilization carriers.