**1. Introduction**

Whey is an industrial by-product resulting from cheese manufacture. Approximately 89% of the milk used for cheese manufacture is processed into whey [1]. Due to its high organic matter content, it holds a high chemical oxygen demand (COD), hence posing a considerable pollution problem [2–5], meaning that currently there are very restrictive legislations regarding whey disposal [6,7]. Hence, a novel trend for finding emerging alternative uses has risen, with one of them being the reuse of whey as a disinfecting agent [8–10]. The increasingly recognized antibacterial potential of whey is mostly due to the presence of lactic acid and antibacterial bioactive peptides [11–13], both resulting

from the cheese fermentation process [14–16]. Being a food product by nature, whey has been suggested to hold particular interest as a disinfectant in freshly-cut fruit and vegetables, or minimally processed produce (MPP), which in turn are becoming major health concerns because of highly frequent pathogen outbreaks [17,18]. Indeed, several studies have reported the presence of strains such as Shiga toxin–producing *Escherichia coli*, *Salmonella* spp., and *Listeria monocytogenes* in MPP, with the last one causing death rates as high as 20% in high risk groups [17–22], followed by *Salmonella* spp., with a reported frequency of 4–8% [23,24]. Moreover, currently, chlorine is the most common disinfectant used in the fresh-cut industry [25] and it poses a serious environmental and health hazard, with strong evidences of carcinogenic problems, mostly due to the formation of toxic derivatives, such as chloramines and trihalomethanes, and restrictions to the use of chlorinated solutions are starting to arise [25–27], which consequentially has led to an increasingly higher demand to find new sanitizers [28,29]. However, as demonstrated by several published works [25,30–32], most of these alternatives are high in cost and induce odor and alterations to the organoleptic properties of the foods to which they are applied. Hence, the development of novel, cost-effective, and natural disinfectants for MPP foods is of very high economical and industrial interest. Overall, whey seems to be a promising alternative to chlorine as a disinfectant [33], and furthermore Lactic Acid Bacteria (LAB) species have been proposed as protective cultures for minimally processed food, because of their great potential for bio-control of pathogenic bacteria [34,35] and their generally recognized safe status (generally recognized as safe - GRAS, Grade One status) [36]. However, using whey or whey permeate as a disinfectant in food would result in the same environmental problem because it still contains a high load of organic matter. This could be partially overcome by fermenting the whey itself, which promotes the removal of proteins as well as lactose, and thus reduces its COD. Another direct effect of this processing is a higher production of lactic acid from lactose, which can increase its antibacterial activity, and in the case of whey proteins, it can also yield specific polypeptide sequences with antibacterial activity [37,38], which can increase its potential as a sanitizer. Nonetheless, most works use up to 24 h of fermentation [33,39], which is not enough to eliminate organic matter content. Also, it is very important to ensure that the fermented whey does not alter the organoleptic properties of the minimally processed (MP) produce, since unprocessed whey, or whey permeate, can induce visual and odor alterations to the product. However, very few works have tested whey, fermented or not, in a realistic manner in MP vegetables, particularly those more sensitive to decay, such as shredded loose-leaf lettuce. Hence, in this work, we aimed to develop a low-cost, scalable way to produce a disinfectant from dairy waste that has very little organic content and high levels of lactic acid, and induces little to no alterations in food quality.

Hence, we developed a fermentation protocol of whey from mixed origin (cow, ewe, goat) based on previous works [40,41] and evaluated its technological potential as a disinfectant in a realistic manner and in several bacterial strains. The goal was to produce the highest amount of lactic acid, whilst reducing lactose in a low-cost and efficient manner, and to determine its applications to control relevant pathogens isolated from vegetable foods, all the while maintaining food quality and safety. We also aimed to determine its efficacy against chlorine in salad disinfection. Overall, our work validated the use of an industrial waste whey as a low-cost, healthy, and efficient disinfectant that can replace chlorine, with potential applications on minimally processed foods.
