In my research on preservatives like Campden (potassium metabisulfite) and potassium sorbate that are commonly used in wine and cider to preserve the cider as well as prevent residual sugar from fermenting, I was always interested in finding alternative methods. Preserving hard cider is generally about stopping various bacteria and yeast that can create bad aromas and flavors. One of the most common is Brettanomyces, which is known to generate farmyard type aromas and flavors. For wine, it is consider a major flaw while beer and hard cider may actually encourage it. It’s most common source for contamination is from processing equipment but it has been found on apples as part of the natural micro flora. Usually, it is treated by adding Campden or potassium metabisulfite after fermentation is complete. Potassium metabisulfite creates sulfur dioxide, SO2, which can kill and suppress the micro flora, including Brettanomyces yeasts. However, people can be sensitive to SO2 and the amount you can add is regulated. Also, it suppresses positive probiotic bacteria and in the US cannot be added for products that are certified organic.
This has caused some researchers to pursue natural preservatives or killer toxins that could naturally suppress microbes like Brettanomyces yeast. Like the killer factor possessed by some Saccharomyces yeast strains, researchers have found other genera of yeast that can create killer toxins. This killer toxins are capable of suppressing and killing off Brettanomyces yeast in wine and hard cider. N.N. Mehlomakulu and his associated assessed 22 Brettanomyces strains commonly found in South African wine with multiple variables including sugar level, alcohol level, temperature, and pH. Many of these variables were assessed at levels similar to hard cider, which I believe could result in similar findings.
They identified two killer toxins created by three Candida yeast strains (C. pyralidae CBS 5035, C. xylopsoci CBS 6037, and C. prunicola CBS 8848) that created these toxins (IWBT Y1140 and IWBT Y1057). They found 16 of the 22 Brettanomyces strains were sensitive to these toxins. It was also noted that these toxins did not appear to impact Saccharomyces strains or lactic acid bacteria that enables malolactic fermentation. The research found that the toxins were active in pH from 3.5 to 4.5 and temperatures from 59-77F (15-25C). It also found that ethanol and sugar did not impact the toxins. For me, this represents a great idea for naturally providing spoilage protection to hard cider. It also supports the idea that many wild ferments could be creating these natural preservatives. Thus, the need for using potassium metabisulfite, Campden, is reduced or eliminated. While natural ferments usually have Candida yeast, they may not have strains that produce these toxins. However, the Candida genera are more readily killed with the addition of Campden to the juice. Ultimately, it sounds like another reason to try and create a cider yeast blend that contains a known diverse micro flora that is capable of leaving some natural residual sweetness, produces great ester and volatile compounds, and could provide some protections against spoilage.
(1) N.N. Mehlomakulu and associates, Characterization of novel killer toxins secreted by wine-related non-Saccharomyces yeasts and their action on Brettanomyces spp., International Journal of Food Microbiology, 188, 83–91, 2014
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