Fermenting apple juice and making hard cider will naturally create compounds that act as preservatives. These compounds, like SO2 and CO2, can be added but, I am focusing on how these compounds are naturally created. The compounds are produced by lactic acid bacteria (LAB) and yeast in your juice and cider. The LAB and yeast can create these compounds through various pathways. LAB can produce bacteriocins while yeast can create peptides. SO2 and CO2 can be created along with ethanol during the fermentation process by yeast but also by bacteria. The pH is related to the type and level of acid in the juice and cider. Again, LAB and yeast can change the pH by creating and processing different acids. So, while you could add many of these compounds to your cider to preserve it, you can also encourage the natural creation of preservatives in your cider. This can result in everything from the elimination of human pathogens to long-term stability and improvements in flavor. Let’s explore the natural preservatives that can be found in hard cider.
- Bacteriocins: polypeptides, which are antibacterial proteins, that are produced by bacteria and suppress the growth of other bacteria – Lactic acid bacteria are known to produce various bacteriocins including nisin. Nisin is approved for use in some food in the US as a preservative. There are many other bacteriocins that have not been fully studied but offer potential benefits as natural preservatives.
- Antimicrobial Peptides: a short chain of amino acids produced by eukaryotes (i.e. yeasts) that are often produced as a defense mechanism against competition and attacks – Killer yeast strains are an example. Some peptides can have toxicity issues at high concentrations, which is called cytotoxicity. This is where high levels of a peptide can become toxic to the cell producing it.
- Sulfur Dioxide/Sulfites (SO2): a highly reactive toxic gas that has antioxidant and antimicrobial properties – Produced by many yeast during fermentation, SO2 binds with compounds that easily oxidize to prevent those oxidative reactions. It can also enter bacteria, mold, or yeast cells through the cell membrane to disrupt enzyme and protein process, which effectively inhibits the cell function. Remember that SO2 is more effective as pH drops.
- Carbon Dioxide (CO2): an acidic yet colorless gas, heavier than air, and having antimicrobial effects against bacteria, mold, and yeast – The effectiveness of CO2 is dependent on many environmental characteristics but pressure is the most impactful. Experiments have shown CO2’s effectiveness is not solely based on oxygen starvation because it also inhibits anaerobic organisms.
- pH: defines the power of positively charged hydrogen ions, which reflect the acidity of the solution – Acid, pH<3.5, can weaken and burst organism cell walls or make the walls more susceptible to attack.
- Ethanol: a colorless psychoactive compound created during fermentation – Ethanol acts as a disinfectant by removing proteins from the organism’s cell. The ethanol tolerance of organisms is strain dependent. The effectiveness also varies based on exposure time and concentration.
(1) Food Process Engineering and Technology, Chapter 25, Chemical Preservation, 2013
(2) J. Cleveland and associates, Bacteriocins: safe, natural antimicrobials for food preservation, International Journal of Food Microbiology 71, 1–20, 2001
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