Ask an Expert: Re-fermentation and Apple Cider Vinegar

The Food Innovation Network comprises a core group of food and beverage industry experts that make up the Network’s Community of Practice (CoP). Anyone in the food value chain can ask a question of the experts and we will endeavour to find an answer.

Today’s question comes from Daryl in the Goulburn Valley. The subsequent dialogue illustrates the level of expertise available and the power of the Food Innovation Network.

Q. I make apple cider vinegar with fruits added. I sanitise the bottles, and wash and then freeze the fruit. The fruit is then added into a small amount of hot vinegar before being put into bottles with the rest of the vinegar. However, I am getting re-fermentation happening weeks later. How can this be stopped, in a safe, natural way, once the product has been bottled?

A1. There are a lot of variables here to deal with to accurately answer the question. How is the apple cider vinegar (ACV) made and does it have a mother? Is it filtered or unfiltered? Is it pasteurised? Has the ACV finished its fermentation before use? Is there residual sugar and what is the pH of the vinegar? The fruits used are not known as is why are they added frozen with warm vinegar added the topped up with cold vinegar. This could be a possible issue. Sanitising is important but working with low pH such as ACV it is not a critical control point. Having dealt with many trouble shooting issues in the food industry, it is always a stretch to answer a question by email or over the phone when the entire process hasn’t been seen in person as in most cases vital information is missed.

To give a very quick opinion on the issue as presented, the quick fix would be to put fruit in the bottle, fill with ACV, seal and then sanitise in a water bath at 90C for a minimum of 20 minutes (timed from when the water reaches 90C). I am assuming it is bottled in a 750ml glass container. Smaller containers will take less time, and larger ones will take longer and times vary if other types of packaging are used.

The only drawback of this method (and I repeat it is an opinion, not a solution) is that this processing method can affect the attributed benefits that raw unfiltered ACV. I hope this is of assistance and good luck to the maker. Patrick Watson, Director, Tarn Food Consulting.

A2. Patrick is on the right track, however things are always more complex than they first appear. Firstly, any pasteurization temperature is highly dependent on pH. In ACV, at around pH 3, temperatures in the range 70-80C could achieve commercial sterilization subject to contact times. However, at higher pH levels 90C may still allow for the survival of spore formers and subsequent spoilage.

Secondly, based on the limited information, the product in its early stages is a two-phased product. That is, ACV at pH 3 and fruit at a pH as high as 4.5-5.0 depending on the fruit and how it was previously processed. Thus, when these are put together and cooked the fruit will be at a higher pH compared to the vinegar phase and the cooking is less effective on the fruit. This has to be accounted for if you are aiming at cooking at the lowest temperature possible that you can get away with.

Thirdly, the final equilibrated pH is important. If a product has been cooked and is low pH, surviving spore formers may not germinate because of the low pH. However, if the product subsequently equilibrates to pH 4 or higher any surviving spore formers may well germinate and cause spoilage. If they are experiencing fermentation and spoilage from non-spore formers their temperature treatment is totally inadequate and needs to be profiled.

It is important to look at these pH differentials and also to investigate spoilage when it occurs to determine what species of bacteria or fungi a causing it. There are some other issues but I do not want to turn this into a never-ending story. I am happy to talk to the manufacturer and advise them based on the equipment and facilities that they might have at their disposal. Derio Comar, Director and Principal consultant, Comar Consulting.

A3. One of our extended team members believes he can solve this problem but feels more information is required. He would be happy to look at it in more detail. He says, “This is not an unusual problem when processing fruits. Unfortunately, not enough details are provided in the question but it seems that an option is to pasteurise the product in the container. The heat treatment needs to be sufficient to kill the microorganisms responsible for the fermentation and at the same time consider the limitations of the packaging (heating and cooling profile). Also, it is important to understand that the heat treatment might alter key organoleptic characteristics of the product (texture, flavour and colour). A more appropriate approach is to prevent the problem in the first place. Identifying the source of contamination and creating a more robust process with the appropriate checks is considerably more complex.” Brenton Dodd, Sales Manager, MEMKO Systems Pty Ltd.

Daryl was asked to provide more information. He responded:

“My process for the ACV is when it gets to a pH of 3 it is ready to use. We keep the mother in it and it is unfiltered and unprocessed. We heat bath the bottles for sanitizing at 90C.

From there we work with raspberries, blueberries, cherries and pineapple. We wash the fruit and then freeze it for seven days prior to use. Then we heat a small amount of the ACV and a portion of the fruit to around 90C. From there we would bottle and top up with cold ACV.

Going by the expert feedback I think I need to stew the fruit separately and then put it into the bottle, thus killing all spores. And then add ACV cold to that, thus taking out the chance of spores surviving the process.

I don’t have a huge commercial kitchen with big equipment but I am currently building a small commercial kitchen in Shepparton.”

Outcome: The Network Coordinator put Daryl in touch with Derio Comar.

We will report on any progress with this case study at a future time.

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Coordinator - Victorian Food Innovation Network

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