About a year ago an American producer and friend of mine said that he had heard from someone who had attended a conference somewhere that yeasts can survive for a long time in bottled extra virgin olive oil and that their activity is detrimental to the oil quality.
I was sceptical. Once bottled, extra virgin olive oil it is thought to be one of the most microbiologically stable foods. I’m pretty sure that if you took your grandmothers olive oil out of her manky pharmaceutical studded cabinet, that was left to her by her grandmother, you could chug down the whole lot of it and not get sick. You may re-acquire a taste for fast food, but you’ll wake up the next day feeling ok.
So when I recently came across the journal article titled
Effects of some oil-born yeasts on the sensory characteristics of Italian virgin olive oil during its storage (Zullo et al. 2013) I thought, ah there is the likely source of my American mates ‘Chinese whisper’ information.
So do olive oil producers really have get yeast counts done on the bottled oil to ensure that their oil doesn’t degrade faster than usual? I have to say that have always treated research reporting potential ‘problems’ that no one has noticed before, even after making the stuff for a couple of thousand years. But with an open mind, I looked at the ins and outs of the very detailed paper closely.
The researchers swabbed some EVOO’s, and ‘plated up’ what they found into a warm lagoon of yeasty heaven which allowed them to grow up enough yeast cells to inoculate a perfectly good extra virgin olive oil to assess the effects of yeast on oil quality during 4 months of bottle storage.
But keep in mind that the number of yeasts found in olive oil is pretty low probably due to the fact that the relatively waterless environment of an olive oil isn’t their favourite hang-out. However, after culturing them up in the laboratory, they added them back at a rate of (wait for it), 25gms of yeast for every 100L of EVOO. That is about the same amount of yeast that winemakers used when they ferment grape juice into wine. In laymans terms ‘bucket loads’. To put the amount they used into an olive oil perspective, they estimated that they added around 100,000 yeast cells to every 1ml of oil. Compare that to the 1,000 odd yeast cells per ml that are naturally found in olive oil (based on a survey of 14 commercial olive oils from the Liguria and Central Italy, Zullo et al. 2010).
So what effect did adding 100x the typical number of yeast to an olive oil and leaving it for 4 months have?
Probably not surprisingly, the yeasts were shown by the researchers to produce all sorts of evocative (to a microbiologist anyway) enzymatic activity – Peroxidase, tyrosinase, phenoloxidase, and b-glucosidase, which could potentially degrade olive oil and affect its quality. But we all have ‘potential’ don’t we?
Here are the effects that matter. The effects on the chemistry and taste ….
Table 1: The effect* of inoculating with these large amounts of yeast and then storing the oil plus yeast for 4 months.
|No yeast added||Yeast added*|
|Secondary oxidation UV232||2.19||1.98|
*The researchers looked at the effects of half a dozen yeast types. I averaged their effects and presented them with changes to the same oil that had not been yeast inoculated and stored for the same length of time.
^Approximate value as the raw data was read off a figure, but it gives you an idea.
They also found that yeast inoculation modified the profile of aroma compounds, with yeasty oils having lower amounts of the grassy tasting C6 compounds. Interestingly though, many other compounds that are normally associated with poor tasting olive oil (C7, C8 and C9’s) did not increase – indeed many of them decreased in the presence of yeast.
There are a few ways of looking at these results, including that you could add yeast to your olive oil to improve its oxidative stability! Lower peroxide values and lower secondary oxidation values are good things right? Well that is what the data suggests. Others would look at the results and emphasise that polyphenols were reduced by 10%, and the oil was less fruity and some yeast produced a faint muddy aroma. But, despite the fact that the researchers appeared to try to use experimental extremes to induce an effect by all manner of yeast types, they barely dented the quality of olive oil.
Where an extra virgin olive oil is bottled immediately after extraction, and is still ‘murky’, then maybe, just maybe these results may apply. However, oils of this type are meant to be consumed immediately as they have a high moisture content and contain relatively large amounts of vegetative solids. It is arguable that they would probably go muddy even without any yeasty action, as keeping vegetable solids in a highly anaerobic (airless) environment will most likely do that anyway.
So hopefully we won’t be seeing tests for yeast cell counts any time soon. We have enough tests already.
Zullo, B.A., Cioccia, G. and Ciafardini, G. (2013) Effects of some oil-born yeasts on the sensory characteristics of Italian virgin olive oil during its storage. Food Microbiology, 36, 70-78.
Zullo, B.A., Cioccia, G. and Ciafardini, G. (2010) Distribution of dimorphic yeast species in commercial extra virgin olive oil. Food Microbiology, 27, 1035-1042.