Slick Extra Virgin

This is the 3rd of a short series aimed at explaining the whys, the how and the what (and the what-nots) of analyses that are commonly applied to extra virgin olive oil. This time, I present what I think is a reliable indicator of olive oil oxidation – K232.

FACT FILE

Name: K232 (short for absorbance at 232 nanometers).

What does it measure?: Level of oxidation.

How does it work? All olive oils contain some polyunsaturated fatty acids. These have a basic chemical structure that has two chemical double bonds amongst a long line of single chemical bonds. In an unoxidised state, the pattern of bonding is (Double-Single-Single-Double). But during oxidation of the fatty acid, the conga line of bonds changes to (Double-Single-Double Single).

Whilst this change in order might seem inconsequential, the oxidized rearrangement (D-S-D-S known as a ‘conjugated diene’) has a particular property in that (unlike its unoxidised D-S-S-D cousin), it strongly absorbs ultraviolet light at a specific wavelength of 232 nanometers.

So shining a beam of UV light at 232nm at an oil and measuring how much of it is absorbed can be used as a measure of the oils oxidation status. This is done using a peice of scientific equipment called a spectrophotometer. A schematic as to how the analysis works is shown in Figure 1.

Figure 1: A schematic showing the basic principle behind the K232 analysis for oxidation.

Unit of Measurement: absorbance units(au).

Does it affect extra virgin olive oil status?: Yes. To be extra virgin, the oil must absorb at less than 2.55 absorbance units at 232nm. In my opinion 2.55 is (like most of the IOC limits), way too lax. Most 2 year old oils can make the grade pretty comfortably. But if that is what they want……

Is it a measure of quality?: Yes. K232 has been shown to be a good predictors of the freshness of extra virgin olive oil in Australian competitions (Australia is the only country that routinely measures the chemistry of competition entries).

Official method degree of difficulty: Easy, but for accuracy, the oil sample needs to be accurately diluted by a technician with a relatively expensive solvent so the cost is moderate. It can be measured using cheaper NIR (Near Infrared Spectroscopy) method. However, like all NIR analysis, the validity of the accuracy depends squarely on the quality of the calibration available to the laboratory.

Typical effects of high readings of UV 232

– The oil displays tiredness and in some cases rancidity.

– Shorter shelf life.

K232 level in extra virgin olive oil is increased by:

– Delays between harvest and processing, incidence of fungal disease, olive fly attack and frost damage increase K232.

– Age.

– Oxidative bottling practices.

– Poor storage conditions.

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This is the first of a short series explaining the common chemical analyses applied to extra virgin olive oil. I’ll start off with the most universal analysis of them all – “Free fatty Acidity” – or known to most of us as ‘Acidity’.

Name: Free Fatty Acidity or FFA

Better known as: ‘Free Acidity’ or simply ‘Acidity’

What does it measure?: The level of breakdown of the fats in extra virgin olive oil.

Analysis debuted: Circa 1890

Use by olive oil producers: Universal

Degree of difficulty: Easy. Can be done using basic lab equipment and little technical expertise. You can see me conducting an FFA analysis here

Official Iimit for extra virgin status: < 0.8%

Unofficial limit strived for by good EVOO producers: <0.2%

Direct effects of high readings of FFA:

• Lower smoke point (the temperature at which the oil starts to smoke when heated)
• Greasy mouth-feel/aftertaste
• Shorter shelf life

Typical indirect effects of high FFA:

• Reduced positive flavours and/or greater defective fermented like flavours.

Common causes of high FFA:

• Delays between harvesting and processing.
• Diseased/damaged olives particularly due to olive fly infestation and anthracnose (fungus).
• Bruising of olives during harvesting followed by processing delays.
• Olives harvested from the ground.
• Frozen/frosted olives.

What is Free Fatty Acidity?

Every fat molecules including that in olive oil is comprises 3 smaller parts called fatty acids that are connected together. Figure 1 is the typical fat molecule found in olive oil – three monounsaturated fatty acids called oleic acid linked together.

Figure 1: Extra Virgin Olive Oil Nirvana. A perfectly formed and intact fat typical of olive oil.

However, having 100% perfectly intact fats like those shown in Figure 1 molecules is “fat Nirvana”. Sort of like that place that the “Watchtower” magazines portray – where humans live in perfect harmony with dopey lions and really small monkeys (both of which are likely to kill you at any given moment, but somehow reassuringly, for different reasons). But, getting back to the point, extra virgin olive oil is a natural product, and therefore is less than perfect. Individual fatty acids can be chopped off the fat molecule if the olives are riddled with fungussy olive pestilence, are being gouged upon by olive fly maggots, bruised and battered during harvesting, or (more typically, and less dramatically) if the olives are left to even slightly decay in the interim between olive harvest and processing.

Figure 2: Formation of free fatty acids. The Pacman represents a lipase enzyme, but if you like, you can think of it as a Pacman.

Nature has devised a few ways of busting up intact fat molecules to form free (aka single) fatty acids. Her chopping utensil of choice are enzymes called lipases. They are found in the olive fruit, but in good fresh undamaged olives the lipases are harmlessly locked up in the olive cell away from the fat molecules that they desperately want to destroy. But when the olive cells are damaged in any way, the enzymes are able to escape from their compartments, and in frenzied attacks dismember the intact fat molecules freeing up fatty acid molecules (Figure 2). The more diseased the olive, or the longer the time gap between olive harvesting and processing, the more enzymatic activity and the higher the free fatty acidity.

Common misinterpretations/misrepresentations of FFA in olive oils.

“FFA is a good indicator of olive oil quality” –  Sort of right.

Free fatty acidity is widely acknowledged as a good index of general olive oil quality. While this is generally true, I have tasted many examples of higher FFA extra virgin olive oils that taste just fine, and conversely I have tasted many low FFA oils that are woefully defective. Take for example what is possibly the worst tasting defect in olive oil called muddy sediment. It has a foul (for olive oil) taste of parmesan cheese, baby vomit, salami, band-aids, horse stable and fetid milk (and combinations of these). An olive oil can pick up these characters from short term exposure to the particulate olive sediment that remains after oil extraction and which later falls to the bottom of a tank. The aroma compounds that contribute to this defect are very potent. Therefore whilst leaving the oil on the sediment may result in only a small break down of the fat (resulting in a small increase in FFA), this small increase in FFA can be accompanied by a large reduction in flavour quality.

“Refined olive oils have high FFA” – Very wrong!

Paradoxically nothing is further from the truth. Refined olive oils (i.e.  those labelled as pure, light and ‘olive oil’ ) have essentially zero FFA. Yep, zero. The confusion probably arises from the fact that badly made olive oils, i.e. oils with very high FFA’s are those destined for refining in the first place. The major purpose of the refining process is to remove the free fatty acids. This is effectively acheived by adding caustic soda which converts the free fatty acids to soap which can then be physically removed from the oil.

“The FFA of an EVOO increases substantially during storage” – Wrong again.

The production of free fatty acids from intact fat molecules is process that mostly involves the action of enzymes found within the olive which act in the presence of water. Once an olive oil has been properly extracted from the olive and most of the water has been removed (<0.1% remaining), further degradation in bottle leading to higher FFA is very unlikely. Typically, a properly processed EVOO with a low FFA of 0.2% at the time of processing, may get up to between 0.22% and 0.25% after 12 months of storage.  Ok, that’s a 10-20% increase, but when you consider that your typical inexpensive supermarket purchased EVOO from the EU typically starts at 0.60-0.80% the 0.02-0.05 increase in FFA due to storage is inconsequential.

“You can’t taste acidity in olive oil” – Got that one right.

The free fatty acids are so weak that they don’t have the ability to bother our otherwise very busy acid taste receptors.

“If the FFA of an olive oil is X% measured as oleic acid , this mean that the olive oil contains X% oleic acid”. – NO it does not mean that.

The “measured as oleic acid” is simply a unit of measurement. For a few anally retentive lipid chemists, it may seem important, but in reality, whichever fatty acid equivalent (pick one, any one), it really has bugger all effect on the final result. So everyone can ignore the “measured as” bit without any loss of meaning. That’s of course unless you are an anally retentive lipid chemist. But to satisfy the inevitable “% bi (oleic acid) curious” readers, I might have a go at explaining it in a forthcoming post.

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Some Background

All edible fats primarily comprise of fat molecules which in turn are made up of fatty acids. The fatty acids are made up of chains of carbon atoms that are held together mostly by single chemical bonds. But every now and then a double bond appears on the chain. The majority of the fatty acids in olive oil (typically 75%+) contain only one double bond, whilst the fatty acids that dominate most other edible oils contain a higher number of double bonds.

The double bonds in fatty acids allow them to be ‘kinked’. Normally they kink in one particular direction (known as the cis form). Upon heating however, a small proportion of these fatty acids kink in the opposite direction (known as the trans form) as the latter form is the most chemically stable. Our bodies have evolved to be able to use the cis type of fat for normal bodily functions, but not the trans type, the consequence being that consumption of the trans form results in cardiovascular disease.

What’s Been Done

A number of scientific studies have looked at the amount of trans fat formation in edible oils during heating. However, most used either temperatures way above those needed to fry, or the oil was heated for lengths of time that you would only use if you accidentally left oil on the stove before heading out for a vacation. These studies were conducted to replicate the type and length of heating used in commercial deep frying operations.

What’s Been Found

I have included all the studies where the temperature was kept to a recommended frying temperature 180-200C and where there was no possibility of the food that was being fried (if that was the case) contained trans fats themselves. Unfortunately, in all the studies excepting one, the length of time that the oil was heated for far exceeded a typical domestic frying time. However, the data they provided do allow estimates of how much trans fat is formed when cooking for shorter periods of time that are more typical of domestic frying.

Not many of these studies used EVOO as it is an oil that is rarely if ever used for continuous deep fat frying in a commercial environment (comparing how fats perform when heated for long periods of time as occurs in commercial fat frying operations is the purpose of most studies, as the results have big $ implications). However, the same basic chemistry of isomerisation apply to all oils, so I have also summarised the results from these.

Table 1: Trans fat formation in edible fats after heating

*Estimated by linear interpolation at t=20 min. ** 1mg = 1/1,000^th of a gram (which is about 1/50th the weight of a drop of water)

Results

The level of trans fats of most of the edible oils, including olive oil, did not increase by heating using typical temperatures needed for frying. In olive oil, the largest increase was 5 x 1,000^th of a gram per kilogram of oil, and in EVOO no increase was observed.

EVOO is theoretically more resistant to trans fat formation, as being mostly monounsaturated it contains fewer double bonds than that of polyunsaturated fats such as sunflower or vegetable (soybean) oil. Also studies including those presented here have shown that natural antioxidants such as tocopherol and polyphenols which are abundant in EVOO inhibit the fat transing to the dark side.

But for some perspective – The average amount of trans fats in a 170 gm serving of french fries made by two major fast food chains (sampled from 20 countries) was found to be 4 grams (Stender et al. 2006). While this high level may be the result of prolonged high temperature heating, it is far more likely due to the high trans fat content (up to 25%) of the partially hydrogenated oils (1) that are commonly used in the fast food industry.

Based on the worst case result shown in Table 1, i.e. a 5 mg/kg increase in trans fat after heating, you would have to eat 310 servings of olive oil cooked French fries (53 kg or 140lbs worth) to get the same trans fat fix as a single serving cooked in an average fast food establishment (2).

Stender and colleagues in the New England Journal of Medicine stated that….

“Owing to the very high content of industrially produced trans fatty acids in certain fast foods, in many countries it is possible to consume 10 to 25 g of these trans fatty acids in one day and for habitual consumers of large amounts of this food to have an average daily intake far above 5 g”

Based on the studies above, heating 1kg of olive oil yields between 0 and 5mg of trans fat.

5 grams = 1,000 x 5 mg !

References:

Albi, T., Lanzo, A., Guinda, M.C., Perez-Camino, C. and Leon, M. (1997) Microwave and conventional heating effects on some physical and chemical parameters of edible fats. J. Agric. Food Chem. 45, 3000-3003.

Caponio, F., Pasqualone, A. and Gomes, T. (2003) Changes in the fatty acid composition of vegetable oils in model doughs submitted to conventional or microwave heating Int. J. of Food Sci. Tech. 38, 481–486

Hrncirik, K. and Zeelenberg, M. (2014) Stability of essential fatty acids and formation of nutritionally undesirable compounds in baking and shallow frying. J Am. Oil Chem. Soc. 91, 591–598 DOI 10.1007/s11746-013-2401-2

Rani, A.K.S., Reddy, S.Y. and Chetana, R. (2010) Quality changes in trans and trans free fats/oils and products during frying. Eur Food Res Technol. 230, 803–811. DOI 10.1007/s00217-010-1225-7123

Stender, S., Dyerberg, J. and Astrup, A. (2006) High levels of industrially produced trans fat in popular fast foods. New Eng. J. Med. 354, 1650-1652.

Kala, A.L., Joshi, V. and Gurudutt, K.N. (2012) Effect of heating oils and fats in containers of different materials on their trans fatty acid content. J. Sci. Food Agric. 92, 2227-2233.

Tsuzuki. W. (2012) Study of the formation of trans fatty acids in model oils (triacylglycerols) and edible oils during the heating process. JARQ 46, 215-220.

Yang, M., Yang, Y., Nie, S. Xie, M. and Chen, F. (2012) Analysis and formation of trans fatty acids in corn oil during the heating process. J. Am. Oil Chem. Soc. 89, 859–867. DOI 10.1007/s11746-011-1974-x

Footnotes:

(1) Hydrogenation is an industrial process mostly applied to polyunsaturated oils to make them semi-solid, and to increase their resistance to oxidation/rancidity. A by-product of partial hydrogenation is trans-fat production.

(2) given the reasonable assumptions of 1) oil retention of 15%, and 2) EVOO that had been heated for 20 minutes at 180C

Disclaimer: The information provided above is intended as general, and is not meant to be viewed as specific health advice.

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What is Extra Virgin Olive Oil?

The ‘oily’ olive fruit juice extracted from fresh olives using a mechanical process which does not involve the application of excessive heat, additives or solvents.

Should I only buy first cold pressed oil?

Most extra virgin olive oil is produced by spinning the lighter oil away from the other heavier components of the olive such as water and olive matter using a centrifuge. So the term ‘cold pressed’ is redundant.

What’s the difference between extra virgin olive oil and other types of olive oil?

Extra virgin olive oil is a whole, unprocessed food and therefore contains higher amounts of healthy antioxidants. Conversely, ‘light’, ‘pure’, ‘olive oil’ and pomace oils are highly processed products.

Is ‘Pure’ olive oil better than Extra Virgin Olive Oil?

No. The word ‘Pure’ (together with ‘Light’) are purely marketing terms applied to refined olive oil. Pure sounds better than refined.

Do ‘light’ olive oils contain fewer calories than Extra Virgin Olive Oil?

No. All olive oils (and indeed all edible oils) have the same energy values.

This oil is bitter and peppery. Is it ok?

The bitterness and pepperyness indicates that the oil contains healthful antioxidants.. These characters complement the taste of strongly flavoured foods.

Is eating Extra Virgin Olive Oil good for my health?

EVOO’s contain significantly higher levels of monounsaturated fats and antioxidants than any other oil. These attributes are sought after by the health conscious.

Does the colour of olive oil indicate quality?

No. High quality extra virgin olive oils can appear emerald green through to golden yellow.

Where should I store Extra Virgin Olive Oil?

They should be stored in a cool dark place. For longer term storage where oils are used infrequently, refrigeration is a good option.

How long does Extra Virgin Olive Oil last?

They are best consumed young as fresh flavours and healthful polyphenols decline over time.  So when the new season oils are released, buy them.

I’ve heard that you can’t cook with Extra Virgin Olive Oil as its smoke point is too low. Is this true?

The temperature at which good quality (low acidity) extra virgin olive oil begins to smoke is significantly higher than what is needed to fry food (180C). So you can fry in EVOO.

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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.

*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 dec