Phenolics in extra virgin olive oil – Recent research results
admin | February 24, 2013For once I have a theme!
But first, I must rid myself of a burden. There are no polyphenols in extra virgin olive oil. I’ve been resisting saying so for years, but now I must now confess. There is nothing poly(merised) about them. Yes, a couple of them could be a bit binary (depending on how you define it), but poly, errr no. Yes, many have a few optional extras like having attached sugars and esters hanging off them, which most certainly gives them interesting biological and health giving properties, but they are generally comprised of single simple phenolic monomers. They remind me of those troglodytes creatures that ruled the world during the Cambrian Period. There were thousands of different types – all very intricate and ecologically specialised, but in the general scheme of things, they were relatively biologically simple critters. Now I’m rid of the weight of this chemo-trivia, here are a few interesting, and practical research results about the phenolics in EVOO.
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The researchers compared the ability of total phenolics and absorbance at 255nm (K255) to predict the perceived bitterness of EVOO’s. The 35 oils from 4 Italian varieties ranged in bitterness from 1 to 7 on the International Olive Council rating scale which corresponded total phenolics of around 200 to 650 mg/L, and to 0.05 to 0.65 in K255. Total phenolics were found to be a poorer predictor of perceived bitterness (correlation 0.69) compared with the almost perfect, K255 (correlation of 0.98). The bitterness of these oils could be predicted by multiplying the K255 measure by 14 and subtracting 0.6
Comment: The K255 index for bitterness was first proposed in 1992 and has been used quite widely by the international industry. Compared to total phenolics, K255 is a very easy and quick and accurate analysis. I’m not sure that the paper broke any new ground except that it provided a link between the K255 measure and bitterness ratings using the IOC rating scale. But of course the relationship given above is wholly dependent on the panel that provided the ratings. While bitterness is one the attributes on the official IOC rating sheet, to the best of my knowledge, unlike the defects, cross panel comparisons for bitterness is not conducted. Well it wasn’t done when I was a panel leader anyway. Regardless of panel differences, K255 still seems to be a good predictor of relative bitterness amongst samples.
The relatively poor relationship between bitterness and total phenolics was unexpected. I have seen other data sets where the correlation was much stronger than reported here. Interestingly, the method used to determine total phenolics was not given which was a major oversight given that the method of extraction of phenolics from the oil plays a crucial role in the outcome. The method is also notoriously subject to variable results (where a 10% error is accepted by those who do them), so doing triplicate or more replications would be needed to ensure that the poor relationship between total phenol measures and bitterness wasn’t due to measurement error.
Faviti et al. (2013) Extra virgin olive oil bitterness evaluation by sensory and chemical analyses. To appear in: Food Chemistry DOI: http://dx.doi.org/10.1016/j.foodchem.2013.01.098
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Crushing between 1 and 10% leaves with ripe olives resulted in oils with higher free fatty acidity, peroxide value and K232 (measures of primary and secondary oxidation respectively). Adding leaves produced oils with higher chlorophyll aka they were greener. Adding over 5% leaves also increased oxidative stability and tocopherol content.
Comment: Lab scale work. Phenolics were not measured which seemed to be an oversight given that both tocopherols and phenolics contribute to oxidative stability. Also personally, I’d like to think that the oil I buy is made from olive fruit rather than leaves. Call me old fashioned.
Malheiro et al. (2013) Effect of olive leaves addition during the extraction process of overmature fruits on olive oil quality. Food Bioprocessing Technology,6, 509–521.
DOI: http://dx.doi.org 10.1007/s11947-011-0719-z
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Malaxing for 40 minutes at 35C caused a 25% increase in both total phenolics and the pungent phenolic oleocanthal compared with malaxing at 25C (Figure 1), which could be explained by a 50% reduction of the activity of the major enzyme (polyphenol oxidase, PPO) responsible for oxidative losses of phenolics at the higher temperature. The longer malaxation time, the lower the total phenolics.
Figure 1: Concentrations (mg/kg) of oleocanthal and total phenolics as a function of malaxation temperature (malaxation time = 40 minutes).
Comment: Mill scale work (yay!). The fact that high temperatures can knock out PPO probably also explains why dipping olives into hot water prior to processing results in oils with higher phenolics.
Bitchy comment: If you believe that EVOO is just a polyphenol dietary supplement (rather than a tasty food), then these results suggest that you should insist on HOT pressed olive oils.
Taticchi et al. (2013) The influence of the malaxation temperature on the activity of polyphenol-oxidase and peroxidase and on the phenolic composition of virgin olive oil. Food Chemistry, 136, 975–983.
http://dx.doi.org/10.1016/j.foodchem.2012.08.071
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12 Spanish people were fed a diet that was devoid of polyphenols except for an olive oil fortified with olive oil phenolics i.e. +250, 500 and 750mg/L. Blood plasma content of the key phenolic substances that had passed from the digestive system increased substantially when the subjects ate the oils with +500mg/L phenolics. Eating oils with +750 mg/L phenolics did not increase blood plasma phenolics over the +500mg/L diet. The authors appeared to recognise that phenolics impact on the taste and therefore the acceptability of EVOO. They stated that “The sensory acceptance is the principal barrier for the acceptance of a functional food”. In the case of EVOO high phenolics relates to high bitterness and/or pepperyness. So they also determined the taste acceptability of the oils with added phenolics. As phenolic content went up, acceptance by the tasters declined. The authors felt that the olive oil with a content of 500 mg/kg of phenolics provided “a good pharmacokinetic response and a good sensory acceptability” (in laymans terms, a good compromise between being healthy and not being overly bitter). The olive oil with 500mg/L was rated on average somewhere between “I like it a bit” and “I neither like nor dislike”. They went on to say that” Moreover, by consuming 30 ml of M-EVOO (+500mg/L) it is easy to reach the 5 mg daily dose of hydroxytyrosol and derivates recommended by the EFSA Panel”
Comment: The authors should be congratulated for recognising that long term consumption of a healthy food is only assured if it has an acceptable taste. EVOO included. But why not consume 60gms of something you really like, rather than 30 gms of something that you only like a bit?
Rubio et al. (2012) Impact of olive oil phenolic concentration on human plasmatic phenolic metabolites. Food Chemistry, 135, 2922–2929.
http://dx.doi.org/10.1016/j.foodchem.2012.07.085
Hi Richard,
First, re your opener: there are at least some true polyphenols in EVOO: there are, at least, apigenin (5,7,4′-trihydroxyflavone) and luteolin (5,7,3′,4′-tetrahydroxyflavone), and potentially others undiscovered. Yes?
Re: the study on crushing olive leaves in with the olives: I’m rather glad to see that someone finally did this study and that it “resulted in oils with higher free fatty acidity, peroxide value and K232”. I’ve been concerned for some time that someone might try to exploit those of us “who believe that EVOO is [mostly] a polyphenol dietary supplement ([and secondarily] a tasty food)” 😉 by spiking the phenolic counts this way. (My concern would be getting the ‘wrong’ phenolic species, and also the possibility of toxic compounds in the leaves). If doing this turns the oil to crap, producers won’t stoop to this, and thus we health nuts will be protected by the delicate sensibilities of you aesthetes 😉 .
Re: the increase in phenolic levels from malaxing at 35ºC http://dx.doi.org/10.1016/j.foodchem.2012.08.071
Bitchy comment: If you believe that EVOO is just a polyphenol dietary supplement (rather than a tasty food), then these results suggest that you should insist on HOT pressed olive oils.
Well, the effects on phenolics and temperature aren’t consistently reported, and if anything it appears to me that at least as many studies have found that extraction is maximal at temperatures more like 25-30ºC, similar to the EU “cold-press” limit of 27ºC, which some hypothesize could be due to increased PPO activity at higher temperatures
(Note the publication date. See also their refs 14, 17, 18, 40, & 41 — older
studies which tend to find higher temps counterproductive)
And the effects on (components of) organoleptics aren’t consistently negative either. Now, much of this is experimental-scale work, in contrast (as you note) to the present study’s work with a real, operating mill — but it’s also quite clear from a lot of this that the temperature optima of the various cultivars’ PPO activity varies by genotype, which likely explains may explain some of the variation between studies, too.
Also, there are very few data on the effects of temperature on oxidation: those of us who consume EVOO mostly for health actually don’t “believe that EVOO is just a polyphenol dietary supplement (rather than a tasty food)”, but also want to see high MUFA content and very low oxidation — and would certainly prefer that the stuff be delicious, too. I’d be concerned that long periods of malaxation, even at 40ºC, could get some bad action going. Ways to hit a sweet spot of high phenolics, low oxidation, and (yes) nice floral notes or whatever 😉 would be ideal, not just setting off Messrs. Folin and Ciocalteu’s reagents.
Re: the study reporting that (as you summarize) As phenolic content went up, acceptance by the tasters declined. The authors felt that the olive oil with a content of 500 mg/kg of phenolics provided… a good compromise between being healthy and not being overly bitter …. They went on to say that” Moreover, by consuming 30 ml of M-EVOO (+500mg/L) it is easy to reach the 5 mg daily dose of hydroxytyrosol and derivates recommended by the EFSA Panel”
RG Comment: The authors should be congratulated for recognising that long term consumption of a healthy food is only assured if it has an acceptable taste. EVOO included. But why not consume 60gms of something you really like, rather than 30 gms of something that you only like a bit?
Well, again, the more educated wing of the health nut brigade really doesn’t regard EVOO as simply a phenolic supplement, such that we’re after 5 mg of HT (or whatever) and would be just as happy to get it from 60 mL of low-dose oil, 30 mL of high-dose oil, or a pill. Part of what makes the phenolics in EVOO healthy is that they’re protecting the oil from oxidizing from malaxation through to ultimate consumption. A higher starting level means a better oil when finally consumed. And then, the concentration of phenolics in association with the triglycerides will determine the transfer rate of those into chylomicrons and thence lipoproteins: the molar ratio actually counts in this.
And, there’s the calories, of which an extra tablespoon has 120.
Thanks, as always, for your valuable dig thru’ the research and analysis.
Hmm… my links on malaxation temperatures vs phenolics seem to have been truncated into nonexistence:
http://onlinelibrary.wiley.com/doi/10.1002/ejlt.200700307/abstract
http://www.aidic.it/IBIC2008/webpapers/103Aliakbarian.pdf
http://www.sciencedirect.com/science/article/pii/S0308814600001941
http://dx.doi.org/10.1007/s00217-008-0977-9 http://www.ncbi.nlm.nih.gov/pubmed/19338278
http://www.aziendabettini.com/filezip/gramolatura.pdf
(This is the study to which I drew attention to their refs 14, 17, 18, 40, & 41 — older
studies which tend to find higher temps counterproductive)
Firstly once again, thank you for your thoughtful reply.
I’ll do it in steps as you raise quite a few things.
Firstly the polyphenol vs phenolic thing. From the outset, it does appear to be just semantics, and it probably is.
The simplest phenolic compound is called “phenol”. Not surprisingly as its basic structure defines the class of compounds that bears its name. Phenol is comprised of 6 carbons joined in a ring with a oxygen+ hydrogen atoms bound to one of the carbons.
In nature, there are lots of compounds that have that basic 6 carbon ring structure with the hydroxyl (OH) group hanging off one, but the basic building block tends to be three 6x carbon rings bound to each other with hydroxyl groups hanging off them in various places. The compounds you mention have exactly this structure. But remember, this is the basic building block provided by nature, so chemists don’t generally think of them as “polyphenols”, rather they are considers as monomers (despite that they are poly looking compared with the basic phenol).
When you get these basic 3 ring molecules hooking up to each other (2-20) to form complexes – which are considered polyphenols. Google something like “proanthocyanidin B3” and you’ll see what mean.
Yes, I’ve seen the max at 27C papers as well, but I’ve seen one or two like this one whereby there appears to be no optimum. As an aside I can’t thing of any other product in which phenolics are being extracted whereby doing it sort of long, and sort of hot, results in maximum extraction.
Incidentally, the optimum functioning temperature for polyphenol oxidase is 25C (and pH 5-6). So go figure.
I’ve tasted a lot of overmalaxed oils in my time. Some taste flat, others metallic (which I equate to bitterness), and others burnt. So I believe that there are a number of interacting factors at play during malaxation. I’m surprised that given the number of studies that the picture re phenolic extraction hasn’t been nailed yet.
But phenolics are interesting creatures. I think of red wine fermentations. If you ferment on skins until almost dryness, the colour isn’t as strong as if you ferment to complete dryness (as you have extracted for a shorter time), but in the longer term the former ends up with better colour as the colour is more stable due to complexation with other phenolics such as tannins. I’ve often wondered if something similar happens in olive oil – which may account for differences in the studies. I’d be surprised if the phenolics coming out of the malaxer are the same quantitavely or qualitatively as those coming out of the decanter. Just speculation I know.
Hi Richard,
There is an active debate here in Greece about the health benefits of Oleocanthal and Oleacein as it relates to our recent ability to directly measure these two compounds in one measurement using Quantitive -NMR developed by Dr. Magiatis at the Athens University of Pharmacognocy. The question that we are attempting to answer is whether this measurement of Oleocanthal and Oleacein would qualify for the Health Claim under the EU regulation 432/2012 which I copy below.
“Olive oil polyphenols
contribute to the protection
of blood lipids from
oxidative stress”
“The claim may be used only for olive oil which contains at least
5 mg of hydroxytyrosol and its derivatives (e.g. oleuropein complex
and tyrosol) per 20 g of olive oil. In order to bear the claim
information shall be given to the consumer that the beneficial
effect is obtained with a daily intake of 20 g of olive oil.”
Thank you for taking the time to read my email and I hope you can shed some light on whether a 5mg of Oleocanthal and/or Oleacein per 20gm (of EVOO) is sufficient to be included in the above mentioned health claim.
Thanks in advance Athan Gadanidis
Hi Athan
It appears that the regulation is saying that the EVOO must have 250mg/kg of total polyphenols (however as there is no mention of the unit of measurement i.e. caffeic acid equivalents or tyrosol equivalents etc this means little). I’m somewhat surprised that a regulation doesn’t specify this.
That “hydroxytyrosol and its derivatives” bit is presumably a technobabble way of just saying total polyphenols as many of the polyphenols in olive oil fall into this class. Nonetheless, oleocanthal is just one of the phenolics that contribute to the overall total. It is unlikely that there would ever be 250mg/kg of just oleocanthal in an oil no matter what unit was decided upon, so the debate you mention is probably a little mute.
RG
Thank you for your thoughtful reply. It is the conclusion of many others here in Greece. Unfortunately the Minister of Rural Development and Food, Professor Tsaftalis in response to a question posed in the Greek Parliament stated that: oleocanthal and oleacein are not included in the EU Regulation 432/2012… I have asked for clarification from the Minister and the EU on this matter as well and I await their response. Unfortunately in Greece right now politics and special interests trumps rational thought.