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