Avocado pigments in ripening fruit

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  • Published: Feb 12, 2007
  • Author: Steve Down
  • Channels: HPLC
thumbnail image: Avocado pigments in ripening fruit

The avocado plant originated in south-west Mexico around 7000-5000 BCE but was not cultivated until several thousand years later. It was named ahuacatl by the Aztecs, a word literally meaning fruit of the avocado tree or testicle due to its shape, but the Spanish explorers could not get their mouths around this, so renamed it aguacate, then avocado. It appeared that the word avocado sounded like the Spanish word avocado (now abogado), meaning a lawyer. The fruit is also known as the avocado pear or alligator pear, due to its shape and rough green skin.

In modern times, the fruit of the avocado is prized as a food, having a strong flavour and a fatty, smooth consistency. In fact, both the fruit and the oil contain high levels of the beneficial mono-unsaturated fats and relatively low levels of the unhealthy saturated fats. They are also rich in vitamin B, E, K and folate, and have 60% more potassium the bananas.

Avocado oil, one of the few vegetable oils derived from pulp rather than seeds, is also in high demand as a cosmetic as it has moisturising and regenerative properties. It is added to many skin care products such as soaps and body oils.

It has been claimed that both the fruit and the oil have beneficial health properties due to the presence of relatively high quantities of chlorophylls, carotenoids and anthocyanins, as well as the vitamins. If so, then the method of preparation might have a significant effect on their contents. The skin and the different sections of the fruit, and the oils produced from these separate portions, could well contain different amounts of the pigments.

This possibility has been studied by scientists in New Zealand and described by senior reporter Allan Woolf from The Horticulture and Food Research Institute of New Zealand. The researchers measured the pigments in Hass avocados, the most popular variety, at various stages of ripening. The fruit skin and dark green flesh (next to the skin) pale green flesh and yellow flesh (next to the seed) were frozen and extracted separately with acetone.

The oils produced from these individual sections were removed from freeze-dried tissue by accelerated solvent extraction with hexane, then the solvent was evaporated off. The carotenoids, chlorophylls and anthocyanins were measured in all samples by HPLC with fluorescence detection.

Lutein, alpha- and beta-carotene, neoxanthin, violaxanthin, zeaxanthin, antheraxanthin, chlorophyll a and b and pheophytin a and b were the major pigments found in the skin, flesh and oil. Chlorophyllide a and b were found in the skin and flesh only. In the oils, water-soluble pigments such as the chlorophyllides and anthocyanins were not detected because they were not extracted into hexane.

As the fruit ripened and changed colour from green to purple, the level of the anthocyanin cyanidin 3-O-glucoside in the skin rose markedly and that of chlorophyllide a decreased. In the flesh, the amounts of chlorophylls and carotenoids changed very little with ripening. In the oils, ripening reduced the levels of some carotenoids.

Overall, the pigment concentrations varied markedly between the skin and flesh, between the different flesh types, and between flesh and oil.

However, the researchers point out that their oil extraction method differs from the commercial method of cold pressing, a point which they are addressing in ongoing work. They will also examine the effects of the amount of skin on the pigment levels and quality of the cold-pressed oils.

Nevertheless, the results to date indicate the broad disparity in the concentrations of the beneficial pigments and antioxidants in the fruit sections and oils, a point which should be borne in mind by oil producers. It appears that those who choose to maximise pigments concentrations should incorporate as much green flesh and skin as possible.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Allan Woolf
 Allan Woolf

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