Exploitation of virgin olive oil by-products (Olea europaea L.): phenolic and volatile compounds transformations phenomena in fresh two-phase olive pomace ('alperujo') under different storage conditions

Abstract

Background: Much effort has recently been spent for re-using virgin olive oil by-products as nutraceutical ingredients for human diet thanks to their richness in bioactive phenols, but their management is not easy for producers. We aimed to provide useful information for a better management of fresh olive pomace before drying, by studying the phenolic and volatile compounds transformations phenomena of fresh olive pomace stored under different conditions planned to simulate controlled and uncontrolled temperature conditions in olive oil mills.

Results: The evolution of the phenolic and volatile compounds was studied by high-performance liquid chromatography-diode array detector mass spectrometry (HPLC-DAD-MS) and headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS). The phenolic profile varied rapidly during storage: the verbascoside content decreased about 70% after 17 days even at 4 °C, while the content of simple phenols such as hydroxytyrosol and caffeic acid increased over time. The low temperature was able to slow down these phenomena. A total of 94 volatile organic compounds (VOCs) were detected in the fresh olive pomace, with a prevalence of lipoxygenase (LOX) VOCs (78%), mainly aldehydes (19 490.9 μg kg^-1 ) despite the higher number of alcohols. A decrease in LOX volatiles and a quick development of the ones linked to off-flavors (carboxylic acids, alcohols, acetates) were observed, in particular after 4 days of storage at room temperature. Only storage at 4 °C allowed these phenomena to be slowed down.

Conclusion: To preserve the natural phenolic phytocomplex of fresh olive pomace before drying and to avoid off-flavors development, storage in open containers must be avoided and a short storage in cold rooms (7-10 days) is to be preferred. © 2021 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Keywords: HS-SPME-GC-MS; nutraceuticals; olive mill by-products; olive pomace storage; polyphenols; secoiridoids.

Figure 1

Evolution over time of the content of phenolic compounds in ‘alperujo’ according to type of storage: at 4 °C in the fridge in full and closed containers (4 °C), at room temperature in full closed containers (close) and at room temperature in open containers (open). Data are the mean of three independent measurements and are expressed on dried matter basis. For each molecule and for each type of storage, different letters indicate a different content over time at P = 0.05.

Figure 2

Evolution over time of the content of total tyrosol (A) and hydroxytyrosol (B) after acid hydrolysis in ‘alperujo’ according to the type of storage: at 4 °C in the fridge (4 °C), at room temperature in full and closed containers (close) and at room temperature in open containers (open). Data are the mean of three independent measurements and are expressed on dried matter basis. For each molecule and for each type of storage, different letters indicate a different content over time at P = 0.05.

Figure 3

Evolution over time of volatile compounds in ‘alperujo’ according to type of storage: at 4 °C in the fridge (4 °C), at room temperature in full and closed containers (close) and at room temperature in open containers (open). Volatiles originated from the LOX pathway (A), (E)‐2‐hexenal (B), 1‐hexanol (C) and volatiles originated from the LOX pathway without C6 alcohols and esters (D). Data are the mean of three independent measurements and are expressed on dried matter basis.

Figure 4

Evolution over time of the sum of VOCs belonging to different chemical classes in ‘alperujo’ according to type of storage: at 4 °C in the fridge (4 °C), at room temperature in full and closed containers (close) and at room temperature in open containers (open). Data are the mean of three independent measurements and are expressed on dried matter basis.