New Research Could Help Reduce Acrylamide in Black Olives

A newly published scientific study sheds new light on how acrylamide forms in table olives, a finding that could help further reduce its presence in these popular products.

“Acrylamide is a compound that is formed in oxidized black olives during sterilization,” Mercedes Brenes-Álvarez, a researcher at the Instituto de la Grasa of the Spanish National Research Council (CSIC) and co-author of the study, told Olive Oil Times. “This thermal treatment is mandatory to ensure their microbiological safety; nonetheless, it promotes the formation of acrylamide.”

The substance is a byproduct of high-temperature processing in a wide range of common foods. Its presence is monitored worldwide due to research identifying potential carcinogenic effects. Understanding how it forms is considered key to improving processing methods.

Oxidized black olives, often referred to as Californian-style black olives, are darkened through an oxidation process before packing.

According to Brenes-Álvarez, among factors intrinsic to the olives themselves, variety is the only one consistently shown to influence acrylamide concentrations in oxidized black olives.

“Among Spanish cultivars, Cacereña exhibits the lowest concentrations, followed by Hojiblanca and then Manzanilla,” she said. “Unfortunately, there is no clear explanation for these differences. In contrast, other factors, such as fruit ripeness and irrigation conditions, have been studied, but the results remain inconclusive.”

Brenes-Álvarez said acrylamide concentrations in commercial products usually range from 300 to 700 micrograms per kilogram and rarely exceed 800 micrograms per kilogram.

For comparison, current European Union benchmark levels for acrylamide — which are not maximum legal limits — include 500 micrograms per kilogram for French fries, 750 for potato crisps, 400 for roasted coffee and 850 for instant coffee. No specific benchmark currently applies to oxidized black olives in the EU.

While the pathways responsible for acrylamide formation in other common foods, including potatoes, cereals, coffee and pizza, have been extensively studied, researchers have struggled to fully explain what happens in table olives.

“This may be because olives constitute a complex matrix, and the classical routes of acrylamide formation in foodstuffs have been ruled out,” Brenes-Álvarez said.

The latest research may help fill part of that gap. The study focuses on 2,4-decadienal, a compound originating from lipid degradation that researchers found could play a significant role in acrylamide formation.

According to Brenes-Álvarez, 2,4-decadienal has previously been detected in black olives. Its formation has been associated with enzymatic processes in other vegetables, but the specific mechanisms through which it forms in olives have not yet been investigated.

In the new study, researchers compared acrylamide formation in olives with several model systems involving different compounds that could contribute to the process. Their model indicated that 2,4-decadienal could account for roughly one-third to two-thirds of the acrylamide formed in the experiments.

The estimated contribution is consistent with the significant variability in acrylamide concentrations observed among olives, particularly among different cultivars, Brenes-Álvarez said.

“The involvement of 2,4-decadienal in acrylamide formation in olives has been demonstrated in our recent study under in vitro conditions and in lyophilized olive samples,” she said.

Lyophilized samples are olive samples that have been freeze-dried before analysis.

The researchers cautioned against translating laboratory findings directly to industrial production, and the newly identified pathway may represent only part of the process.

“These findings open a new potential pathway for acrylamide formation in olives, which should be further validated under real processing conditions,” Brenes-Álvarez said.

“It is possible that 2,4-decadienal is not the main pathway but rather an intermediate step,” she added. “Alternatively, multiple pathways may also contribute to acrylamide formation in olives.”

Identifying the compounds and reactions responsible for acrylamide formation could enable researchers to develop mitigation strategies targeting the processes that lead to it, an important focus of ongoing olive research.

“If 2,4-decadienal is ultimately shown to play a major role in acrylamide formation, specific mitigation strategies should be developed to target this compound,” Brenes-Álvarez said. “For example, the addition of specific inhibitors of 2,4-decadienal could help to reduce acrylamide formation.”

Researchers at the Instituto de la Grasa have already explored several other ways to reduce acrylamide levels. In one study, Brenes-Álvarez and her colleagues found that packing oxidized black olives at a higher pH resulted in reductions of at least 20 percent.

However, the experiment also illustrated one of the problems that has repeatedly emerged in the search for mitigation measures: reducing acrylamide may alter qualities that define the product for consumers.

The higher-pH treatment slightly affected the olives’ characteristic black color and firmness. The researchers suggested that compounds such as lactic acid could help counteract some of those effects.

“The main challenge is not merely modifying the organoleptic characteristics of the olives, but rather the fact that oxidized black olives are a well-established product with distinctive sensory attributes, particularly their characteristic black color and flavor,” Brenes-Álvarez said.

“In all studies conducted to date, the application of acrylamide mitigation strategies has compromised these attributes to some extent, representing a major limitation for acrylamide reduction in this product.”

In another study, researchers investigated whether packing oxidized black olives in an acidified medium could allow producers to replace sterilization with pasteurization, which requires milder thermal conditions.

The approach aims to minimize acrylamide formation by avoiding more intensive sterilization while maintaining the product’s microbiological safety, although some of its characteristics may be altered.

The European Commission is currently working toward establishing a benchmark level for acrylamide in oxidized black olives. Such a benchmark would not constitute a maximum legal limit. Instead, regulators and food businesses use benchmark levels as performance indicators to assess the effectiveness of measures aimed at keeping acrylamide concentrations as low as reasonably achievable.

“Hence, it is necessary to gain knowledge on the mechanisms involved in acrylamide formation in olives along with the real application of the mitigation measures studied,” Brenes-Álvarez said.

Her team is currently investigating the influence of oxygen during processing on acrylamide concentrations in the final product.