Virgin olive oil and biophenols in oil-in-water food emulsions: stability and interactions in relation to the release of aroma compounds

The use of olive oil in several food products as been increasing in the past few years due to its healthy fatty acid composition, content of phenolic compounds and appreciated flavour. The addition of natural phenolic compounds in foods is also an interesting issue for researchers and food industry, as several challenges have to be addressed, such as lipid oxidation and the effects on the physical stability over storage. Usually, emulsions used as sauce or dressing creams are not formulated with olive oil, mainly due to its price, but its use is interesting for some niche markets. With specific regard to olive oil, there is usually the aim is to preserve its characteristics and its appreciated flavour, both in terms of aroma and taste. However, little is known about the possible interaction of constituent phenolic compounds of virgin olive oil on the emulsions properties, both in terms of physical stability and oxidation rate, but also for its impact on aroma release. The present thesis deals with the use of olive oil as fat phase in mode oil-in-water (O/W) emulsions, with also the addition of phenolic compounds extracted and recovered from olive mill wastewater. Indeed, the presence of valuable phenolic compounds also in the by-products of olive oil extraction, i.e. olive mill wastewater, is of great importance in terms of environmental sustainability for its high pollutant risk, but it is also interesting for the recovery and application of these biophenols to be further used in food products to design functional foods. Moreover, olive phenolic compounds have been shown to protect some key aroma compounds in virgin olive oils, with consequent impact on the headspace aroma release was demonstrated. The effect of ingredient interactions on the release of aroma compounds and therefore on the final aroma and flavour of the product was also addressed in the present thesis. The first chapter presents an overview of the most up-to-date researches on olive oil chemistry and flavour, on food emulsions characterisation and on the emulsion destabilisation phenomena. The interaction of emulsion ingredients, particularly phenolic compounds and proteins (especially milk proteins) has been detailed, with emphasis on their effect on emulsion stability and on aroma compounds. The principle of “molecular gastronomy”, i.e. the use of new ingredients but also the application of an uncommon combination of ingredient in a food product must be studied to assess the final flavour effect, based on the food pairing theory, has been detailed. Being the study of aroma release particularly important, the latest research papers about food aroma release and model systems simulating mouth conditions, also regarding the interaction of phenolics and proteins, have been reviewed. The third chapter presents an experimental study where a model olive O/W emulsion has been created by using refined olive oil and the following ingredients: olive mill wastewater phenolic extracts, whey protein isolate, xanthan gum. The phenolic compounds extracts were added to create a functional food product with possibly effect on the lipid oxidation status and potential human health benefits, but the effects on physical stability should be also studied. For this reason, the creaming rate, mean particle size, particle size distribution and other physical properties were assessed and reported in order to verify the effects of the hydrocolloid, proteins and phenolic added. A mathematical model has been then created and presented in chapter four to study the emulsion behaviour by means the response surface methodology to better describe the effect and interaction of these ingredients. Not discrete concentrations but ranges were used for each of the compound in the same emulsions system, by using the statistical design known as central composite design. The model performance and the full description of the emulsions was reported and discussed. Finally, in chapter five the headspace aroma release from emulsions/dispersions was the object of the study. Dispersed systems were created by using olive oil with added olive phenolic compounds and whey protein isolate. Their effect on the aroma release was studied by solid-space micro-extraction and analysed using gas-chromatography coupled with mass-spectroscopy. Moreover, a retronasal aroma simulator system was applied in order to evaluate the possible retronasal release in vitro. This system used human saliva and therefore also the effects of salivary proteins and other components on the mixture was studied, in relation to the release of olive oil aroma compounds