Cross-modal interactions in complex food matrices

Abstract

In the light of increasing rates of nutrition related diseases, such as obesity, diabetes, high blood pressure and stroke it is necessary to reduce sugar, salt and fat contents of industrial produced food. Reduction of those components generally leads to changes of the sensory properties of the products and rejection by the consumer. Traditional compensation strategies like the use of taste enhancers, artificial sweeteners or fat replacers often lead to off-flavours and consequently to consumer rejection. This thesis describes cross-modal interactions as an alternative strategy to reduce sugar, salt and fat. In the past cross-modal interactions have been describe to influence taste and texture perception, however up to now research focused on simple model systems lacking in perceptual, as well as chemical complexity. This thesis aimed on the application of cross modal interactions involving aroma modification to complex food systems, such as apple juice and cheese. It was investigated whether aromas are able to modify taste perception in complex food matrices and what the driving factors are to which extend aromas can enhance taste perception. Furthermore different cross modal approaches were combined in order to investigate possible additive or synergistic effects on taste enhancement. At first, the question whether individual aroma components can be identified that have the capacity to enhance taste perception in a complex beverage was addressed [chapter 2]. Apple juice was choosen to represent a complex matrix where interactions between taste and aroma are a natural product characteristic. Ethylhexanoate was identified to significantly enhance sweetness in apple juice, while three other esters selected on the same basis did not show sweetness enhancement. Ethylhexanoate induced sweetness enhancement appeared to be concentration dependent. Concentrations of 5 ppm were found to be most effective to enhance sweetness in this specific system. However, next to sweetness undesired attributes such as flowery and synthetic were also increased significantly. As a conclusion it has to be noticed that flavour balance in complex food is fragile and has to be carefully altered in order to use odour induced taste enhancement as a tool in sugar, salt and fat reduction. Chapter 3 describes the masking of off-flavours induced by ethylhexanoate as described in chapter 2. It was hypothesised that ethylhexanoate induced off-flavours can be masked by restoring flavour balance by equally adding combinations of all four esters. A combination of ethylhexanoate, ethylbutanoate and ethyl-2-methylbutanoate was found to be most effective to restore flavour quality while maintaining ethylhexanoate induced sweetness enhancement. It was concluded that all components that are naturally part of an aroma are needed to achieve a balanced product. Optimisation of odour presentation time in order to achieve maximum taste enhancement was subject to the study described in chapter 4. Swallowing is the key to aroma release during food consumption. It was demonstrated that aroma is most presented most effectively either 1.7 – 2.5 seconds before or 2.6 – 3.6 seconds after the moment of swallow. Aroma presented directly at the moment of swallow was found to be least effective to enhance taste. It is assumed that olfactory receptor neurons (ORN) do not transmit the activation signal to the brain at the exact moment of swallow in order to spare energy and function most effective, as during normal food consumption the aroma is released shortly after swallowing. This hypothesis was further tested in an fMRI study. Preliminary results support this theory, however at the moment of completion of this thesis data evaluation was still in progress. In chapter 5 further optimisation of odour/taste interactions was studied, by investigating the influence of temperature on odour induced taste enhancement. Consumption temperatures differ strongly among products. As aroma release strongly depends on the temperature of the food, it was hypothesised that the magnitude of aroma effects on taste perception changes over consumption temperature. Subjects consumed a sweet and a savoury system at four different temperatures (7, 25, 37 and 50°C). Stimuli temperature and odour presentation were fully controlled by temperature optimised gustometry and olfactometry. Both aroma/taste systems were known to have induced taste enhancement in the past (sweet: apple flavoured tea/ethylhexanoate; savoury: broth/sotolon). No significant effect of temperature was found for either of the stimulus pairs. Results indicate that the complexity of the experiment led to confusion by the panelists under fully randomised stimuli delivery conditions. It was therefore concluded, that temperature effects on the magnitude of odour induced taste enhancement need to be studied in a reduced experimental design. Temporal contrast as a strategy to enhance salty taste was studied in the experiments described in chapter 6. Salty solutions of different NaCl concentrations were presented in alternating sequence by a gustometer, creating a sensory contrast of low-in salt and high-in salt pulses. It was demonstrated that the sensory contrast induced by the pulsed delivery led to significant taste enhancement. It was shown that high concentrations of NaCl delivered in short pulses were most effective to enhance salty taste. It was concluded that pulsed stimulus delivery can be an additional tool to reduce salt and sugar concentrations in industrial produced food. Chapter 7 combines both cross-modal strategies to enhance taste in this thesis so far, odour induced taste enhancement and temporal contrast of stimulus delivery. Subjects were presented with taste and aroma pulses timed via a gustometer. The aroma was either presented in-phase or out-of-phase with the taste stimulus. A cumulative effect of aroma/taste interactions and temporal contrast of tastant delivery was found, resulting in higher taste enhancement than each of the strategies alone. Overall highest sweetness enhancement was observed when aroma and taste pulses were presented out-of-phase. Texture modification is a third cross-modal strategy to enhance taste perception. Combinatory effects of texture modification and odour induced taste enhancement are subject to the study reported in chapter 8. Apple juice containing gels were engineered differing in textural properties, aroma and sugar concentration. In contrast to the results presented in chapter 7, only an additive but no synergistic effect was found for those strategies. Texture modification was found to be more effective than aroma modification. Chapter 9 studies effects of aroma on the taste and the texture of cheese and dairy model gels. In the first study subjects consumed different types of cheese with and without a nose-clip. This way the contribution of the aroma phase on the flavour and texture perception of cheese was studied. Saltiness was influenced significantly by the aroma. It was concluded that the aroma of cheese strongly contributes to a cheeses salty taste. Furthermore it was observed that a decrease in cheese firmness strongly correlated with an increase in buttery aroma notes. This was further studied in fully controlled dairy model gels. For gels tasting of cream cheese a significant decrease in firmness was found upon increasing aroma concentration. No effect of butter aroma on firmness was found for yoghurt-like gels. This once more demonstrates the importance of congruency between stimuli. Only congruent sensory impressions can influence each other. In addition, an increase in creaminess was observed with increasing aroma concentration. The results described in chapter 9 clearly show that aromas cannot only modify taste perception, but also are a valid tool for texture modification. This thesis demonstrates for the first time, that flavour and texture modification and subsequently the reduction of sugar, salt and (possibly also) fat can be achieved by using cross modal interactions in complex food matrices. New methods have been developed and existing methods have been combined to study aroma/taste interactions in fully controlled settings. Its multidisciplinary approach combines chemical, physical and psychological knowledge in order to discuss and explain results. It furthermore shows that combinations of different strategies are most effective in order to achieve healthier products reduced in sugar, salt and fat. </p