Introduction:
Adaptation in sensory evaluation of food refers to the slow drop of sensation (in constant magnitude) after the stimulus has been fully switched on (Lawless & Heyman, 1993). They also added that after a certain time this sensation can completely disappear or in some instances the taste persists that shows a degree of taste adaptation. This idea usually occurs when the perceived taste intensity of one product reduces the other different product. For example, water can taste sour or bitter after a salt adaptation. On the other hand, taste interaction masks the interaction between mixtures of different tastes (Lawless & Heyman, 1993). A solution of sucrose (sweet) and quinine (bitter) will taste less sweet than a pure sucrose solution of the same strength.
According to Lawless and Heyman (1993), these sensations are distinguished by taste buds present on the tongue namely fungiform, circumvallate and foliate. They contain 30 to 50 taste receptor cells (TRCs). TRCs protrude hair-like cilia called microvillae to the surface of the taste buds that form the taste pore. This then comes into contact with the saliva. It is believed that flavoursome molecules from food bind to microvillae at the top of the taste cells. After binding, the taste-producing compounds (tastants) transfer sensations to the tongue and beyond by different pathways. For chemicals that produce sweet, umami and bitter tastes they use similar receptors called G- protein coupled receptors (GPCR). Basically, GPCR is an inner membrane protein receptor that is bound to a ligand, in this case sucrose, which causes the G-protein to dissociate that eventually open an ion channels resulting a cell to depolarise and send an action potential to the brain. While sour and bitter rely on ion channels. The citric acid in this case binds to an ion receptor that causes it to open and allow positive ions to flow in, that eventually synapse an action potential to the brain (Lawless & Heyman, 1993).
This experiment is divided into four sections and it begins with evaluating the sour intensities using six coded samples in the first two sections. However, the sixth sample of second section is sweet. Then, the 3rd section is composed of two experiments which test for sweetness intensities. Finally, the 5th experiment has three phases (sub-sections). The first is testing the taste intensity by holding your nose. Secondly is identifying while releasing the nose- hold and finally, is to scale the taste intensity by sniffing the unknown liquid sample.

Aim: To study the physiological changes in taste adaption and mixture interaction and how it affects the tasters’ perception.
Materials:
Experiment 1 (sample codes): 0.11% Citric acid monohydrate | Experiment 2(sample codes): 0.11% Citric acid monohydrate | Experiment 3 (sample codes): 5% Sucrose + 0.05% Citric acid | Experiment 5 (sample code): 10% Vanilla solution | 496 | 584 | 565 | 932 | 382 | 272 | Experiment 4 (sample codes): 5% sucrose + 0.05% citric acid | | 129 | 391 | 401 | | 756 | 706 | | | 901 | 687 | | | 243 | 423* | | |
423* is a solution of 5% Sucrose + 0.05% Citric acid.

Method:
Participants performed four experiments. Firstly, they evaluated sourness intensity in five samples, holding each solution in their mouths for five seconds. They spat each into a plastic container and washed their mouths with water. Then, for the sixth solution in the first experiment, participants waited for one minute before they evaluated the sourness. The sourness was scored numerically 0 -10, the higher number being the most sour.
In the second experiment the participants assessed sourness intensity of five differently coded samples similarly to the first experiment. However, the sixth sample was evaluated according to its sweetness in the range 0-10.
The third and fourth experiments evaluated the sweet intensity (0 – 10) of sample coded 565 and 401 respectively; participants washing their mouths between tastings.
For the last experiment, the participants were given a liquid and rated the taste and flavour intensities while holding their noses. They repeated the process after releasing their noses and drinking simultaneously. The participants were then required to identify the unknown liquid by smelling.

Results:
Figure 1: Results from experiment 1

Figure 1 depicts average sourness scores and error bars from experiment 1 for each of the six solutions. There were 64 participants

Figure 2: Results from experiment 2

Figure 2 from experiment 2 summarises the average sourness scores and error bars for the first five samples (64 participants). For sample 423* the average sweetness score is given, both scales being 0 – 10
Figure 3: Results from experiment 3 and 4

Figure 3 gives the average sweetness score with error bars (n=64) for samples 565 (experiment 3) and sample 401 (experiment 4).

Figure 4: Results from experiment 5

Figure 4 gives the average taste intensities observed for each phase for the unknown liquid sample 932 for the whole class; 64 participants. Phase 1 is recording the taste intensity (0 – 10) by holding the nose. Phase 2 is recording the taste intensity (0 – 10) by swallowing and releasing the nose simultaneously. Phase 3 is the taste intensity (0 – 10) by sniffing the liquid sample.
Discussion:
All samples in experiment 1 had the same concentration of citric acid. Figure 1 shows that the taste intensity perceived by the participants decreased as they went through the first five samples. This is because taste adaptation for sourness has taken place. The constant stimulation of the taste receptors has caused responsiveness to decrease. However, after a minute break the receptors had recovered and the sourness perception of the participants returned to normal.
The first five samples in experiment were the same strength of citric acid as experiment 1. Sourness perception started lower but continued in a downward trend as shown in figure 2. Figures 1 and 2 are similar in this respect, but it is postulated that full recovery from the last sample (243) of experiment 1 had not occurred before starting experiment 2. The sixth sample (423*), containing sucrose, was readily tasted for sweetness because sweetness uses different taste receptors which were not adapted by earlier sucrose solutions. Thus, the sensitivity of participants to sour taste has decreased while to other tastes such as sweet are not affected. Other tastes like salty and bitter will presumably not be affected by sourness adaption.
Figure 3 displays that average scores for samples 565 and 401 are the same. These results are true as both samples are made up of the same concentration of citric acid and sucrose. In comparison to 423*, samples 565 and 401 gained higher scores. This means that the sucrose was able to mask the sour taste of citric acid. According to Shallenberger (1993), the sweetness of sucrose suppresses sourness. He added that the effect of sucrose and citric acid may have an underlying chemical basis. Since increased concentration of sucrose decreases water activity, weak acids such as citric are unable to dissociate. This suppression caused citric acid to taste less sour than usual and sweetness becomes more dominant.
Figure 4 showed an increased taste perception from phase 1 to phase 2 to a solution containing vanilla. In the first trial the nostrils were pinched so volatile materials were restricted from retronasal stimulation by failing to pass from the back of the oral cavity through the nasopharynx area to the olfactory epithelium (Lawless & Heyman, 1993). When the nose was not pinched, normal volatile sensations in the lining of the nose returned, thus the sensation to vanilla solutions was stronger. Phase 3 of this experiment allowed volatile vanilla compounds to enter the nose by the orthonasal route. Odorous molecules unrestrictedly enter the nares of the nose and activate the sensory cells, thus giving a strong response as depicted in figure 4 (phase 3).

Conclusion: The experiments have shown that taste is readily diminished by repeated exposure of taste buds to sour solutions of citric acid. This adaption can be reversed by removing the taste stimulation for a period of time to allow the receptors to recover. Deactivation of taste sensations to sour stimulation does not impact on sweetness because different tastes have different taste transduction process involving different biochemical pathways. Mixing the tastes of sweetness and sourness caused changes in taste perception, the sweetness inhibiting sourness sensitivity. This and other interactions between the five taste elements are expected to be very important in food acceptability.
Experiments performed here with vanilla have also shown the importance of smell to taste. Volatile molecules activate sensors in the nasal region, contributing strongly to overall sensitivity.
References:
Shallenberger, R. S. (1993). Taste Chemistry. Alden Multimedia: Northampton, UK
Lawless, H. T & Heyman, H. (1999). Sensory Evaluation of Food: Principles and Practices. Aspen Publishers, Inc.: Gaithersburg, MA