Abstract
Taste perception is an important factor affecting human life. It plays an undisputed role in public health through its effects on food choice, food preference, dietary intake, nutritional status, and subsequently quality of life. Previous researchers have extensively studied taste perception and suggested that various factors (e.g., genetics, age, gender, culture, ethnicity, emotional status, diseases, treatments, smoking behaviour, alcohol, and drug usage) significantly influence taste perception. Owing to the well-known association of taste perception with eating behaviour and health it is of importance to identify factors that can affect taste perception.
The effect of diet on health is becoming an increasingly popular topic of debate due to the high prevalence of lifestyle diseases (e.g., diabetes, cardiovascular diseases, cancers, and obesity), and the role of a person’s diet in promoting/or preventing diseases. Hence, interest in what is perceivedin being healthier diets in particular plant-based diets (e.g., vegan or vegetarian diets) have recently increased. Shifting to vegan or vegetarian diets and demands for vegan or vegetarian products is increasing worldwide due to concerns around health, sustainability, and animal ethics. Changing consumers' food behaviour is a challenge for food and beverages manufacturers who need a wide range of information to optimise food product development. Various aspects of vegan and vegetarian diets such as health benefits, nutritional advantages or disadvantages, alternative food and beverage products, meal recipes, environmental or economic benefits have been investigated by researchers and have provided
more knowledge around these factors. A few studies have recently highlighted the influence of frequent/regular consumption of certain food types (e.g., fast food, sweetened beverages, or high-salted food) on taste perception. Overall, it remains unclear whether a person’s diet also can influence taste perception, or individuals following a vegan or vegetarian diet have different taste perceptions relative to omnivores? Understanding the possible impact of diet on taste perception is important to inform food manufacturers about product formulation, the effect of ingredient change, or designing food products with higher acceptability and liking matched with taste requirements of different populations and target consumers.
Taste perception is frequently evaluated by the assessment of taste sensitivity through the measurements of taste detection thresholds of individuals. The measurement of taste detection thresholds needs to be not only accurate but also convenient for people. These methods have traditionally been carried out in laboratory settings. The requirement of laboratory context poses challenges for assessing the taste sensitivity of individuals who are unable to attend laboratories due to physical (health/age) or mental conditions. Moreover, the testing process requires relies upon the provision of adequate testing facilities for these individuals (e.g., space, and equipment). These long-standing research challenges indicate a need for developing and establishing home-based taste sensitivity methods in sensory science that are convenient and accessible for use.
In the context of the limited knowledge on the influence of diet on human taste perception and the lack of a home-based taste sensitivity method, the current study aimed to investigate the influence of diet on the taste sensitivity of individuals across vegan, vegetarian,
or omnivorous diets. The research also aimed to develop a home-based taste sensitivity method. Accordingly, the first objective in this thesis was the development, evaluation, and validation of a home-based method for determining taste detection thresholds. This method was designed and developed using the two-Alternative Forced Choice (2-AFC) procedure prescribed by ASTM international standard practice E-679 (ASTM E-679). The newly developed method was evaluated by conducting a comparison between the home-based method and the laboratory-based method using the Best Estimated taste detection Threshold (BET) approach. In a cross-sectional trial with a within-subjects design, 60 healthy participants (females=38, and males=22) aged 21-65 years tasted and detected four different tastes: sweet (sucrose), salty (Sodium chloride), bitter (caffeine), and metallic (Iron II sulphate heptahydrate) through both the home-based and the laboratory-based methods. The obtained results from both methods were analysed and their total BETs were compared. The total BETs from the laboratory-based and the home-based method were 1.81±1.35 g/L and 1.99±1.37 g/L respectively for sweet taste, 0.20±0.26 g/L and 0.19±0.26 g/L respectively for salty taste, 0.026 g/L and 0.028 g/L respectively for bitter taste, and 0.0056 g/L and 0.0058 g/L respectively for metallic taste. No statistically significant difference was found between the two methods indicating the similar efficiency of the home-based method with the laboratory-based method. The results of analysis of data through test-retest reliability using Intra-class Coefficient Correlations (ICC), and assessment of RMSE (Root Mean Square Error) demonstrated that there was no significant difference in the accuracy of both methods in the measurement of taste sensitivity. Key factors for the successful design of the home-based method included providing a clear design, accurate instruction, and having participants carry out the test in a place free of noise, odour, and other distractions.
The second objective was to compare the taste sensitivity of individuals on either meat-based (omnivorous diet) or plant-based diets (vegetarian or vegan diets) using the home-based method through a between-subjects study. The BET to sweet, salty, bitter, sour (citric acid), umami (MonoSodium Glutamate or MSG), and a metallic taste were determined for NZ European female participants aged 18-45 years on either a vegetarian (n=23), vegan (n=22), and omnivorous (n=35) diet. Individuals following a vegetarian diet for at least one year had a lower BET to bitter taste compared to vegans and omnivores indicating a higher sensitivity to bitter taste in vegetarians, with no significant differences detected in BET for the other tastes tested across the three diets.
The third objective assessed and compared energy and nutrient intake of NZ European female participants aged 18-45 years on either vegetarian (n=18), vegan (n=21), and omnivorous (n=22) diets using a 4-day dietary intake record. A significantly lower intake of protein, carbohydrate, glucose, fructose, and zinc, as well as a higher intake of caffeine (caffeinated products), was found among individuals following a vegetarian diet compared to vegans and omnivores. Vegans were found with a lower intake of cholesterol, and higher intake of polyunsaturated fatty acids, dietary fibre, potassium, magnesium, manganese, copper, iron, total folate, thiamine, vitamin C, and vitamin E compared to both vegetarians and omnivores. The results, therefore, demonstrated differences in dietary intake between “vegetarians and vegans” and omnivores but also between vegans and vegetarians.
This study speculated on the links between observed differences in dietary intakes and taste sensitivities to six different tastes including sweet, salt, sour, bitter, umami, and metallic
tastes obtained in the second objective. It was suggested that the higher intake of caffeine (caffeinated products), may be associated with a lower bitter taste perception in people on a vegetarian diet. A vegetarian diet may affect carbohydrates, glucose, and fructose consumption through the effect on bitter taste perception.
In conclusion, the results from the research presented in this thesis have provided new insights into the effect of diet on human taste perception. The differences in dietary intake and their association with taste perception were described across the three diets. The findings in this thesis will help food professionals developing food products designed for individuals following a specific diet.