Abstract/Summary

Cheese is a popular ingredient in cooked dishes (such as pizza, toppings on baked dishes and in ready-meal sauces) and is an important commodity for the UK dairy industry. Fat contributes a large percentage of the composition of cheeses, leading some consumers to have health concerns regarding consumption of cheese. The production of reduced-fat variants which function comparably to full-fat cheeses in cooked applications is a focus for the dairy industry. However, little information is available in published literature on the compounds responsible for the flavour of cooked cheese, the volatile changes which occur when cheese is cooked, or the effect of reducing-fat on cooked cheese flavour. The first aim of this research was to characterise cooked cheese flavour, including volatiles (especially odorants) and selected non-volatiles (tastants and precursors). Additionally, this work aimed to investigate the role of fat in development of cooked cheese flavour. Using solid phase microextraction (SPME) the volatile profiles of six cooked cheeses (mozzarella, Parmesan, mature Cheddar, mild high-fat Cheddar, mild medium-fat Cheddar, mild low-fat Cheddar) were characterised and compared to their uncooked counterparts (chapter three). Fatty acids and esters decreased in concentration during cooking, while many other volatile classes including 2-methylketones, pyrazines, Strecker aldehydes, lipid-derived aldehydes and furanones increased during cooking. GC-O was performed on the mature Cheddar using SPME, which identified odorants responsible for cooked cheese flavour. Many odorants in cooked cheese (including Strecker aldehydes, furanones, sulfur compounds, fatty acids and 2-methylketones) have been detected previously in uncooked cheese, but were significantly (p < 0.05) The flavour of cooked cheese September 2022 Rosa C. Sullivan higher in concentration in cooked cheeses. Others ((3-methyl-2-butene-1-thiol, (furan-2-yl)methanethiol, cyclotene, 3-ethyl-2,5-dimethylpyrazine, 2-ethyl-3,5- dimethylpyrazine, 2-methyl-3-methyldithiofuran and (E)-2-decenal) have not been reported previously as odorants in uncooked Cheddar. A solvent assisted flavour evaporation (SAFE) approach for comparison of matrices with substantially differing fat contents was developed (chapter five) and used during the solvent extraction and SAFE of cooked mild high (HF), medium (MF) and low-fat (LF) cooked Cheddars (chapter six). GC-O was performed on SAFE extracts from HF and LF cooked Cheddars and compared to determine the role of fat in cooked cheese flavour. Far fewer odorants were detected in the LF cheese than the HF, which reflects the lower concentration of the majority of volatiles in the LF cooked Cheddar. Comparison of SPME and SAFE data showed similar trends for comparison of HF, MF and LF cheese. Selected non-volatiles were quantified in the six cheeses, both uncooked and cooked (chapter four). Amino acids, sugars and γ-glutamyl-dipeptides all decreased in concentration during cooking, which is consistent with their participation in the Maillard reaction. Diketopiperazines (DKPs) increased in concentration during cooking and were above their taste threshold in some cooked cheeses while below threshold in uncooked cheeses. In particular, more extensively aged cheeses formed more DKPs when cooked than younger cheeses, which is likely to be due to the formation of DKP precursors during maturation. As DKPs are bitter and metallic, they may contribute to bitter flavour in cooked aged cheeses. Scanning electron microscopy (SEM) on the cooked Cheddars confirmed substantial structural differences (chapter six), which may contribute to the differences in generation of flavour.