The term ‘Swiss-type cheese’ such as Emmentaler PDO, Leerdammer or Fol Epi is used for cooked and ripened cheeses which undergo a propionic acid fermentation usually achieved by adding a culture of selected Propionibacterium freudenreichii strains. The most distinct, desired visual characteristic of Swiss-type cheeses are the holes or eyes often immediately associated with a Swiss-type cheese. An elastic texture of Swiss-type cheeses, strongly related to the calcium content, is a prerequisite for an optimal eye formation. The typical flavour of Swiss-type cheeses is described as sweet and nutty. Especially, the sweet flavour note, supposed to be related to the content of propionic acid, is often more pronounced compared to other hard cheese varieties. In addition, free fatty acids (FFA) derived from lipolytic activity by propionic acid bacteria (PAB) and its metabolites derived from catabolism during cheese ripening are also considered to have a high impact on the typical flavour notes of Swiss-type cheeses. Compared to other cheese varieties, Swiss-type cheeses are often perceived as rather low in saltiness due to the usually low-to-medium salt concentration ranging from 0.2 to about 1.6% of sodium chloride. One of the early systematic studies more than half a century ago on volatiles contributing to the flavour of Swiss-type cheese identified more than 50 different compounds encompassing alcohols, aldehydes, ketones, esters, hydrocarbons, lactones and sulphides. Nevertheless, up to date no odour active compounds uniquely attributed to the activity of P. freudenreichii and other species of propionic bacteria could be determined in Swiss-type cheeses. It can be assumed that the flavour characteristics specific for Swiss-type cheeses result from a balanced mix of a vast number of aroma active volatile compounds and their interactions with the composition and structure of the cheese. In addition, the flavour seems to depend highly on the strain composition of both the lactic acid bacteria and PAB used for the production of Swiss-type cheeses. Often the sensory data is linked to chemical analyses of volatile compounds in order to get information on the impact of specific compounds on flavour perception or to build models to predict sensory characteristics from instrumental data. Nowadays, Swiss-type cheeses are manufactured all over the world using a broad range of production technologies resulting in cheeses often quite different from Emmentaler, the original Swiss-type cheese. In-depth knowledge on the influence of key factors crucial to the sensory character such as milk quality, choice of bacterial cultures, production parameters or ageing strategies allows for control and optimisation of the quality of Swiss-type cheeses. Thereby, an accurate and reproducible analysis of the sensory properties – as desired by producers and by consumers – is of great importance to guarantee a high and consistent product quality.
