Istanbul Technical University
Filiz Altay has completed her Ph.D at the age of 31 years from University of Wisconsin-Madison (USA) at Biological Systems Engineering. She has become a faculty at the Istanbul Technical University, Department of Food Engineering (Istanbul, Turkey). She has studied on rheological properties and modeling of foods. Dr. Altay has also supervised two ongoing COST projects related to electrospun nanofibers and encapsulation of active food ingredients.
Fermented food products have been important in human diet around the world due to their health benefits including increased bioavailability and digestibility of proteins and carbohydrates in their contents. Organoleptic qualities of fermented food beverages which are strongly related to rheological properties are also important as they affect their preferences as well as their processing. The combination of fermentation process with the current technological tools may be considered as controlled fermentation. There are numerous studies about how and how many starter cultures should be used and how they should be applied to increase fermentation yield, there is little research on how structure-function concept can be applied to fermentation process to monitor the texture of fermented food products. Especially non-alcoholic lactic acid beverages have been taking great attention due to their probiotic characteristics. Naturally studies have been focused on their health beneficent, but not on quality characteristics such as rheological properties. Even if they do, they have suggested composition-related solutions to the quality-related problems of fermented beverages. However, based on literature search, depending on fermentation conditions the amount of exopolysaccharides (EPs) that are produced during fermentation can be adjusted. If the amount of EPs may adjust then their rheological properties may also be modified accordingly. Actually this is an understanding of a structure-function and/or property relations of individual components in a mixed system containing protein and polysaccharide. This approach can be extended to fermented food products. For instance, both ayran and yoghurt are made of milk which is a Newtonian fluid. The same starter cultures are used for their lactic acid fermentation. During yoghurt fermentation, milk as a liquid turns into yoghurt which is a three dimensional viscoelastic gel; somewhere in between ayran can be obtained as a pseudoplastic or thixtropic fluid depending on its fat content. Pseudoplasticity, thixtropy and viscoelasticity are very different rheological behaviors; their flow characteristics are different as well as their perceptions in the mouth. If the structure–function relations for fermentation process would be constituted then it can be determined that at which point and under what conditions these transitions take place. The problems in these beverages such as serum separation/solid particle sedimentation, acidity development and low viscosity affect consumer acceptance in terms of mouthfeel and appearance, and are related to quality characteristics during manufacturing, transportation and storage. It appears that most of the quality problems are rheology-related which are affected by initial raw materials and composition of product, fermentation conditions, type of microorganisms and their numbers and ratios. The fermentation ability and fermentation products such as EPS vary depending on microorganism species, leading fermented products with different rheology. By using LAB and yeast isolates as starter cultures, controlled fermentation studies can be carried out to optimize quality for obtaining stable products. The selection of strains with probiotic and antimicrobial properties also enhance the functional properties of traditional foods. Apparently, more studies are needed to investigate the fermentation, rheology and their relations for improving quality and safety of fermented beverages.