Storage study and characterization of an yoghurt fermented under pressure

• Main Author: Vieira, Ana Patrícia Pinheiro
• Publication Date: 2019
• Format: Master thesis
• Language: eng
• Source: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Download full: http://hdl.handle.net/10773/27324
• topic: Fermentation; Yoghurt; Storage; Pressure; Lactic acid bacteria

This work aimed to study refrigeration storage (4°C for 23 days) of yoghurt produced at 43°C under sub-lethal high pressure, at 10, 20, 30 and 40 MPa, in comparison with the fermentation process at atmospheric pressure (0.1 MPa). Lactic acid bacteria (Streptococcus thermophilus e Lactobacillus delbrueckii spp. Bulgaricus, LAB) and quality parameters like pH, titratable acidity, syneresis and colour were evaluated, along with sensorial and textural analyses to infer how pressure would impact the obtained yoghurt along storage. Moreover, it was done a metabolite fingerprinting by nuclear magnetic resonance (NMR), sugars and organic acids assessment by high performance liquid chromatography (HPLC), total fatty acids (TFA) determination and quantification by gas chromatography with a flame ionization detector (GC-FID) and determination of the principal volatile compounds by gas chromatography electron ionisation mass spectrometry (GC-EI-MS). Higher fermentation pressures resulted in slightly lower LAB loads (a maximum of 1.01 Log (CFU/mL)) and increased the fermentation time (a maximum of 3h 25 min), syneresis (a maximum of 44 %), all for 40 MPa and firmness (a maximum of 2.5-fold) for 30 MPa. Under refrigeration, LAB were more active during the first 15 days of storage in yoghurts fermented under pressure (increasing loads up to 0.54 Log (CFU/mL)). Colour, pH and titratable acidity parameters were not affected by pressure or storage. Metabolomic analyses by NMR permitted to verify that just 2,3-butanediol, acetoin, diacetyl and formate vary with the increase of pressure and probably pressure active diacetyl reductase, acetoin reductase and acetolactate decarboxylase. Pressure also affect lactose consumption and the content of total fatty acids. Yoghurts fermented at 40 MPa had the less content in lactose (39.7 % of total sugar reduction) and the less content in TFA (12301.5 μg/g of yoghurt, 56.1 % less than the fermented under 0.1 MPa). So, the consumption of these yoghurts (fermented under pressure) can be a good consumer choice because of their low content in sugars and fats, but also the lipid quality indices obtained were very good and may contribute to healthy diet. The method applied to determine volatile compounds allowed to identify 131 compounds and some of them were only found in the yoghurts fermented under higher pressures and may be characteristic of them. Finally, and despite of the low number of participants in the sensory analysis, it is important to highlight the higher acceptability and preference of the yoghurt fermented under pressure, mainly at 10 and 20 MPa. Further research is of interest to ascertain the biotechnological potential of fermentation processes under sub-lethal high pressure in general and in particular for yoghurt production