Fresh cheese made from concentrated milk: Tailoring the calcium content of milk retentates by means of microfiltration to modulate the bitter taste

The current manufacture of fermented fresh cheese varieties, such as quarg or cottage cheese, according to the fermentation-concentration process, involves the two main steps: i) Fermentation; gelation of milk by means of fermentation, ii) concentration; the curd-whey mixture is separated into acid whey and fresh cheese of a pre-defined protein content. Due to several reasons, in particular a technically complex processing, the valorization of acid whey stays difficult. In order to completely avoid the production of acid whey, attempts were made to fully concentrate the milk prior to fermentation using ultrafiltration. However, the fresh cheeses, made from the fermented retentates, showed several sensory defects, especially a bitter taste. It was postulated that an elevated mineral content, in particular calcium ions, is responsible for the bitterness. Thus, the hypothesis of this thesis was that the bitterness level of fresh cheese, made from retentates obtained by means of microfiltration, is reduced by decreasing the calcium content prior to fermentation. In order to avoid calcium triggered bitterness as observed in cream cheese, a threshold of < 1800 mg/kg for the calcium content was suggested and aimed in this thesis. According to the hypothesis, the procedure was as follows: Firstly, a technical process to remove calcium from the casein micelles by means of microfiltration to enable the manufacture of skim milk retentates with a calcium content below 1800 mg/kg was developed. During processing acid whey production was avoided and a permeate (pH ≥ 6.0) that can be processed similarly to sweet whey was generated. Secondly, an experimental setup to ferment milk retentates in the cup was proposed. Thirdly, in order to support the bitterness evaluation of concentrated-fermented fresh cheese by means of a sensory panel, an instrumental method for the identification and quantification of key bitter peptides in fresh cheese was developed. Fourth and finally, to test the hypothesis, a feasibility study to manufacture fresh cheese from skim milk retentates with different calcium contents was carried out at technical scale. The study demonstrated that a calcium reduction prior to fermentation, lowered the perceived bitterness level significantly and the bitter peptide content of the concentrated–fermented fresh cheese. Thus, the hypothesis was confirmed.