Progressive ultrastructural changes in the casein matrix during the ripening of inadequately acidified feta cheese

摘要

This study investigated the ultrastructural changesunderlying the undesired softening of insufficientlyacidified feta cheese during cold storage. Experimentalfeta cheeses with a range of pH values before briningwere manufactured by allowing the cheese blocks to fermentovernight at 3 temperatures (35, 20, and 3°C),which resulted in pH values of 4.80, 4.88, and 5.17,respectively. Cheese blocks were stored in pH-adjustedwhey brine solutions for up to 120 d, at which pointsignificant decreases in the cheese firmness were confirmedwith compression and shear tests. Samples fortransmission electron microscopy were taken duringthe make procedure, after overnight fermentation, andafter 7 and 90 d of cold storage. Increasing the initialpH from 4.80 to 5.17 resulted in a fundamentally differentultrastructure at d 90, with the protein matrixas the continuous phase having markedly decreaseddensity compared with the typically open porous anddiscontinuous protein matrix of high density in the lowpHcontrol feta cheese. Ultrastructural changes wereprogressive, and the first signs were evident after only20 h (the overnight fermentation), when fine, proteinaceousmaterial dissociated from the edges of the caseinstrands into the serum phase. By d 7, the serum phasewas completely filled with the loosely aggregated caseinclosely surrounding the spheroidal fat globules. A furtherbreakdown of the protein matrix was observed after90 d, with the complete loss of open porous networkstructure. Image analysis quantitatively confirmed theprogressive and significant decrease in density of theprotein matrix. In summary, this is the first studyto provide a comprehensive and in-depth view of theprogressive and most likely irreversible ultrastructuralchanges that lead to this textural defect.