# Proteomics Analysis in Dairy Products: Cheese, a Review
## 7 Starters and Proteomics
### Lactococci  
_Lactococci_ is another family of bacteria used as starters or developed during ripening in cheeses. Studies conducted on Feta cheese have reported the heterogeneity of \"wild\" _Lactococci_ strains used as starters at the production chain in different regions. Identification of the isolates was performed by comparison of their protein patterns to the fingerprints of the reference strains by means of phenotypic criteria and the SDS-PAGE of cellular proteins assay. The non-starter lactic acid bacteria (NSLAB) that are used in household level for the manufacture of Feta in many areas, are usually _Lactococci_ strains. During maturation the NSLAB microflora may differ depending from the area of origin and display various activities, with the most interesting being the inhibition of undesirable bacteria through competitiveness, which plays a key role in food and consumers' safety. Moreover, principal component analysis (PCA) confirmed that acidifying, proteolytic and autolytic activities vary according to the district of origin, leading to different maturation and flavor traits of the produced Feta. Thus, a link seems to be formed between the area of production, the composition of NSLAB and their biochemical properties, which allows the selection of the strain used for starter for cheese production [26].  
_Lactococcus_ spp. are known to hydrolyze caseins and their micelles [87]. The formation of as1-CN (f40-49) is difficult to explain on the basis of the known specificity of chymosin toward Leu40-Ser41, but not toward Glu39-Leu40 [88]. The nearest bond that has been reported to be hydrolyzed in solution was Val37-Asn38 by a PIII-type proteinase of _Lc. Lactis_ ssp. _cremoris_ SK11 [89]. Although residue 37 is Ile in the ovine polypeptide chain [87], the Ile37-Asn38 bond would likely be still cleaved by a starter proteinase, and, thereafter, the two residues Asn38 and Glu39 would be released by aminopeptidase or aminopeptidases, several of which are present in _Lactococcus_ spp. [29; 90; 91]. The peptide eluted in peak 5(I) was b-CN (f164-180). The N-terminus suggests that this peptide originated from cleavage of Leu163-Ser164, a probable site for chymosin action in solution and in cheese [29]. This bond is also susceptible to the action of lactococcal cell-wall proteinases [29]. In ovine b-CN, the residues Pro179 and Tyr180 are deleted. Therefore, Gln180-Arg181 in the primary sequence of ovine b-CN corresponds to the Gln182-Arg183 of bovine b-CN, the latter being cleaved by the cell-wall-associated proteinases of _Lactococci_[29]. The isolation of k-CN peptides from cheese has not been reported. In the water soluble fraction of Feta cheese, however, a peptide corresponding to k-CN (f96-105) was found and presumably originated from para-k-CN (k-CN (f1-105)), and its formation could be the result of the action of lactococcal proteinase at Met95-Ala96, which exhibits the characteristics of a susceptible cleavage site for such an enzyme [29; 92]. This histidine-rich peptide is of particular physiological significance because His is known to be an essential amino acid for _L. lactis_.  
In a metaproteomic analysis with use of isobaric tags for relative and absolute quantitation (iTRAQ) labeling reagents, it was found that proteins that originated from bacteria were present in Swiss-type cheese [11; 53]. The concurrent presence of proteins that originated from milk and cheese microbiota complicated the metaproteomic approach. The protein content coming from starter bacteria (_L. helveticus_ and _S. thermophilus_) increased during ripening [57; 93]. The reference map of mixed proteins belonging to microbial starter and the cheese proteins 2 DEmap comparison showed that some peptidases from _L. helveticus_ and _S. thermophilus_ were released into the cheese. Thus, metaproteomics approaches were proposed as a means for the determination of the microbial impact on the human health and disease progress [94; 95; 96].