Psychrotrophic bacteria, capable of growing in milk during refrigerated storage
before processing, produce thermostable proteinases that strongly influence the
keeping quality of long life dairy products (Cousin, 1982). This is the case with UHT
milk, where the activity of residual or reactivated bacterial proteolytic enzymes is
linked to flavour deterioration and gelation. Furthermore, since most of these
enzymes are acid proteinases with a broad specificity and particularly active against
κ-casein (Fairbairn & Law, 1986), their hydrolysis products can interfere with
detecting the fraudulent addition of rennet whey (López-Fandiño et al. 1993a, b;
Recio et al. 1996, 2000). This is usually based upon detecting, by reversed-phase (RP-)
HPLC (Olieman & van Riel, 1989; Van Riel & Olieman, 1995a) or capillary
electrophoresis (CE) (Van Riel & Olieman, 1995b), caseinmacropeptide (CMP), the
106–169 fragment of κ-casein released by chymosin during the initial stages of
cheesemaking (for review, see López-Fandiño & Olano, 1999). Nevertheless, it is not
yet clear whether degradation of κ-casein by psychrotrophic enzymes gives rise to
hydrolysis products that are identical to those produced by chymosin or just
indistinguishable by CE (Van Riel & Olieman, 1995b; Recio et al. 1996).
In the present investigation, we studied CMP-like degradation products produced
by the action of extracellular psychrotrophic proteinases on κ-casein by RP-HPLC
and CE. Electrospray ionization–mass spectrometry (ESI–MS) was used for peptide
identification. Characterization of breakdown products specific to psychrotrophic
enzymes may help the investigation of their residual activity remaining in milk after
processing, as well as provide an indication of the suitability of CMP as an indicator
of adulteration of UHT milk with whey solids.