termoestabilidad de sustancias antimicrobianas en la leche - RiuNet

Summary
HPLC-PDA for cephalosporins and tetracyclines. In the case of the sulphonamides, the
milk samples were spiked at a concentration equivalent to 200 µgKg and were
analysed using HPLC-MS/MS. At the different temperatures and after different
treatment times, each substance loses concentration and these losses were analysed
by applying the first order kinetic model and the Arrhenius equation. From these
analyses, the degradation percentages of each antimicrobial substance were estimated
for the heat treatments that are widely used both in the control laboratories and the
dairy industry.
From the results obtained in this study, it can be concluded that it is suitable to
apply the first order kinetic model to study heat degradation of antimicrobial substances
in milk. It may also be deduced, from the calculation of the activation energy and the
half-lives, that cephalosporins are the most unstable molecules, whereas tetracyclines
and sulphonamides are much more heat resistant. As regards the estimation of
degradation percentage as influenced by different treatments, it may be seen that
those used in laboratories (40 ºC-10 min and 83 ºC-10 min ) produce hardly any
effects, except for the treatment at 83 ºC for 10 min on some cephalosporins.
Concerning the industrial treatments (63 ºC-30 min, 72 ºC-15 sec, 120 ºC-20 min y 140
ºC-10 sec), it seems that most of them have no effect upon antimicrobial degradation,
with two exceptions; the treatment at 63 ºC for 30 minutes has a degrading effect on
cephalosporins, reaching percentages ranging between 14% (cefuroxime) and 36.2%
(cefapirin) and the milk sterilization (120 ºC-20 min) which leads to significant losses
(37.4-100%) in all the betalactam, tetracycline and sulphonamide antibiotics, except for
sulfadimethoxine (6.5%) and sulfathiazole (9.9%).
It can be concluded from this that milk sample storage conditions, both
refrigeration and freezing, can affect the stability of betalactam and tetracycline
antibiotics. This is why it is advisable to keep the milk samples for the shortest possible
time prior to analysis (both at 4 ºC and -20 ºC) and also to study the possibility of
storing extracts of those molecules that are mores stable then the milk samples. In this
way, the aim is to reduce the possible variability in the results of the analysis carried
out on the same milk sample over a period of time and which may be behind the
contradictory results obtained during the different control stages.
Regarding the heat treatments, neither those performed in the laboratories nor
the ones used in the dairy industry, produce hardly any degradation effects on
antimicrobial substances, except for milk sterilization (120 ºC-20 min). This means that,
as most of the heat treatments do not act as a barrier preventing antimicrobial
substances from reaching the consumer, it is necessary to apply the preventative and
control methods correctly throughout the whole milk production chain to avoid any
potential risk caused by the presence of these substances.
The results of this work establish a starting point for future studies and it would
be of interest to analyse the stability of other antimicrobial groups that are being used
more and more in milk. It is also relevant to study the molecular changes that can be
brought about in the antibiotics by the heat treatments and the possible toxicological
and/or technological effects of their metabolites. All of this contributes towards
improving one of the basic principles of food safety: consumer protection.