termoestabilidad de sustancias antimicrobianas en la leche - RiuNet

SUMMARY
Summary
The presence of antimicrobial residues in milk constitutes a public health risk as
they may provoke allergies, resistance to antibiotics and intestinal alterations, among
other things. It is, moreover, one of the dairy industry’s main concerns, as they may
have a negative effect on the manufacturing process of some dairy products, such as
yoghurt and cheese. The amount of information available on the heat stability of the
residues in milk is very limited, particularly so when referring to the refrigeration and
freezing temperatures used throughout the production stages, transport and analysis,
as well as the heat treatments used in the control laboratories and dairy industries.
Therefore, the aim of this study is to carry out a two-fold analysis. On the one
hand, to evaluate the influence of refrigerated and frozen storage conditions on the
presence of antimicrobial substances belonging to groups of betalactams, tetracyclines
and sulphonamides and, on the other, to analyse the effect that heating milk to different
temperatures has on the stability of these substances.
The influence that refrigeration (4 ± 2 ºC) and freezing (-20 ± 2 ºC) has on
antimicrobial presence in milk has been studied by means of analysing samples of 20
substances that have been spiked at Maximum Residue Limits (MRL) equivalent
concentrations. These substances belong to the betalactam (penicillins and
cephalosporins), tetracycline and sulphonamide antimicrobial groups and their
corresponding extracts. HPLC-MS/MS equipment was used to carry out the
chromatographic analysis of the milk samples and extracts after 0, 1, 2, 3, 7, 10, 15
and 30 days of storage. How much refrigeration and freezing time and the type of
matrix (milk and extracts) influenced the loss in concentration of the antimicrobial
substances was determined by applying the multiple linear regression model and
estimating the degradation percentages of each substance.
From the obtained results, it can be concluded that refrigeration and freezing do
affect the betalactam and tetracycline antibiotics present in both the extracts and the
milk samples, but have no effect on the stability of sulphonamides. It may also be seen
that the stability of each substance depends on temperature, storage time and the
interaction with the type of matrix (milk and extracts). Although the estimated losses in
the milk samples refrigerated for 7 days ranged between 14 and 90% for every
substance, the most unstable molecules were the betalactam antibiotics (with the
exception of cephalexin). Nearly all the extracts experienced smaller losses. When
frozen, most of the antimicrobial samples were more stable than the extracts. After 7
days of frozen storage, the degradation percentages ranged between 6.7 and 68.4%,
and once again, the betalactams were the antibiotics that degraded the most, except
for cephalexin and cefquinome.
To analyse the effect of heating on antimicrobial stability in milk, an experiment
was performed consisting of 5 heat treatments at 60, 70, 80, 90 and 100 ºC for different
periods of time on samples spiked with 22 antimicrobial substances. In this study, the
antimicrobial substances belonging to the betalactam and tetracycline antibiotic groups
were analysed by preparing milk solutions that have been spiked at a concentration of
5000 µg/Kg and by analysing the samples treated for different periods of time at each
of the temperatures, using two pieces of equipment: HPLC-UV for penicillins and