Protein oxidative changes in whole and skim milk after ultraviolet or fluorescent light exposure

We investigated how protein changes occur, at the primary or higher structural levels, when proteins are exposed to UV or fluorescent (FL) light while in the complex matrix, milk. Whole milk (WM) or skim milk (SM) samples were exposed to FL or UV light from 0 to 24h at 4°C. Protein oxidation was eva...

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Bibliographic Details
Published in:Journal of dairy science 2010-11, Vol.93 (11), p.5101-5109
Main Authors: Scheidegger, D., Pecora, R.P., Radici, P.M., Kivatinitz, S.C.
Format: Article
Language:English
Subjects:
Animal productions
Animals
Biological and medical sciences
carbonyl compounds
chemical bonding
digestibility
dityrosine bonds
Fluorescence
fluorescent lighting
Food Handling - methods
Food industries
Fundamental and applied biological sciences. Psychology
kynurenine
Kynurenine - analogs & derivatives
Kynurenine - analysis
light exposure
Milk - chemistry
Milk and cheese industries. Ice creams
milk protein
milk proteins
Milk Proteins - chemistry
molecular weight
N-formylkynurenine
oxidation
Oxidation-Reduction
polymerization
protein aggregates
Protein Carbonylation
protein degradation
protein fragmentation
protein oxidation
protein structure
proteolysis
skim milk
Terrestrial animal productions
tyrosine
Tyrosine - analogs & derivatives
Tyrosine - analysis
ultraviolet radiation
Ultraviolet Rays
Vertebrates
whole milk
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Summary:We investigated how protein changes occur, at the primary or higher structural levels, when proteins are exposed to UV or fluorescent (FL) light while in the complex matrix, milk. Whole milk (WM) or skim milk (SM) samples were exposed to FL or UV light from 0 to 24h at 4°C. Protein oxidation was evaluated by the formation of protein carbonyls (PC), dityrosine bond (DiTyr), and changes in molecular weight (protein fragmentation and polymerization). Oxidative changes in AA residues were measured by PC. Dityrosine and N′-formylkynurenine (NFK), a carbonylation derivative of Trp, were measured by fluorometry. Protein carbonyls increased as a function of irradiation time for both WM and SM. The initial rate for PC formation by exposure to FL light (0.25 or 0.27nmol/h for WM and SM, respectively) was slower than that following exposure to UV light (1.95 or 1.20 nmol/h, respectively). The time course of NFK formation resembled that of PC. After 24h of UV exposure, SM had significantly higher levels of NFK than did WM. In contrast, WM samples irradiated with UV had higher levels of DiTyr than did SM samples, indicating different molecular pathways. The formation of intra- or intermolecular DiTyr bonds could be indicative of changes in the tertiary structure or oligomerization of proteins. The existence of NFK suggests the occurrence of protein fragmentation. Thus, proteolysis and oligomerization were analyzed by sodium dodecyl sulfate-PAGE. After 24h of exposing WM to UV or FL light, all the proteins were affected by both types of light, as evidenced by loss of material in most of the bands. Aggregates were produced only by UV irradiation. Hydrolysis by pepsin and enzyme-induced coagulation by rennet were performed to evaluate altered biological properties of the oxidized proteins. No effect on pepsin digestion or rennet coagulation was found in irradiated SM or WM. The oxidative status of proteins in milk and dairy products is of interest to the dairy industry and consumers. These findings provide knowledge that could be useful in determining the optimal lighting conditions in the dairy industry in general and in cheese making in particular.
ISSN:0022-0302
1525-3198
DOI:10.3168/jds.2010-3513