Quantification of lactosylation of whey proteins in stored milk powder using multiple reaction monitoring

Cross-linking reactions in food proteins and proteomic approaches for their detection

Various protein cross-linking reactions leading to molecular polymerization and covalent aggregates have been described in processed foods. They are an undesired side effect of processes designed to reduce bacterial load, extend shelf life, and modify technological properties, as well as being an expected result of treatments designed to modify raw material texture and function. Although the formation of these products is known to affect the sensory and technological properties of foods, the corresponding cross-linking reactions and resulting protein polymers have not yet undergone detailed molecular characterization. This is essential for describing how their generation can be related to food processing conditions and quality parameters. Due to the complex structure of cross-linked species, bottom-up proteomic procedures developed to characterize various amino acid modifications associated with food processing conditions currently offer a limited molecular description of bridged peptide structures. Recent progress in cross-linking mass spectrometry for the topological characterization of protein complexes has facilitated the development of various proteomic methods and bioinformatic tools for unveiling bridged species, which can now also be used for the detailed molecular characterization of polymeric cross-linked products in processed foods. We here examine their benefits and limitations in terms of evaluating cross-linked food proteins and propose future scenarios for application in foodomics. They offer potential for understanding the protein cross-linking formation mechanisms in processed foods, and how the inherent beneficial properties of treated foodstuffs can be preserved or enhanced.

Contents variation analysis of free amino acids, nucleosides and nucleobases in semen sojae praeparatum fermentation using UFLC-QTRAP MS

UFLC-QTRAP MS was used to develop a sensitive and rapid method of evaluating content variation during Semen sojae praeparatum (SSP) fermentation. It did this through the simultaneous quantification of 22 free amino acids and 16 nucleosides and nucleobases in the raw materials and processed products of SSP. The method was shown to be reproducible and accurate. The limits of detection and quantity values were 0.09-168.75 and 0.31-562.50 ng/mL for the 38 analytes, respectively. The data were examined through principal components analysis to compare the content variations. The quantitative results showed that the ingredients were properly determined in most of the samples and were converted regularly throughout the SSP fermentation process. These results correspond to the morphologic changes and principal components analysis results.

Influence of storage and heating on protein glycation levels of processed lactose-free and regular bovine milk products

Thermal treatment preserves the microbiological safety of milk, but also induces Maillard reactions modifying for example proteins. The purpose of this study was evaluating the influence of consumer behaviors (storage and heating) on protein glycation degrees in bovine milk products. Lactosylation and hexosylation sites were identified in ultra-high temperature (UHT), lactose-free pasteurized, and lactose-free UHT milk (ULF) and infant formula (IF) using tandem mass spectrometry (electron transfer dissociation). Overall, 303 lactosylated and 199 hexosylated peptides were identified corresponding to 170 lactosylation (31 proteins) and 117 hexosylation sites (25 proteins). In quantitative terms, storage increased lactosylation up to fourfold in UHT and IF and hexosylation up to elevenfold in ULF and threefold in IF. These levels increased additionally twofold when the stored samples were heated (40°C). In conclusion, storage and heating appear to influence protein glycation levels in milk at similar or even higher degrees than industrial processing.

Dairy products and the Maillard reaction: A promising future for extensive food characterization by integrated proteomics studies

Heating of milk and dairy products is done using various technological processes with the aim of preserving microbiological safety and extending shelf-life. These treatments result in chemical modifications in milk proteins, mainly generated as a result of the Maillard reaction. Recently, different bottom-up proteomic methods have been applied to characterize the nature of these structural changes and the modified amino acids in model protein systems and/or isolated components from thermally-treated milk samples. On the other hand, different gel-based and shotgun proteomic methods have been utilized to assign glycation, oxidation and glycoxidation protein targets in diverse heated milks. These data are essential to rationalize eventual, different nutritional, antimicrobial, cell stimulative and antigenic properties of milk products, because humans ingest large quantities of corresponding thermally modified proteins on a daily basis and these molecules also occur in pharmaceuticals and cosmetics. This review provides an updated picture of the procedures developed for the proteomic characterization of variably-heated milk products, highlighting their limits as result of concomitant factors, such as the multiplicity and the different concentration of the compounds to be detected.

Identification and quantification of bovine protein lactosylation sites in different milk products

The microbiological safety of milk is typically guaranteed by thermal treatments, such as pasteurization and ultra high temperature (UHT) treatment, whereas infant formula (IF) is often produced at even harsher conditions including a drying process. Thermal treatments have raised concerns, as they may denature proteins and initiate protein modifications. Previous studies identified already many lactosylation sites in milk and showed that the lactosylation degree of some proteins correlates to thermal treatment conditions. Here, we studied the glycation degrees of 124 lactosylation sites in 28 bovine milk proteins in raw milk, three brands of pasteurized milk, three brands of UHT milk, and five brands of IF. Whereas, the glycation degree of many lactosylation sites increased from raw milk, to pasteurized milk, UHT milk, and IF, several modification sites showed a different behavior indicating that global measures do not correctly reflect the reactivity of distinct sites. Interestingly, the glycation degrees varied considerably among the brands of UHT milk and IF indicating that specific production processes of a company have to be considered and not only the classification of milk as pasteurized or UHT. Thus, proper adjustments of the technical processes should allow reducing the lactosylation levels in both UHT milk and IF.

SIGNIFICANCE

It is well established that thermal treatment of milk triggers protein modifications, such as lactosylation of lysine residues in several proteins, although the extent of lactosylation has not been quantitatively compared for a broad panel of protein lactosylation sites among different commercial products. The current study extends previous reports by relatively quantifying 124 confirmed lactosylation sites in 28 bovine milk proteins including several low abundant proteins. Whereas, glycation is generally assumed to be an unspecific chemical reaction with the modification degrees depending on the protein and sugar concentrations, we could show that each protein and even each lactosylation site in a given protein is differently affected by thermal processes indicating that the global lactosylation degrees will not allow predicting the influence of a technical process on individual proteins and lactosylation sites. Additionally, we could show that brands of each milk product differ significantly in their glycation degrees with UHT milk brands for example spanning the whole range from the relatively low lactosylation degree of pasteurized milk to the rather high lactosylation degree of IF. Similar differences were obtained for IF that generally showed the highest glycation degree. The targeted quantification approach established and validated here will be useful to reveal technical processing steps that trigger individual lactosylation sites and thus can help to prevent such unwanted reactions. Even slight changes of the technical processes might allow reducing the lactosylation degree of milk proteins significantly without challenging the microbiological safety or affecting consumer behavior.

Modified peptides as indicators for thermal and nonthermal reactions in processed milk

Site-specific relative quantification of β-lactoglobulin modifications in heated milk and dairy products was performed to determine their thermal and nonthermal origins and to evaluate marker candidates for milk processing. Therefore, formation kinetics of 19 different structures at 26 binding sites were analyzed by ultrahigh-performance liquid chromatography-tandem mass spectrometry with multiple reaction monitoring (UHPLC-MS/MS/MRM) after specific protein hydrolysis. The results indicate that (i) site-specific analysis of lactulosyllysine may be a more sensitive marker for mild heat treatment than its overall content; (ii) N(ε)-carboxymethyllysine, N-terminal ketoamide, and asparagine deamidation are of thermal origin and may be good markers for rather intensive heat treatment, whereas N(ε)-carboxyethyllysine reflects thermal and nonthermal processes; (iii) the relevance of methylglyoxal-derived arginine modifications is low compared to that of other modifications; (iv) oxidation of methionine and cysteine is a rather weak indicator of thermal impact; and (v) the tryptophan modifications formylkynurenine and kynurenine are of nonthermal origin and further degraded during processing.

Effects of lactose-free milk in adjunctive treatment of infants with rotavirus enteritis and lactose intolerance: An analysis 30 cases

AIM: To explore the effects of lactose-free milk in adjunctive treatment of infants with rotavirus enteritis and lactose intolerance (RV-SLI).

METHODS: Ninety patients with RV-SLI were randomly divided into three groups: a zinc gluconate group (n = 30), a lactose-free milk group (n = 30) and a combination group (n = 30). All the patients were treated with conventional oral rehydration solution, oral lactase, and bifid triple viable. On the basis of this treatment, the zinc gluconate group was treated with zinc gluconate, the lactose-free milk group was treated with lactose-free milk powder, and the combination group was treated with zinc gluconate and lactose-free milk powder. The clinical effects, total course, time to relief of clinical symptoms, and myocardial enzyme spectrum [aspartate aminotransferase (AST), lactic dehydrogenase (LDH), creatine kinase (CK), creatine kinase-MB (CK-MB)] were compared before and after treatment for the two groups.

RESULTS: The effective rates rate for the two monotherapy groups were significantly lower than that for the combination group (83.33%, 80.00% vs 96.67%, P < 0.05). The total course and time to relief of clinical symptoms were significantly longer in the two monotherapy groups than in the combination group (7.32 d ± 2.14 d, 7.41 d ± 2.03 d vs 5.52 d ± 1.81 d; 5.15 d ± 0.46 d, 5.23 d ± 0.44 d vs 3.18 d ± 0.77 d; P < 0.05). AST, LDH, CK and CK-MB levels were significantly lower after treatment than prior treatment in all the three groups (39.66 U/L ± 3.28 U/L vs 64.02 U/L ± 5.75 U/L, 108.25 U/L ± 10.18 U/L vs 161.08 U/L ± 12.17 U/L, 117.27 U/L ± 9.86 U/L vs 153.59 U/L ± 12.85 U/L, 32.55 U/L ± 3.50 U/L vs 57.45 U/L ± 6.77 U/L, 40.17 U/L ± 3.31 U/L vs 63.89 U/L ± 5.63 U/L, 112.27 U/L ± 11.08 U/L vs 160.12 U/L ± 12.49 U/L, 120.07 U/L ± 9.92 U/L vs 153.62 U/L ± 12.80 U/L, 33.06 U/L ± 3.52 U/L vs 57.68 U/L ± 6.73 U/L, 26.75 U/L ± 2.68 U/L vs 64.11 U/L ± 5.88 U/L, 86.88 U/L ± 6.30 U/L vs 159.62 U/L ± 12.30 U/L, 87.20 U/L ± 6.34 U/L vs 154.10 U/L ± 12.97 U/L, 24.11 U/L ± 2.62 U/L vs 57.56 U/L ± 6.39 U/L; P < 0.05). AST, LDH, CK and CK-MB after treatment were significantly higher in the two monotherapy groups than in the combination group (39.66 U/L ± 3.28 U/L, 40.17 U/L ± 3.31 U/L vs 26.75 U/L ± 2.68 U/L; 108.25 U/L ± 10.18 U/L, 112.27 U/L ± 11.08 U/L vs 86.88 U/L ± 6.30 U/L; 117.27 U/L ± 9.86 U/L, 120.07 U/L ± 9.92 U/L vs 87.20 U/L ± 6.34 U/L; 32.55 U/L ± 3.50 U/L, 33.06 U/L ± 3.52 U/L vs 24.11 U/L ± 2.62 U/L; P < 0.05).

CONCLUSION: Lactose-free mild has good effects in adjunctive treatment of infants with rotavirus enteritis and lactose intolerance, and it can shorten the total course and improve clinical symptoms and myocardial enzyme spectrum.