Geospatial Fluid Milk Processing Preferences: Is Consumer Taste Perception the Key Factor?

Consumption of ultrapasteurized milk is marked by extensive global variability; in the U.S. and U.K., less than 10% of fluid milk is marketed as ultrapasteurized, compared to >80% in France, Spain and Portugal. Fluid milk taste perception amongst U.S. consumers is that high-heat treatment of milk, as in ultrapasteurization, generates undesirable differences in taste compared to low-temperature/conventional pasteurized treatment. Although highly trained experts can distinguish characteristics in controlled studies, it remains unknown if general consumers can detect a difference or are subject to confirmation bias. In testing sensory perception in a defined untrained population, our findings indicate that the general consumer is unable to distinguish ultrapasteurized from pasteurized milk. On this basis, we conclude that presumptive “consumer taste perception” that speciously impacts fluid milk processing types in the U.S. market precludes noted benefits to ultrapasteurization not only in flexibility for storage and distribution, but also in reduction of public health risks.

Simultaneous quantification of the most common and proteolytic Pseudomonas species in raw milk by multiplex qPCR

Abstract

The heat-stable peptidase AprX, secreted by psychrotolerant Pseudomonas species in raw milk, is a major cause of destabilization and premature spoilage of ultra-high temperature (UHT) milk and milk products. To enable rapid detection and quantification of seven frequent and proteolytic Pseudomonas species (P. proteolytica, P. gessardii, P. lactis, P. fluorescens, P. protegens, P. lundensis, and P. fragi) in raw milk, we developed two triplex qPCR assays taking into account species-dependent differences in AprX activity. Besides five species-specific hydrolysis probes, targeting the aprX gene, a universal rpoB probe was included in the assay to determine the total Pseudomonas counts. For all six probes, linear regression lines between C_q value and target DNA concentration were obtained in singleplex as well as in multiplex approaches, yielding R² values of > 0.975 and amplification efficiencies of 85–97%. Moreover, high specificity was determined using genomic DNA of 75 Pseudomonas strains, assigned to 57 species, and 40 other bacterial species as templates in the qPCR. Quantification of the target species and total Pseudomonas counts resulted in linear detection ranges of approx. 10³–10⁷ cfu/ml, which correspond well to common Pseudomonas counts in raw milk. Application of the assay using 60 raw milk samples from different dairies showed good agreement of total Pseudomonas counts calculated by qPCR with cell counts derived from cultivation. Furthermore, a remarkably high variability regarding the species composition was observed for each milk sample, whereby P. lundensis and P. proteolytica/P. gessardii were the predominant species detected.

Key points

• Multiplex qPCR for quantification of seven proteolytic Pseudomonas species and total Pseudomonas counts in raw milk

• High specificity and sensitivity via hydrolysis probes against aprX and rpoB

• Rapid method to determine Pseudomonas contamination in raw milk and predict spoilage potential

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11109-0.

Genetic Organization of the aprX-lipA2 Operon Affects the Proteolytic Potential of Pseudomonas Species in Milk

Psychrotolerant Pseudomonas species are a main cause of proteolytic spoilage of ultra-high temperature (UHT) milk products due to the secretion of the heat-resistant metallopeptidase AprX, which is encoded by the first gene of the aprX-lipA2 operon. While the proteolytic property has been characterized for many different Pseudomonas isolates, the underlying aprX-lipA2 gene organization was only described for a few strains so far. In this study, the phylogenomic analysis of 185 Pseudomonas type strains revealed that the presence of aprX is strongly associated to a monophylum composed of 81 species, of which 83% carried the aprX locus. Furthermore, almost all type strains of known milk-relevant species were shown to be members of the three monophyletic groups P. fluorescens, P. gessardii, and P. fragi. In total, 22 different types of aprX-lipA2 genetic organizations were identified in the genus, whereby 31% of the species tested carried the type 1 operon structure consisting of eight genes (aprXIDEF prtAB lipA2). Other genetic structures differed from type 1 mainly in the presence and location of genes coding for two lipases (lipA1 and lipA2) and putative autotransporters (prtA and prtB). The peptidase activity of 129 strains, as determined on skim milk agar and in UHT-milk, correlated largely with different aprX-lipA2 gene compositions. Particularly, isolates harboring the type 1 operon were highly proteolytic, while strains with other operon types, especially ones lacking prtA and prtB, exhibited significantly lower peptidase activities. In conclusion, the phylogenomic position and the aprX-lipA2 gene organization specify the proteolytic potential of Pseudomonas isolates. In addition, however, an interplay of several environmental factors and intrinsic traits influences production and activity of AprX, leading to strain-specific proteolytic phenotypes.

Antilisterial activity of dromedary lactoferrin peptic hydrolysates

The aim of this study was to explore the antibacterial peptides derived from dromedary lactoferrin (LFc). The LFc was purified from colostrum using a batch procedure with a cation exchange chromatography support and was hydrolyzed with pepsin to generate peptic digest. This peptic digest was fractionated by cation exchange chromatography, and the antilisterial activity of LFc, peptic digest, and obtained fractions was investigated using the bioscreen method. The growth of Listeria innocua ATCC 33090 and LRGIA 01 strains was not inhibited by LFc and its hydrolysates. Two fractions of dromedary lactoferrin peptic hydrolysate were active against both strains. A tandem mass spectroscopy analysis revealed that the 2 active fractions comprised at least 227 different peptides. Among these peptides, 9 found in the first fraction had at least 50% similarity with 10 known antimicrobial peptides (following sequence alignments with the antimicrobial peptide database from the University of Nebraska Medical Center, Omaha). Whereas 9 of these peptides presented homology with honeybee, frog, or amphibian peptides, the 10th peptide, F₁₅₂SASCVPCVDGKEYPNLCQLCAGTGENKCACSSQEPYFGY₁₉₂ (specifically found in 1 separated fraction), exibited 54% homology with a synthetic antibacterial peptide (AP00481) derived from human lactoferrin named kaliocin-1. Similarly, the second fraction contained 1 peptide similar to lactoferrampin B, an antibacterial peptide derived from bovine milk. This result suggests that peptic hydrolysis of LFc releases more active antimicrobial peptides than their protein source and thus provides an opportunity for their potential use to improve food safety by inhibiting undesirable and spoilage bacteria.

Microbiological quality of raw milk attributable to prolonged refrigeration conditions

Refrigerated storage of raw milk is a prerequisite in dairy industry. However, temperature abused conditions in the farming and processing environments can significantly affect the microbiological quality of raw milk. Thus, the present study investigated the effect of different refrigeration conditions such as 2, 4, 6, 8, 10 and 12 °C on microbiological quality of raw milk from three different dairy farms with significantly different initial microbial counts. The bacterial counts (BC), protease activity (PA), proteolysis (PL) and microbial diversity in raw milk were determined during storage. The effect of combined heating (75 ± 0·5 °C for 15 s) and refrigeration on controlling those contaminating microorganisms was also investigated. Results of the present study indicated that all of the samples showed increasing BC, PA and PL as a function of temperature, time and initial BC with a significant increase in those criteria ≥6 °C. Similar trends in BC, PA and PL were observed during the extended storage of raw milk at 4 °C. Both PA and PL showed strong correlation with the psychrotrophic proteolytic count (PPrBC: at ≥4 °C) and thermoduric psychrotrophic count (TDPC: at ≥8 °C) compared to total plate count (TPC) and psychrotrophic bacterial count (PBC), that are often used as the industry standard. Significant increases in PA and PL were observed when PPrBC and TDPC reached 5 × 104cfu/ml and 1 × 104cfu/ml, and were defined as storage life for quality (SLQ), and storage life for safety (SLS) aspects, respectively. The storage conditions also significantly affected the microbial diversity, wherePseudomonas fluorescensandBacillus cereuswere found to be the most predominant isolates. However, deep cooling (2 °C) and combination of heating and refrigeration (≤4 °C) significantly extended theSLQandSLsof raw milk.

Review: Selected indicators of the quality of thermal processed milk / Revisión: Indicadores seleccionados para el control de calidad de la leche tratada térmicamente

Selected indicators of the quality of processed milk are reviewed in three sections: indices of heat treatment, detection of adulterations and assessment of shelf life. The characterization of the thermal process to which milk was submitted can be achieved by measuring either the formation of new compounds (lactulose, furosine) or the degradation of thermolabile constituents (enzymes, whey proteins). The presence of certain compounds may indicate fraudulent additions committed for eco nomic reasons. Finally, residual or reactivated heat stable enzymes may cause serious storage defects in UHT milk and, therefore, the proteolytic and lipolytic activities and the degree of protein and lipid degradation are useful predictors of the shelf life. Different analytical methods for the determination of the selected quality indicators are also summarized.

The effect of deamidation on the structural, functional, and rheological properties of glutelin prepared from Akebia trifoliata var. australis seed

The characteristics of glutelin samples from Akebia trifoliata var. australis seeds (AG) that had been deamidated by malic acid (MDAG) and by citric acid (CDAG) were investigated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed high-molecular-weight subunits that were degraded into smaller fragments, and FTIR indicated a decrease in the number of β-sheet groups and an increase in the amount of β-turns in the deamidated samples. These results could be caused by the cleaving of partial disulfide bonds to form new sulfhydryl groups during deamidation. Citric acid was found to be more effective at deamidation and hydrolysis, resulting in a higher solubility and emulsifying activity for CDAG, and MDAG also exhibited some improvement in terms of surface hydrophobicity and emulsion ability. Rheology showed that the gelation point for deamidated samples was increased, and the gel network was strengthened. The amounts of essential amino acids that were well-preserved and the improved solubility, emulsification, and rheology properties of AG after acid-heating deamidation show that this technique can be useful for treating other plant-based food ingredients in the future.

Microbial biodiversity, quality and shelf life of microfiltered and pasteurized extended shelf life (ESL) milk from Germany, Austria and Switzerland

Information on factors limiting the shelf life of extended shelf life (ESL) milk produced by microfiltration and subsequent pasteurization is very limited. In this study, three different batches of ESL milk were analyzed at different stages of the production process and during storage at 4 °C, 8 °C and 10 °C in order to evaluate the changes in bacterial cell counts, microbial diversity and enzymatic quality. Additionally, detailed biodiversity analyses of 250 retail ESL milk packages produced by five manufacturers in Germany, Austria and Switzerland were performed at the end of shelf life. It was observed that microfiltration decreased the microbial loads by 5-6 log₁₀ units to lower than 1 CFU/mL. However, bacterial counts at the end of shelf life were extremely variable and ranged between <1 and 8 log₁₀ CFU/mL. 8% of all samples showed spoilage indicated by cell counts higher than 6 log₁₀ CFU/mL. The main spoilage groups of bacteria were Gram-negative post-process recontaminants (Acinetobacter, Chryseobacterium, Psychrobacter, Sphingomonas) and the spore formers Paenibacillus and Bacillus cereus, while other spore formers and Microbacterium spp. did not reach spoilage levels. Paenibacillus spp. and B. cereus apparently influenced enzymatic spoilage, as indicated by increased free fatty acid production, pH 4.6 soluble peptide fractions and off-flavors. In some cases, enzymatic spoilage was observed although microbial counts were well below 6 log₁₀ CFU/mL. Thirteen B. cereus isolates were characterized for their toxin profiles and psychrotolerance. Hbl, nhe, and cytK toxin genes were detected in ten, thirteen, and four isolates, respectively, whereas the ces gene was always absent. Interestingly, only three of the thirteen isolates could be allocated to psychrotolerant genotypes, as indicated by the major cold shock cspA gene signature. Generally, large discrepancies in microbial loads and biodiversity were observed at the end of shelf life, even among packages of the same production batch. We suggest that such unexpected differences may be due to very low cell counts after ESL treatment, causing stochastic variations of initial species distributions in individual packages. This would result in the development of significantly different bacterial populations during cold storage, including the occasional development of high numbers of pathogenic species such as B. cereus or Acinetobacter.

Comparison of methods for analysis of proteolysis by plasmin in milk

Sensitive methods that are currently used to monitor proteolysis by plasmin in milk are limited due to their high cost and lack of standardisation for quality assurance in the various dairy laboratories. In this study, four methods, trinitrobenzene sulphonic acid (TNBS), reverse phase high pressure liquid chromatography (RP-HPLC), gel electrophoresis and fluorescamine, were selected to assess their suitability for the detection of proteolysis in milk by plasmin. Commercial UHT milk was incubated with plasmin at 37°C for one week. Clarification was achieved by isoelectric precipitation (pH 4·6 soluble extracts) or 6% (final concentration) trichloroacetic acid (TCA). The pH 4·6 and 6% TCA soluble extracts of milk showed high correlations (R2 > 0·93) by the TNBS, fluorescamine and RP-HPLC methods, confirming increased proteolysis during storage. For gel electrophoresis, extensive proteolysis was confirmed by the disappearance of α- and β-casein bands on the seventh day, which was more evident in the highest plasmin concentration. This was accompanied by the appearance of α- and β-casein proteolysis products with higher intensities than on previous days, implying that more products had been formed as a result of casein breakdown. The fluorescamine method had a lower detection limit compared with the other methods, whereas gel electrophoresis was the best qualitative method for monitoring β-casein proteolysis products. Although HPLC was the most sensitive, the TNBS method is recommended for use in routine laboratory analysis on the basis of its accuracy, reliability and simplicity.

Acidification test based on the use of biomass for screening ofLeuconostoc. Application toLn. mesenteroidesstrains isolated from French raw milk cheeses

Unlike that of other lactic acid bacteria, the growth ofLeuconostocspp. in milk is poor and the resulting acidification cannot be used to distinguish different strains. An acidification test based on the use of high initial numbers (109cfu/ml) has been developed and proved to be an efficient tool for discriminating between 110Leuconostocstrains isolated from French raw milk cheeses. The pH values after 24 h ranged from 6.55 to 4.05 and distinguished four acidification groups. All 34% of the strains that acidified milk to at least pH 5.1, coagulating it and should be considered as Lac+. The differences in rates and degrees of acidification could not be related to the proteolytic activity which was, from all 27 representative strains tested, similar to that ofLactococcus lactisPrt–variants.

A simple procedure using trinitrobenzenesulphonic acid for monitoring proteolysis in cheese

A simple, rapid and sensitive spectrophotometric assay was developed and evaluated for monitoring proteolysis during cheese ripening, based on the fact that α-amino groups released by hydrolysis of cheese proteins react with trinitrobenzenesulphonic acid to form products that absorb strongly at 420 nm. A linear relationship was shown to exist between A420 and concentration of free α amino groups up to 0·5 HIM (r = 0·999, 38 df, P < 0·001). Repeatability of the method was satisfactory. The coefficient of variance was 0·53% for amino acid solutions and 1·19% for cheese extracts. Average recovery of glycine added to the cheese was 104 ± 2·9%. A comparison of the above method with that of determination of water-soluble N to total N ratio showed that there was good agreement between these two methods of assessment of proteolysis in cheese (r = 0·857, 32 df, P < 0·001). Mainly Feta and Teleme cheese were examined, but a similar correlation was obtained with hard Greek cheeses. Analytical conditions of the procedure are discussed.

Characterization and in vitro digestibility of bovine beta-lactoglobulin glycated with galactooligosaccharides

Galactooligosaccharides (GOS) are well-known prebiotic ingredients which can form the basis of new functional dairy products. In this work, the production and characterization of glycated beta-lactoglobulin (beta-LG) with prebiotic GOS through the Maillard reaction under controlled conditions ( a(w) = 0.44, 40 degrees C for 23 days) have been studied. The extent of glycation of beta-LG was evaluated by formation of furosine which progressively increased with storage for up to 16 days, suggesting that the formation of Amadori compounds prevailed over their degradation. RP-HPLC-UV, SDS-PAGE, and IEF profiles of beta-LG were modified as a consequence of its glycation. MALDI-ToF mass spectra of glycated beta-LG showed an increase of up to approximately 21% in its average molecular mass after storage for 23 days. Moreover, a decrease in unconjugated GOS (one tri-, two tetra-, and one pentasaccharide) was observed by HPAEC-PAD upon glycation. These results were confirmed by ESI MS. The stability of the glycated beta-LG to in vitro simulated gastrointestinal digestion was also described and compared with that of the unglycated protein. The yield of digestion products of glycated beta-LG was lower than that observed for the unglycated protein. The conjugation of prebiotic carbohydrates to stable proteins and peptides could open new routes of research in the study of functional ingredients.

Charged casein-derived oligopeptides competitively inhibit the transport of a reporter oligopeptide by Lactococcus lactis

AIM

To study the effect of casein-derived peptides, accumulated during growth of Lactococcus lactis in milk, on its oligopeptide transport (Opp) function.

METHODS AND RESULTS

This effect was estimated by analysing the ability of casein-derived peptides to compete for the transport of a reporter peptide by whole L. lactis cells. The transport of the reported peptide was monitored by determining the intracellular concentrations of the corresponding amino acids by means of reverse-phase high-performance liquid chromatography (HPLC). Uptake of the reporter peptide was competitively inhibited by casein-derived peptides. The competition was only because of charged casein-derived peptides, including anionic peptides. The design of specific pure peptides made it possible to evidence for a positive (or negative) influence exerted by the positively (or negatively) charged side chain of the N-terminal amino acid on the competition.

CONCLUSIONS

Charged casein-derived peptides impaired the oligopeptide transport function of L. lactis.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results demonstrate an inhibition of Opp when too many peptides are produced by the proteinase. Peptide transport by Opp therefore represents a bottleneck for increasing the growth rate of L. lactis in milk.

Extracellular enzymatic activity profiles in yeast and yeast-like strains isolated from tropical environments

AIMS

The objective of this study was to investigate the extracellular enzymatic activity (EEA) profile of yeasts isolated from tropical environments of the Brazilian rain forest. This screening survey could constitute the first approach in selecting yeast strains of environmental origin potentially exploitable as enzyme producers.

METHODS AND RESULTS

In this study, 348 yeast (193 ascomycetes and 155 basidiomycetes) and 46 yeast-like strains (Aureobasidium pullulans) were screened for their EEA profile. The spread occurrence of extracellular amylases, esterases, lipases, proteases, pectinases and chitinases appeared to be a strain-related character.

CONCLUSIONS

Yeasts isolated from tropical environments could represent a promising source of EEA. Selected strains showed maximum levels of EEA under acidic or neutral conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrated the potential for yeasts isolated from extreme environments as sources of industrially relevant enzymes for biotechnological purposes.

Overproduction of an inducible extracellular serine protease improves biological control of Pythium ultimum by Stenotrophomonas maltophilia strain W81

Stenotrophomonas maltophilia W81 can protect sugar beet against PYTHIUM:-mediated damping-off disease through the production of an extracellular protease. Here, the proteolytic enzyme of W81 was purified by anion-exchange chromatography and characterized as a serine protease. The purified enzyme was fungicidal against PYTHIUM: ultimum in vitro. Its synthesis was inducible by casein in W81, and mutagenesis of this strain using the luciferase (luxAB) reporter transposon Tn5-764cd resulted in the isolation of two mutant derivatives (W81M3 and W81M4) capable of producing significantly increased levels of extracellular protease in the presence of casein. Strain W81M4 also exhibited increased chitinolytic activity. The luxAB fusions in strains W81M3 and W81M4 were highly expressed in the absence of casein but not in its presence, suggesting that the corresponding loci were involved in down-regulating extracellular protease production. Extracellular protease production in the W81 wild-type strain and protease overproduction in mutants W81M3 and W81M4 were also induced in the presence of the autoclaved fungal mycelium. In soil microcosms naturally infested by PYTHIUM: spp., inoculation of sugar beet seeds with W81M3 or W81M4 resulted in improved biocontrol of PYTHIUM:-mediated damping-off disease compared with W81, and the level of protection achieved was equivalent to that conferred by chemical fungicides. The wild-type W81 and its mutant derivatives did not differ in rhizosphere colonization. Therefore, the improved biocontrol ability of W81M3 and W81M4 resulted from their capacity to overproduce extracellular serine protease.

Seasonal changes in protein composition of whey from commercial manufacture of caprine and ovine specialty cheeses

Pooled whey from the production of one variety of ovine cheese and two varieties of caprine cheeses was studied for gross composition and individual whey protein composition over one production season. Individual proteins were quantified by sodium dodecyl sulfate-PAGE and digital imaging technology. The mean proportion of alpha-lactalbumin (LA) from caprine wheys from the manufacture of Chevre and Cheddar-type cheeses was higher than values previously reported for bovine whey from Cheddar cheese; proportions of serum albumin, immunoglobulin (Ig)G, and beta-lactoglobulin (LG) were lower. Ovine whey from Manchego-type cheese showed a higher proportion of beta-LG, about the same proportion of alpha-LA, and lower proportions of serum albumin and IgG than did the bovine whey. Relative amounts of alpha-LA decreased throughout the season, but beta-LG rose in midlactation and then gradually decreased toward the end of lactation. Relative proportions of serum albumin remained fairly stable throughout the year, and IgG decreased.

Biological control of Pythium ultimum by Stenotrophomonas maltophilia W81 is mediated by an extracellular proteolytic activity

Stenotrophomonas maltophilia strain W81, isolated from the rhizosphere of field-grown sugar beet, produced the extracellular enzymes chitinase and protease and inhibited the growth of the phytopathogenic fungus Pythium ultimum in vitro. The role of these lytic enzymes in the interaction between W81 and P. ultimum was investigated using Tn5 insertion mutants of W81 incapable of producing extracellular protease (W81M1), extracellular chitinase (W81M2) or the two enzymes (W81A1). Lytic enzyme activity was restored in W81A1 following introduction of a 15 kb cosmid-borne fragment of W81 genomic DNA. Incubation of P. ultimum in the presence of commercial purified protease or cell-free supernatants from cultures of wild-type W81, the chitinase-negative mutant W81M2 or the complemented derivative W81A1 (pCU800) resulted in hyphal lysis and loss of subsequent fungal growth ability once re-inoculated onto fresh plates. In contrast, commercial purified chitinase or cell-free supernatants from cultures of the protease-negative mutant WS1M1 or the chitinase- and protease-negative mutant W81A1 had no effect on integrity of the essentially chitin-free Pythium mycelium, and did not prevent subsequent growth of the fungus. In soil microcosms containing soil naturally infested by Pythium spp., strains W81, W81M2 and W81A1(pCU800) reduced the ability of Pythium spp. to colonize the seeds of sugar beet and improved plant emergence compared with the untreated control, whereas W81A1 and W21M1 failed to protect sugar beet from damping-off. Wild-type W81 and its mutant derivatives colonized the rhizosphere of sugar beet to similar extents, it was concluded that the ability of S. maltophilia W81 to protect sugar beet from Pythium -mediated damping-off was due to the production of an extracellular protease.

Changes in furosine and proteins of UHT-treated milks stored at high ambient temperatures

Changes in furosine, undenatured whey protein content, and proteolysis during 90 days storage at 20, 30, and 40 degrees C of UHT-processed milk were studied. Furosine increased as the temperature and storage time increased. The peptide concentration increased during the storage period, being faster at 30 degrees C than at 20 or 40 degrees C. High-performance liquid chromatographic analysis of undenatured whey proteins showed considerable changes in the shape of the peaks during storage.