Lactose oxidase: Enzymatic control of Pseudomonas to delay age gelation in UHT milk

Shelf-stable milk is consumed worldwide, and this market is expected to continue growing. One quality challenge for UHT milk is age gelation during shelf life, which is in part caused by bacterial heat-stable proteases (HSP) synthesized during the raw milk storage period before heat processing. Some Pseudomonas spp. are HSP producers, and their ability to grow well at refrigeration temperature make them important spoilage organisms for UHT processors to control. Previous studies have shown that lactose oxidase (LO), a natural and commercially available enzyme that produces hydrogen peroxide and lactobionic acid from lactose, can control bacterial growth in raw milk. In this research, we investigated the ability of LO to control HSP producer outgrowth, and thus delay age gelation in UHT milk. Six strains of Pseudomonas spp. were selected based on their ability to synthesize HSP and used as a cocktail to inoculate both raw and sterile (UHT) milk at a level of 1 × 10⁵ cfu/mL. Groups were treated with and without LO, stored for 4 d at 6°C, and monitored for cell count and pH. Additionally, a sample from each was tested for HSP activity via particle size analysis (average effective diameter at 90° angle and 658 nm wavelength) and visual inspection on each day of the storage period. The HSP activity results were contrasted using Tukey's HSD test, which showed that in UHT milk, a LO treatment (0.12 g/L) effectively prevented gelation as compared with the control. In raw milk, however, a concentration of 0.24 g/L of LO was needed to obtain a similar effect. This test was scaled up to 19-L pilot plant batches of raw milk where they were challenged with Pseudomonas cocktail, treated with LO for 3 d, and then UHT processed. Resulting UHT milk bottles were monitored for gelation. Significant differences in particle size between the LO-treated samples and the control were observed as early as 1 mo after processing, and gelation was not detected in the LO-treated samples through 6 mo of storage. These results demonstrated that LO can be used to delay age gelation in UHT milk induced by HSP-producing Pseudomonas spp., representing an opportunity to improve quality and reduce postproduction losses in the shelf-stable milk market sector.     

Lactose oxidase: A novel activator of the lactoperoxidase system in milk for improved shelf life

The lactoperoxidase system (LS), an antimicrobial system naturally present in milk that is activated by H₂O₂, has been used to inhibit microbial outgrowth in raw milk in areas where refrigeration is not viable. This study evaluated lactose oxidase (LO) as a novel activator of the LS. Lactose oxidase oxidizes lactose and produces H₂O₂ needed for the activation of the LS. The antimicrobial effect of different concentrations of LO with and without components of the LS, thiocyanate (TCN) and lactoperoxidase (LP), was evaluated in model systems and then applied in pasteurized milk and raw milk. In general, an increase in LO caused greater reductions of Pseudomonas fragi in the model systems and treatments were more effective at 6°C than at 21°C. At 6°C, the LO solution at 0.12 and 1.2 g/L showed significantly higher microbial reduction than the control when both added alone and combined with LS components. At 21°C, treatments with 1.2 g/L of LO solution achieved a reduction of >2.93 log cfu/mL in 24 h, but at lower levels there was not a significant reduction from the control. Higher concentrations of TCN led to a greater P. fragi reduction at both temperatures when LO was added alone but not when combined with LP. In pasteurized milk, the LO solution at 0.12 g/L caused a reduction of approximately 1.4 log of P. fragi within 24 h when added alone and a reduction of approximately 2.7 log when combined with LP and TCN. Bacterial counts remained at significantly lower levels than the control during storage, and the TCN-supplemented milk exhibited an approximately 6-log difference from the control by d 7. In raw milk, the total bacterial growth curve showed a longer lag phase when the LS was activated by LO (11.3 ± 1.4 h) compared with the control (4.0 ± 1.0 h), but it was not different from the recommended method (9.4 ± 1.0 h). However, the total bacterial count after 24 h for the sample treated with LO and TCN (5.3 log cfu/mL) was significantly lower compared with the control (7.2 log cfu/mL) and the recommended method (6.1 log cfu/mL). Results from this study suggest that LO is an alternative source of H₂O₂ that enhances the microbial inhibition achieved by the LS. Lactose oxidase could be used to develop enzyme-based preservation technologies for applications where cold chain access is limited. This enzymatic approach to improving the shelf life of dairy products also represents a novel option for clean label spoilage control.     

Lactose in blood plasma and the ability of dairy cows to tolerate once-daily milking in terms of milk loss and milk recovery

This experiment described the variability among cows with the aim of studying the ability of dairy cows to tolerate once-daily milking (ODM) in terms of milk losses and milk recoveries observed when cows are switched from twice- to once-daily milking and then back to twice-daily milking (TDM). It also aimed to investigate whether or not lactose in blood plasma, which indicates the mammary epithelium permeability, is correlated with milk losses and recoveries and, consequently, a potential candidate assessor of dairy cow tolerance to ODM. The study used 86 crossbred dairy cows (Holstein × Normande) split into 5 groups over 2 yr. The trial consisted of 3 successive periods: a 1-wk control period of TDM, then 3 wk of ODM, followed by 2 wk of TDM. Blood samples were collected 1 to 1.5 h before the morning milking and 5 to 6 h after milking on d −3, 0, 1, 3, 7, 21, 23, and 28, where d 0 is the last d of the control period. Milk losses measured as kilograms per day were higher in cows with highest control-period milk yields (r = −0.66). When expressed in relative terms (%), milk losses were weakly correlated with the control-period milk yield (r = −0.22). When switched back to TDM, cows recovered only 47% of the milk lost. Milk recovery (kg/d) was weakly correlated with control milk yield (r = 0.34) and not correlated with ODM milk yield. Milk recovery was correlated with milk yield losses: the higher the milk yield losses (kg/d or %), the higher the milk recovery (kg/d; r = −0.59 and −0.52, respectively). Rate of milk recovery expressed as the quantity of milk recovered per kilogram of milk lost, was not correlated with milk loss expressed as a percentage. This means that to be considered well-adapted to ODM scheduling, a cow must be well adapted to milk loss, and be well adapted to milk recovery. Blood plasma lactose concentration (log₁₀-transformed data) was not correlated with milk production levels in either control, ODM, or back-to-TDM periods. It was poorly correlated with milk loss but was positively correlated with milk recovery (kg/d) and rate of milk recovery. We conclude that blood plasma lactose needs to be coupled with other indicators for evaluating dairy cow tolerance to ODM.     

Evaluation of lactose oxidase as enzymatic antifungal control for Penicillium spoilage in yogurt

In this study, we investigated the antifungal activity of lactose oxidase (LO) as a potential biopreservative in dairy products. Our study objectives were to screen antifungal activity of LO against common mold strains, to detect the minimum inhibitory level of LO against the same strains, and to understand how LO affects the pH and lactic acid bacteria (LAB) counts in set yogurt. Five mold strains (Penicillium chrysogenum, Penicillium citrinum, Penicillium commune, Penicillium decumbens, and Penicillium roqueforti) were used throughout study. These strains were previously isolated from dairy manufacturing plants. Throughout the study, yogurts were stored at 21 ± 2°C for 14 d. Antifungal activity of LO was screened using 2 enzyme levels (1.2 and 12 g/L LO) against selected strains on the surface of a miniature laboratory set-yogurt model. For all tested strains, no visible mold growth was detected on the surface of yogurts covered with LO compared with control yogurt without LO. The minimum inhibitory level of LO against each strain was further investigated using 4 enzyme levels (0.12, 0.48, 0.84, and 1.2 g/L LO) on the miniature laboratory set-yogurt model. We detected 0.84 g/L LO as the minimum level inhibiting visible hyphal growth across strains. The minimum inhibitory level of LO varied for each individual strain. To study the effect of LO on the pH of yogurt, miniature laboratory set-yogurt models were covered with different enzyme levels (0.12, 0.48, 0.84, 1.2, and 12 g/L LO). At d 14, a difference was detected comparing pH values of treatments to control with no LO. Commercial low-fat set yogurt was used to study the effect of LO on LAB survival when yogurt surface was covered with 0.84 g/L LO under the same experimental conditions. Control with no LO was included. At d 14, 3 levels of catalase were added (0, 0.01, and 0.1%) to each treatment. To enumerate LAB, homogenized samples were plated on de Man, Rogosa, and Sharpe agar and incubated. Yogurts with 0.84 g/L LO had lower LAB counts compared with control yogurts, and catalase level did not have a significant effect on LAB counts. Our results demonstrated potential antifungal efficacy of LO against common spoilage organisms in dairy products with residual lactose and relatively low pH. Manufacturers should establish efficacy of LO against mold strains of interest and determine the effects of LO on organoleptic properties and LAB survival in set yogurt.     

中老年低乳糖低脂营养牛奶的研制

针对中老年人的生理特点及营养需求,合理补充钙铁锌硒及维生素等,适当调整脂肪含量,并通过添加乳糖酶将乳糖部分水解,缓解＂乳糖不耐受症＂,采用合理的杀菌方式,全面保持牛奶的营养成分.     

乳糖异构化制备乳果糖的研究

本文介绍了一种生产食品和医药添加剂乳果糖的可行性方法。采用牛乳超滤液作为乳糖来源，蛋壳作为催化剂。这是利用工业废物的一种可供选择的方法。研究了催化剂填料，乳糖浓度以及pH值对乳糖异构化的影响。温度在98℃，采用6mg／ml的催化剂填料，反应60min就可以获得乳果糖的最佳产品。乳果糖的量为1．18g／100ml，通过这种反应条件可以生产出低水平的二级产品（表乳糖，半乳糖以及有机酸）。确定了从65％-92％的乳果糖糖浆中脱除有色副产品的方法。     

环介导恒温扩增方法快速检测乳中单增李斯特菌

建立了一种快速检测灭菌乳中单增李斯特菌的环介导恒温扩增(Loop-Mediated Isothermal Amplification,LAMP)方法。以hlyA基因作为靶基因,对人工污染乳中单增李斯特菌进行了LAMP方法的灵敏度试验,同时与PCR方法进行比较。并对单增李斯特菌和7种其他乳中常见致病菌进行了LAMP检测,以验证该方法的特异性。结果表明,LAMP检测单增李斯特菌的特异性强,检出限为42 mL-1,其灵敏度比普通PCR高10倍。并且检测时间比PCR更短,在1.5 h内即可完成扩增反应。此方法快速、特异、简单、灵敏,具有较高的推广价值。     

单增李斯特菌的生长特性及其在冷藏牛奶中的预测模型

通过测定单增李斯特菌菌株在不同培养条件下菌液的OD600值,绘制生长曲线,分析培养基成分、温度、p H、Na Cl浓度、接种量以及转速对单增李斯特菌生长的影响,并选取Gompertz方程、Logistic方程和Hill方程建立单增李斯特菌在冷藏牛奶中的生长模型。结果表明:菌株在TSB-YE和MRS培养基中生长良好;能够耐受0.5%³%的Na Cl浓度范围;最适生长温度为37℃,最适生长p H为8,对氧气的需求量表现不明显。菌株可在4℃冷藏的全脂牛奶中缓慢生长,达到稳定期的菌数有2 lg CFU/m L的增长,Gompertz和Logistic方程模型可用于预测其生长情况。      

Control of Listeria monocytogenes in whole milk using antimicrobials applied individually and in combination

Dairy product recalls and dairy-related illnesses are often the result of contamination with Listeria monocytogenes, which can occur throughout the dairy production and supply chains. The use of antimicrobial compounds is one practical approach for controlling pathogen survival and growth in foods. The goal of this study was to use fluid milk as a model system to identify listeristatic or listericidal treatments that show promise for application in fluid milk and for further evaluation in other dairy products (e.g., cheese). Caprylic acid (CA), ε-polylysine (EPL), hydrogen peroxide, lauric arginate (LAE), and sodium caprylate (SC) were added individually or in combination to whole milk inoculated with L. monocytogenes at ?4 log₁₀ cfu/mL. Samples were stored at 7°C for 21 d, and L. monocytogenes counts were determined weekly. Inhibitory concentrations of LAE (800 mg/L) and EPL (100–400 mg/L), as well as SC and CA (3,200 mg/L each), were identified. The addition of EPL at 800 mg/L reduced L. monocytogenes counts by >3 log₁₀ cfu/mL from initial inoculation levels after 21 d. Addition of hydrogen peroxide to milk reduced counts by >3 log₁₀ cfu/mL from initial inoculation within 24 h (400 and 800 mg/L) or by d 7 (200 mg/L). Although the combinatory treatments of EPL + CA, EPL + LAE, and LAE + SC were characterized as indifferent, EPL + SC worked synergistically to reduce L. monocytogenes populations in milk over 21 d. Overall, these data identify potential antimicrobial treatments to control L. monocytogenes in milk and serve as a foundation for the continued development of antimicrobial controls for L. monocytogenes in dairy products.     

Dynamic kinetic analysis of growth of Listeria monocytogenes in pasteurized cow milk

The objective of this study was to develop a dynamic model for predicting the growth of Listeria monocytogenes in pasteurized cow milk under fluctuating temperature conditions during storage and temperature abuse. Six dynamic temperature profiles that simulated random fluctuation patterns were designed to change arbitrarily between 4 and 30°C. The growth data collected from 3 independent temperature profiles were used to determine the kinetic parameters and construct a growth model combining the primary and secondary models using a 1-step dynamic analysis method. The results showed that the estimated minimum growth temperature and maximum cell concentration were 0.6 ± 0.2°C and 7.8 ± 0.1 log cfu/mL (mean ± standard error), with the root mean square error (RMSE) only 0.3 log cfu/mL for model development. The model and the associated kinetic parameters were validated using the data collected under both dynamic and isothermal conditions, which were not used for model development, to verify the accuracy of prediction. The RMSE of prediction was approximately 0.3 log cfu/mL for fluctuating temperature profiles, and it was between 0.2 and 1.1 log cfu/mL under certain isothermal temperatures (2–30°C). The resulting model and kinetic parameters were further validated using 3 growth curves at 4, 7, and 10°C arbitrarily selected from ComBase (www.combase.cc). The RMSE of prediction was 0.8, 0.4, and 0.5 log cfu/mL, respectively, for these curves. The validation results indicated the predictive model was reasonably accurate, with relatively small RMSE. The model was then used to simulate the growth of L. monocytogenes under a variety of continuous and square-wave temperature profiles to demonstrate its potential application. The results of this study showed that the model developed in this study can be used to predict the growth of L. monocytogenes in contaminated milk during storage.     

Lactose hydrolysis and lactase activity in fermented mixtures containing mare's,cow's,sheep's and goat's milk

The degree of lactose hydrolysis and exogenous lactase activity of fermented mare's milk (MM) mixed with cow's (CM), sheep's (SM) and goat's milk (GM) was investigated. Moreover, the overall sensory desirableness of the novel products was also evaluated. A mixture MM + CM, or MM + GM, or MM + SM at a 1:1 ratio, significantly reduced the amount of contained lactose (P < 0.05). Fermented MM + SM mixture may be considered the product with the greatest potential consumer desirability. This particular product is preferable for patients with secondary lactose intolerance. Its additional advantage is connected with the predicted course of fermentation assessed on the basis of changes in pH, the count of mesophilic LAB and yeast as well as their survival rates during storage.     

Identification of lactose ureide, a urea derivative of lactose, in milk and milk products

With the widespread consumption of milk, the complete characterization of the constituents of milk and milk products is important in terms of functionality and safety. In this study, a novel nonreducing carbohydrate was separated from powdered skim milk and was identified using electron spray ionization-mass spectrometry (m/z 385.1[M + H⁺]), ¹H, ¹³C, ¹H¹H-correlation spectroscopy, and heteronuclear single quantum-nuclear magnetic resonance spectra. The carbohydrate was identified as a lactose derivative of urea, N-carbamoyl-o-β-d-galactopyranosyl-(1–4)-d-glucopyranosylamine (lactose ureide, LU). For the HPLC analysis of LU in milk and milk products, benzoylated LU, hepta-o-benzoyl lactose ureide (melting point 137–139 °C; m/z 1,113 [M + H⁺]; wavelength of maximum absorption, λ_max, 229 nm; molar extinction coefficient, ε, 8.1037 × 10⁷), was used as a standard. The crude nonreducing carbohydrate fraction from raw milk, thermally processed milk, and milk products such as powdered milks were directly benzoylated and subjected to HPLC analysis using an octadecylsilyl column to determine the quantity of LU. The content of LU in 10% solutions of powdered skim milk and powdered infant formula (5.0 ± 1.1 and 4.9 ± 1.5 mg/L, respectively) were almost 3-fold higher than that of UHT milk (1.6 ± 0.5 mg/L) and higher than that of low-temperature, long-time-processed (pasteurized at 65 °C for 30 min) milk (1.2 ± 0.3 mg/L) and the fresh raw milk sample (0.3 ± 0.1 mg/L). A time-course of the LU content in raw milk during heating at 110 °C revealed that LU increased with time. From these results, it is likely that LU is formed by the Maillard-type reaction between the lactose and urea in milk and milk products. Because the concentration of LU in milk increased with the degree of processing heat treatment, it could serve as an indicator of the thermal deterioration of milk. Although it is known that the human intestine is unable to digest LU, the gastrointestinal bacteria in human subjects are able to digest and utilize urea nitrogen in formation of essential amino acids that are available to the host human. These findings suggest that LU in milk might have a functional role in human health.     

单增李斯特菌在牛奶中生长曲线的拟合和预测模型的建立

通过生长动力学曲线拟合,建立牛奶中单增李斯特菌的生长预测模型。比较了5、15、25和35℃条件下Gompertz方程、Logistic方程和Hill方程的拟合情况,确定Gompertz方程为单增李斯特菌在牛奶中最优的初级模型。采用Belehradek方程描述温度对最大生长速率和延滞时间的影响,结果表明它们之间成良好的线性关系(R2分别为0.9721和0.9747),并建立了相应的二级模型。对20℃条件下由预测模型构建的微生物生长动力学方程进行了验证,同时对模型的可靠性进行了分析,结果表明预测值和真实值的残差均小于0.2,模型的偏差度为1.04,准确度为1.13,表明建立的模型有效且可靠。在已知初始菌数的情况下,可以根据模型快速预测5~35℃条件下不同贮藏时间点单增李斯特菌在牛奶中的生长情况。      

酶法低乳糖牛奶水解率测定及酶解前后牛奶脂肪酸分析

采用乳糖酶水解液体牛奶并采用Willstarter和Schudel醛糖测定方法测定乳糖水解率，在2h和4h后乳糖水解率分别为20．2％～28．39％和47．03％～66．57％，乳糖水解过程乳的脂肪酸没有显著变化。有少量短链和长链脂肪酸游离含量增加。     

Staphylococcus aureus and Listeria monocytogenes in Norwegian raw milk cheese production

The aim of this study was to survey the presence of Staphylococcus aureus and Listeria monocytogenes during the cheese making process in small-scale raw milk cheese production in Norway.The prevalence of S. aureus in bovine and caprine raw milk samples was 47.3% and 98.8%, respectively. An increase in contamination during the first 2-3 h resulted in a 73.6% prevalence of contamination in the bovine curd, and 23 out of 38 S. aureus-negative bovine milk samples gave rise to S. aureus-positive curds. The highest contamination levels of S. aureus were reached in both caprine and bovine cheese after 5-6 h (after the first pressing). There was no contamination of L. monocytogenes in caprine cheeses and only one (1.4%) contaminated bovine cheese.This work has increased our knowledge about S. aureus and L. monocytogenes contamination during the process of raw milk cheese production and gives an account of the hygiene status during the manufacture of Norwegian raw milk cheeses.     

乳糖诱导重组大肠杆菌表达胆固醇氧化酶的研究

采用乳糖诱导胆固醇氧化酶(COD)基因在大肠杆菌BL21(DE3)中表达,研究了培养基成分、乳糖浓度、诱导时间和诱导温度对胆固醇氧化酶表达的影响。结果显示,在对诱导条件进行优化控制的前提下,胆固醇氧化酶酶活达到15.2 U/mL。研究结果为乳糖作为诱导剂最终应用于重组基因工程药物的工业化生产提供了有益的参考和借鉴。     

Short communication: Persistent contamination by Listeria monocytogenes of bovine raw milk investigated by whole-genome sequencing

Following the persistent detection of Listeria monocytogenes in raw bovine milk sold through a vending machine, the 120 lactating cows of the herd producing the milk were subjected to bacteriological investigation. A single cow with subclinical mastitis (1.2–1.3 × 10⁵ somatic cells/mL) and persistent L. monocytogenes excretion was detected. The cow was subjected to antimicrobial therapy, but L. monocytogenes excretion remained high (>3.0 × 10² cfu/mL). Following culling of the infected cow, L. monocytogenes disappeared from the tank milk, and further isolates were recovered from the mammary parenchyma and lymph nodes of the infected cow. To investigate the clonal nature of the contamination, all isolates recovered in the study (n = 13) were analyzed by serogroup PCR, pulsed-field gel electrophoresis, and whole-genome sequencing. Our results demonstrated the clonal nature of the contamination. All isolates belonged to lineage II, serogroup IIa, sequence type 37, clonal complex 37 and harbored some virulence determinants. This case showed that, although relatively rare, prolonged milk contamination by L. monocytogenes can originate from subclinical and persistently infected cows, posing a health risk to consumers.     

Membrane Fractionation Processes for Removing 90% to 95% of the Lactose and Sodium from Skim Milk and for Preparing Lactose and Sodium‐Reduced Skim Milk

ABSTRACT: Pilot‐scale microfiltration (MF), microfiltration‐diafiltration (MDF), ultrafiltration (UF), ultrafiltration‐diafiltration (UDF), and nanofilration (NF) membrane fractionation processes were designed and evaluated for removing 90% to 95% of the lactose and sodium from skim milk. The study was designed to evaluate several membrane fractionation schemes as a function of: (1) membrane types with and without diafiltration; (2) fractionation process temperatures ranging from 17 to 45 °C; (3) sources of commercial drinking water used as diafiltrant; and (4) final mass concentration ratios (MCR) ranging from about 2 to 5. MF and MDF membranes provided highest flux values, but were unsatisfactory because they failed to retain all of the whey proteins. UDF fractionation processes removed more than 90% to 95% of the lactose and sodium from skim milk. NF permeate prepared from UDF cumulative permeate contained sodium and other mineral concentrations that would make them unsuitable for use as a diafiltrant for UDF applications. A method was devised for preparing simulated milk permeate (SMP) formulated with calcium, magnesium, and potassium hydroxides, and phosphoric and citric acids for use as UDF diafiltrant or for preparing lactose and sodium reduced skim milk (L‐RSM). MF retentates with MCR values of 4.7 to 5.0 exhibited extremely poor frozen storage stabilities of less than 1 wk at ?20 °C, whereas MCR 1.77 to 2.95 MDF and UDF retentates and skim milk control exhibited frozen storage stabilities of more than 16 wk. L‐RSM exhibited a whiter appearance and a lower viscosity than skim milk, lacked natural milk flavor, and exhibited a metallic off‐flavor.     

牛乳中乳过氧化物酶的分离纯化

采用强阳离子交换色谱梯度洗脱法,对牛乳中的乳过氧化物酶进行了分离和纯化,利用SDS-PAGE定性检测。结果表明,分离出的乳过氧化物酶显示为单一区带,相对分子质量为76186u。该酶酶活回收率为83.07%,纯化倍数达到44.63倍。