Detection and inactivation of saxitoxin in skim milk

In this study, saxitoxin dihydrochloride in skim milk was reacted with sodium hydroxide and hydrogen peroxide to yield nontoxic 8-amino-6-hydroxymethyl-iminopurine-3(2H)-propionic acid (AHIPA), which was quantified by fluorescence spectroscopy using excitation and emission wavelengths of 330 and 425 nm, respectively. Samples of saxitoxin dihydrochloride (in 20% ethanol, vol/vol) were used as controls. The limits of detection of AHIPA, based on the concentration of saxitoxin prior to inactivation, were 5 and 10 μg/ml for the control and skim milk, respectively. These values are considerably below the concentration of saxitoxin that corresponds to the lethal dosage of 1 mg for an adult of average weight (70 kg). The inactivation of saxitoxin proceeded at a lower rate in skim milk than in the control, as its reaction rate constant was only 0.004 min(-1) compared with 0.011 min(-1) for the control. We were unable to detect AHIPA in 2% milk contaminated with saxitoxin because of possible interference from what we believed were products of secondary reactions involving milk fat and sodium hydroxide. Our results also indicated that the conversion of saxitoxin to AHIPA increased initially with temperature up to 40°C but decreased thereafter. We observed a decrease in the formation of AHIPA when the concentration of hydrogen peroxide was increased except at 22°C, where there was an initial increase in AHIPA formation between 1.2 and 2.4 mg/ml hydrogen peroxide but its formation decreased thereafter.     

灭菌乳中嗜热脂肪芽孢杆菌商业无菌的最低可视浓度的测定

目的 测定灭菌乳中嗜热脂肪芽孢杆菌的商业无菌检出限。方法 随机购买3种不同品牌的灭菌乳,分别添加10_⁵、10_⁴、10_³、10_²、10_¹、10_⁰ CFU/mL浓度的嗜热脂肪芽孢杆菌制成模拟样品,分别对培养4 d、7 d、9 d、10 d的样品按照GB 4789.26-2013《食品安全国家标准 食品微生物学检验 商业无菌检验》的检验方法检测。结果 3种品牌牛乳的模拟样品商业无菌的最低检出浓度均为10_¹ CFU/mL,同时发现培养10 d后样品中的嗜热脂肪芽孢杆菌并无明显的增殖也无菌落进入休眠状态。对培养10 d后的模拟样品进行菌落计数验证以上结论。结论 灭菌乳中嗜热脂肪芽孢杆菌商业无菌检出限为10_¹ CFU/mL,如果灭菌乳中存在嗜热脂肪芽孢杆菌浓度＜10_¹ CFU/mL时,可能在检验过程中对灭菌乳的芽孢菌出现漏检。灭菌乳中嗜热脂肪芽孢杆菌在培养后无明显增殖,符合商业无菌相对无菌标准。     

Effect of thermosonication,pulsed electric field and their combination on inactivation of Listeria innocua in milk

The impact of thermosonication (TS) and pulsed electric field (PEF), individually and combined, on the survival of Listeria innocua 11288 (NCTC) in milk was investigated. TS (400 W, 160 s) without pre-heating reduced L. innocua by 1.2 log₁₀ cfu mL^?1, while shorter treatment times produced negligible inactivation, suggesting TS to be a hurdle rather than an effective standalone treatment. PEF (30 and 40 kV cm^?1, 50 μs) at 10 °C caused a reduction of L. innocua of 1.1 and 3.3 log cycles, respectively. The highest field strength (40 kV cm^?1) combined with TS (80 s) led to 6.8 log₁₀ cfu mL^?1 inactivation. Milk pre-heated to 55 °C (over 60 s) prior to TS followed by PEF (30 and 40 kV cm^?1) showed inactivation between 4.5 and 6.9 log₁₀ cfu mL^?1, the latter being comparable (P > 0.05) with thermal pasteurisation. The data indicate that TS followed by PEF represents a valid alternative for L. innocua inactivation in milk.     

NaCl对嗜热脂肪芽孢杆菌耐热性影响的研究

为了确定在100℃以下NaCl对嗜热脂肪芽孢杆菌耐热性的影响,本研究以体积个数为107 CFU/mL的嗜热脂肪芽孢杆菌菌悬液为热处理时的初始菌液,在菌悬液的中心温度在约150 s达到70℃、80℃、90℃条件下,研究不同浓度的NaCl制备的菌悬液经热处理后,嗜热脂肪芽胞杆菌的残存体积个数和热致死率,并对嗜热脂肪芽孢杆菌的残存体积个数初步建立各温度和各NaCl浓度条件下的热失活模型。研究结果表明,嗜热脂肪芽孢杆菌菌悬液在各条件下的失活曲线均可用Origin 8.0软件中Dose Resp模型拟合,且判定系数R2均可达到0.96以上。对嗜热脂肪芽孢杆菌在不同温度、不同时间点和不同浓度NaCl下的热致死率进行单因素方差分析,空白在不同热处理温度前40 s致死率均达到95%以上,而不同浓度氯化钠在70℃下前120 s均达到95%以上,在80℃下前60 s均达到93%以上,在90℃下前60 s均达到98以上,可见不同浓度的NaCl对嗜热芽孢杆菌具有保护作用,其中2%NaCl的保护作用显著大于其它浓度的NaCl。     

热-超声波联合对鼠伤寒沙门氏菌的杀菌效果研究

研究了32⁵2℃条件下,热处理以及热-超声波联合处理对鼠伤寒沙门氏菌的杀菌效果,分析了温度对热-超声波联合杀菌效果的影响。结果表明:32⁵2℃热处理杀菌效果远远达不到卫生标准;而热-超声波联合作用杀菌效果显著增强,在52℃下,190、380、570W分别处理40、30min和30min即可达到平板菌落计数法检测限;在热-超声波联合处理中,较低温度（32⁴7℃）对鼠伤寒沙门氏菌的杀菌效果贡献较小,杀菌效果主要取决于超声波作用;而致死温度（52℃）体现协同杀菌效应,增强杀菌效果,提高杀菌速率。      

Influence of calcium salt supplementation on calcium equilibrium in skim milk during pH cycle

Calcium is a mineral essential for humans, especially for bone constitution. Yet most of the worldwide population does not satisfy their Ca needs. Hence, Ca supplementation is of major importance, even in western countries where some specific populations at risk do not satisfy the recommended daily intake of Ca. More than 70% of dietary Ca comes from dairy products. Calcium supplementation of naturally Ca-rich sources such as skim milk is then of special interest. To our knowledge, few data are available concerning milk Ca (MC) supplementation of milk, particularly when followed by pH cycle. In this paper, MC supplementation is studied and compared with Ca chloride (CC) supplementation as a well-known source of Ca. The effect of Ca salt supplementation followed by pH cycle was studied in reconstituted skim milk. Calcium supplementation was carried out with CC and MC at 25 mmol of Ca/kg of skim milk. Ionized Ca concentration and turbidity variations were followed in situ by Ca ion selective electrode and turbidimetry using light reflection. From normalized data on ionized Ca concentration and turbidity vs. pH, it appeared that hysteresis areas were smaller for CC-supplemented milk, whereas unsupplemented milk and MC-supplemented milk behaved similarly. For these 3 dairy systems, pH cycles to pH 5.0 led to a larger hysteresis area than pH cycles to pH 5.5. The shrinkage of the hysteresis area could be interpreted as a reinforcement of casein micelles with Ca ions over the pH cycle.     

Enhancing inactivation of Staphylococcus aureus in skim milk by combining high-intensity pulsed electric fields and nisin

High-intensity pulsed electric fields (HIPEF) can be used as a nonthermal preservation method that is believed to enhance the effect of nisin on microorganisms such as Staphylococcus aureus. The survival of S. aureus inoculated into skim milk and treated with nisin, with HIPEF, or with a combination of nisin-HIPEF was evaluated. Nisin dose, milk pH, and HIPEF treatment time were the controlled variables that were set up at 20 to 150 ppm, pH 5.0 to 6.8, and 240 to 2,400 micros, respectively. HIPEF strength and pulse width were kept constant at 35 kV/cm and 4 micros, respectively. No reduction in S. aureus concentration was observed in skim milk at its natural pH after treatment with nisin, but 1.1 log units were recovered after 90 min of treatment at pH 5.0 with 150 ppm nisin. A reduction in viable S. aureus counts of 0.3 and 1.0 log unit in skim milk treated with HIPEF at its natural pH was observed at 240 and 2,400 micros, respectively. The nisin-HIPEF treatment design was based on a response surface methodology. The combined effect of nisin and HIPEF was clearly synergistic. However, synergism depended on pH. A maximum microbial inactivation of 6.0 log units was observed at pH 6.8, 20 ppm nisin, and 2,400 micros of HIPEF treatment time, whereas a reduction of over 4.5 log units was achieved when pH, nisin concentration, and HIPEF treatment times were set at 5.0, 150 ppm, and 240 micros, respectively.     

热-超声波联合对鼠伤寒沙门氏菌的杀菌效果研究

研究了32~52℃条件下,热处理以及热-超声波联合处理对鼠伤寒沙门氏菌的杀菌效果,分析了温度对热-超声波联合杀菌效果的影响。结果表明:32~52℃热处理杀菌效果远远达不到卫生标准;而热-超声波联合作用杀菌效果显著增强,在52℃下,190、380、570W分别处理40、30min和30min即可达到平板菌落计数法检测限;在热-超声波联合处理中,较低温度(32~47℃)对鼠伤寒沙门氏菌的杀菌效果贡献较小,杀菌效果主要取决于超声波作用;而致死温度(52℃)体现协同杀菌效应,增强杀菌效果,提高杀菌速率。     

Pressure–ohmic–thermal sterilization: A feasible approach for the inactivation of Bacillus amyloliquefaciens and Geobacillus stearothermophilus spores

The efficacy of a pressure–ohmic–thermal sterilization (POTS) for Bacillus amyloliquefaciens and Geobacillus stearothermophilus spore inactivation was investigated. Spores (2.5 × 10⁸ cfu/ml) were inoculated in 0.1% NaCl solution (pH 5.0 and 7.0), green pea puree (pH 6.1), carrot puree (pH 5.0) or tomato juice (pH 4.1). Samples were ohmically (50 V/cm) treated at 600 MPa and 105 °C for various holding times using a laboratory-scale high-pressure processor. B. amyloliquefaciens and G. stearothermophilus spores suspended in 0.1% NaCl solution (pH 7.0) were inactivated by 4.6 and 5.6 log, respectively, for a 30-min holding time. B. amyloliquefaciens and G. stearothermophilus spores in tomato juice were reduced by 3.1 and 4.8 log, respectively, for a 10-min holding time. Spore germination was highest in the G. stearothermophilus suspended in 0.1% NaCl solution (pH 7.0). POTS treatment appears to be a potent method for inactivating pressure–thermal resistant bacterial spores.Industrial RelevanceFood industry is interested in developing superior quality low-acid shelf-stable foods. This study evaluated the pressure–ohmic–thermal sterilization (POTS) for the inactivation of Bacillus amyloliquefaciens and Bacillus stearothermophilus endospores. The impact of food matrices and acidity on the spore resistance was also investigated. Knowledge gained from the study will help the food processors for understanding the importance of various POTS treatment parameters for sterilization of low-acid foods.     

Influence of denaturation and aggregation of β-lactoglobulin on rennet coagulation properties of skim milk and ultrafiltered milk

Skim milk and ultrafiltered milk were heated at temperatures in the range 80–140 °C for 4s in a UHT plate heat exchanger. The extent of denaturation of β-lactoglobulin and its association with the casein micelles increased with increase in heating temperature; the extent of association was markedly less than the amount that denatured. There was no noticeable difference in the extent of denaturation and association of β-lactoglobulin between skim milk and the corresponding ultrafiltered milk. The coagulation properties of renneted heated milks were measured by dynamic oscillation rheometry. Gelation times and storage modulus (G′) could be related to the extent of denaturation of β-lactoglobulin and its association with the casein micelles. Denaturation of β-lactoglobulin up to 60% had no effect on the gelation time, but additional denaturation markedly increased the gelation times. In contrast, G' decreased gradually with increase in the extent of denaturation of β-lactoglobulin and its association with casein micelles. The effects of β-lactoglobulin denaturation and association on coagulation properties were less pronounced in ultrafiltered milk. G' values for ultrafiltered milk were markedly higher than the corresponding skim milks under all heating conditions.     

低热脱脂奶粉的生产现状

综述了我国脱脂奶粉的的市场和生产现状,提出了向低热脱脂奶粉发展的方向,论述了加工条件对脱脂奶粉蛋白变性程度的影响以及国际上对奶粉热分级的规定和乳粉加工中乳清蛋白热变性及聚合物产生机理的研究进展。     

Physico-chemical characterization of phosphate-added skim milk

The addition of potassium phosphate (KH₂PO₄) to milk is a common practice in the dairy industry but the changes resulting from such addition have not been fully elucidated. Physico-chemical changes were evaluated after addition of KH₂PO₄ at concentrations from 0 to 160 mm with or without adjustment of pH. At pH adjusted to 6.8, the addition of phosphate to milk-induced formation of insoluble calcium phosphate salt, decrease in lightness and solubilization of casein molecules. As a result of these biochemical changes, especially at high phosphate concentrations (about 120 mm and more), the formation of large aggregates occurred. At the same time, the amount of water combined with the modified casein strongly increased. When the pH was not adjusted, the same biochemical changes occurred but they were quantitatively lower. In this case, no destabilization of casein micelles occurred.     

Partitioning of nitroxynil,oxyclozanide and levamisole residues from milk to cream,skim milk and skim milk powder

Veterinary drugs are necessary to control fluke in animals, and if not properly used, residues of these drugs may be found in milk. The aim of this study was to determine whether residues of nitroxynil, levamisole and oxyclozanide in milk partition into skim milk powder during processing. Milk targeted to contain high, medium or low levels of residue was obtained following treatment of trial animals. On separation of cream and skim milk, > 90% of the residue partitioned with the skim milk in all cases. During powder processing, the residues were not degraded with almost 90% of the residue detected in the powder.     

Microstructure of fat globules in whole milk after thermosonication treatment

The structure of fat globules in whole milk was studied after heat and thermosonication treatments to observe what happens during these processes at the microscopic level using scanning electron microscopy. Raw whole milk was thermosonicated in an ultrasonic processor-Hielscher UP400S (400 W, 24 kHz, 120 microm amplitude), using a 22-mm probe at 63 degrees C for 30 min. Heat treatment involved heating the milk at 63 degrees C for 30 min. Color and fat content were measured to correlate the images with analytical measurements. The results showed that the surface of the fat globule was completely roughened after thermosonication. Ultrasound waves were responsible for disintegrating the milk fat globule membrane (MFGM) by releasing the triacylglycerols. Furthermore, the overall structure of milk after sonication showed smaller fat globules (smaller than 1 microm) and a granular surface. This was due to the interaction between the disrupted MFGM and some casein micelles. Minor changes in the aspect of the globules between thermal and raw milks were detected. Color measurements showed higher L* values for sonicated samples. Sonicated milk was whiter (92.37 +/- 0.20) and generally showed a better degree of luminosity and homogenization compared to thermal treated milk (88.25 +/- 0.67) and raw milk (87.82 +/- 0.18). Fat content analysis yielded a higher value after sonication (4.24%) compared to untreated raw milk (4.04%) because fat extraction is more efficient after sonication. The advantages of thermosonicated milk are that it can be pasteurized and homogenized in just 1 step, it can be produced with important cost savings, and it has better characteristics, making thermosonication a potential processing method for milk and most other dairy products.     

Short communication: Growth of dairy isolates of Geobacillus thermoglucosidans in skim milk depends on lactose degradation products supplied by Anoxybacillus flavithermus as secondary species

Thermophilic bacilli such as Anoxybacillus and Geobacillus are important contaminants in dairy powder products. Remarkably, one of the common contaminants, Geobacillus thermoglucosidans, showed poor growth in skim milk, whereas significant growth of G. thermoglucosidans was observed in the presence of an Anoxybacillus flavithermus dairy isolate. In the present study, we investigated the underlying reason for this growth dependence of G. thermoglucosidans. Whole-genome sequences of 4 A. flavithermus strains and 4 G. thermoglucosidans strains were acquired, with special attention given to carbohydrate utilization clusters and proteolytic enzymes. Focusing on traits relevant for dairy environments, comparative genomic analysis revealed that all G. thermoglucosidans strains lacked the genes necessary for lactose transport and metabolism, showed poor growth in skim milk, and produced white colonies on X-gal plates, indicating the lack of β-galactosidase activity. The A. flavithermus isolates scored positive in these tests, consistent with the presence of a putative lactose utilization gene cluster. All tested isolates from both species showed proteolytic activity on milk plate count agar plates. Adding glucose or galactose to liquid skim milk supported growth of G. thermoglucosidans isolates, in line with the presence of the respective monosaccharide utilization gene clusters in the genomes. Analysis by HPLC of A. flavithermus TNO-09.006 culture filtrate indicated that the previously described growth dependence of G. thermoglucosidans in skim milk was based on the supply of glucose and galactose by A. flavithermus TNO-09.006.     

Influence of feed temperature to biofouling of ultrafiltration membrane during skim milk processing

The temperature of the feed is known to be an important factor affecting the formation and the growth rates of bacterial communities on dairy filtration membranes. However, decades after the integration of filtration processes in the dairy industry, there is still questioning whether filtration should be performed at cold (<15 °C) or hot (>48 °C) temperature. A biofilm reactor designed to mimic a filtration system was used to provide answers to this question. Bacteria adhering and growing on ultrafiltration membranes in contact with pasteurised skim milk were characterised at 15 °C and 50 °C through a metabarcoding approach. Our results suggested that the processing time should be limited to 10 h at 50 °C to avoid the exponential growth of thermophilic spore-former bacteria, while the use of 15 °C combined with daily cleaning procedures appeared the best way to retard the formation of biofilms on membranes.     

Acidification of lactoferrin-casein micelle complexes in skim milk

Lactoferrin electrostatically bound to the casein micelles when added to milk, which caused the absolute zeta potential to decrease and the micelle size to increase. On acidification, the lactoferrin progressively dissociated from the micelles, which, at pH below ∼5.0, caused the zeta potential of the casein micelles to be the same as those in milk without added lactoferrin. Acidification caused increased levels of casein to dissociate from the casein micelles at ∼pH 5.0 as the level of added lactoferrin in the milk increased. Lactoferrin decreased the pH at which the milk gelled and caused the G′ and yield stress of the set gels to increase at low levels of added lactoferrin, decrease at intermediate levels of added lactoferrin and increase again at high levels of added lactoferrin. This unusual effect of lactoferrin on gelation was hypothesised to be due to combined effects of dissociated casein and the lower gelation pH.     

State of unesterified fatty acids in skim milk

By a multiple extraction procedure the unesterified fatty acids of skim milk have been shown in at least three entities: a) free dissociated and undissociated fatty acids, b) fatty acids associated with the membrane material in skim milk, and c) fatty acids of unknown origin requiring acidification to pH 1.5 for extraction.