The status of fluoride added to bovine milk

The use of bovine milk as a carrier for fluoride has been advocated as a prophylactic measure. The effect of the addition of sodium fluoride to cold milk, up to 1000 ppm, was examined by particle micro electrophoresis, centrifugation, and dialysis.
The fluoride had little effect on the electrokinetic potential, and no sedimentation calcium fluoride occurred. Even with centrifugation at 30 000 g , the bulk of the fluoride remained in solution. Dialysis showed that the fluoride could easily be removed, and that the eluent contained fluoride in the ionic (free) form.
The addition of fluoride to calcium phosphate free (CPF, milk showed that the bulk of the fluoride remained with the protein, even after addition of excess calcium chloride. Thus the fluoride does not affect the stability of milk and the element forms a reversible ionic complex. The evidence suggests that it is held in solution by the smaller milk proteins.     

Measurement of ionic calcium in milk

There are many reports in the literature regarding the effects of ionic calcium on reactions related to casein micelle stability, such as heat stability, ethanol stability and susceptibility to gelation, sediment formation and fouling. However, experimental evidence supporting these assertions is much less readily available.
This paper evaluates three selective ion electrode systems for measuring ionic calcium directly in milk as well as looking at the effects on pH reduction and addition of calcium chloride.
The best electrode system was the Ciba Corning 634 system, which was designed for blood but has been modified for milk. This was found to be reproducible and stable when calibrated daily and allowed direct measurements to be taken on milk in 70 s. This has been found to perform well now for 3 years. The other systems were not so useful, as they took longer to stabilize, but may be useful for higher ionic calcium concentrations, which are found in acidified milk products.
Reducing the pH increased ionic calcium and reduced ethanol stability. Calcium chloride addition reduced pH, increased ionic calcium and reduced the ethanol stability. Readjusting the pH to its value before calcium addition reduced the ionic calcium, but not back to its original value. Milks from individual cows showed wide variations in their ionic calcium concentrations.
This establishes the methodology for a more detailed investigation on measurement of ionic calcium in milks from individual cows and from bulk milks, to allow a better understanding of its role in casein micelle stability.     

Composition and calcium status of acid whey from pasteurized,UHT‐treated and in‐bottle sterilized milks

Whey extracts were obtained from pasteurized, UHT‐treated and in‐bottle sterilized milks. After acidic precipitation of casein the concentration of protein, NPN, lactose, lipid, calcium, magnesium and potassium was determined. Among the parameters examined, protein content was significantly reduced in the whey extracts from UHT‐treated and in‐bottle sterilized milks compared with that from pasteurized milk, while lactose content was increased. Calcium extracted in whey was at least 80% of total calcium of the milk. The total calcium to protein ratio of whey was increased as a function of the thermal treatment of milk, while ionic calcium was about 50% of total calcium in all whey extracts. In vitro protein digestibility was found to be significantly lower in whey from UHT‐treated and in‐bottle sterilized milks than in that from pasteurized milk. Parallel estimation of the percentage of ionic calcium and of the solubility of proteins in the pH range 2–10 indicated that calcium was not involved in the pH‐dependent solubility of proteins extracted in the whey, the extent of solubility being essentially a function of the thermal treatment of milk. The results suggest that calcium was not responsible for the formation of soluble protein macroaggregates with impaired digestibility that are present in whey from milk subjected to heat treatment of increasing intensity.     

Changes in the surface protein of the fat globules during homogenization and heat treatment of concentrated milk

The changes in milk fat globules and fat globule surface proteins of both low-preheated and high-preheated concentrated milks, which were homogenized at low or high pressure, were examined. The average fat globule size decreased with increasing homogenization pressure. The total surface protein (mg m-2) of concentrated milk increased after homogenization, the extent of the increase being dependent on the temperature and the pressure of homogenization, as well as on the preheat treatment. The concentrates obtained from high-preheated milks had higher surface protein concentration than the concentrates obtained from low-preheated milks after homogenization. Concentrated milks heat treated at 79 degrees C either before or after homogenization had greater amounts of fat globule surface protein than concentrated milks heat treated at 50 or 65 degrees C. This was attributed to the association of whey protein with the native MFGM (milk fat globule membrane) proteins and the adsorbed skim milk proteins. Also, at the same homogenization temperature and pressure, the amount of whey protein on the fat globule surface of the concentrated milk that was heated after homogenization was greater than that of the concentrated milk that was heated before homogenization. The amounts of the major native MFGM proteins did not change during homogenization, indicating that the skim milk proteins did not displace the native MFGM proteins but adsorbed on to the newly formed surface.     

Diffusing-wave spectroscopy investigation of heated reconstituted skim milks containing calcium chloride

The effect of heat treatment on reconstituted 10 wt% skim milks containing up to 20 mM added CaCl₂ at different pH values (pH 6.0–7.2), was investigated both in situ and after cooling of the heat treated milks. Measurements of pH in situ showed that pH decreased at high temperature, that the decrease in pH increases with the increase in the initial pH and that the magnitude of the decrease in pH was greater for milks with added CaCl₂. Marked increases in the viscosity at 25 °C of heated milks indicated that milks without added CaCl₂ with initial pH ≤6.2 and milks with 10 mM added CaCl₂ with initial pH ≤6.4, heated at 90 °C were not heat stable. At a given heating temperature, it was possible to superimpose the measured viscosity of samples with or without added CaCl₂ on the same curve when these were plotted as a function of the pH at that temperature instead of the initial milk temperature. To further demonstrate this finding a DWS experimental set-up was built and in situ measurements were performed on the milk samples heated at 75 °C for 10 min. The DWS measurements showed that ηa, the product of the viscosity with particle size, can also be superimposed for all the measured samples, when plotted as a function of the pH at the heating temperatures. Both the viscosity and DWS data demonstrate the importance of the pH at the heating-temperatures in influencing heat-induced changes in milk.     

Effects of ultra-high pressure homogenization on microbial and physicochemical shelf life of milk

The effect of ultra-high pressure homogenization (UHPH) on microbial and physicochemical shelf life of milk during storage at 4°C was studied and compared with a conventional heat preservation technology used in industry. Milk was standardized at 3.5% fat and was processed using a Stansted high-pressure homogenizer. High-pressure treatments applied were 100, 200, and 300 MPa (single stage) with a milk inlet temperature of 40°C, and 200 and 300 MPa (single stage) with a milk inlet temperature of 30°C. The UHPH-treated milks were compared with high-pasteurized milk (PA; 90°C for 15 s). The microbiological quality was studied by enumerating total counts, psychrotropic bacteria, lactococci, lactobacilli, enterococci, coliforms, spores, and Pseudomonas. Physicochemical parameters assessed in milks were viscosity, color, pH, acidity, rate of creaming, particle size, and residual peroxidase and phosphatase activities. Immediately after treatment, UHPH was as efficient (99.99%) in reducing psychrotrophic, lactococci, and total bacteria as was the PA treatment, reaching reductions of 3.5 log cfu/mL. Coliforms, lactobacilli, and enterococci were eliminated. Microbial results of treated milks during storage at 4°C showed that UHPH treatment produced milk with a microbial shelf life between 14 and 18 d, similar to that achieved for PA milk. The UHPH treatments reduced the L* value of treated milks and induced a reduction in viscosity values of milks treated at 200 MPa compared with PA milks; however, these differences would not be appreciated by consumers. In spite of the fat aggregates detected in milks treated at 300 MPa, no creaming was observed in any UHPH-treated milk. Hence, alternative methods such as UHPH may give new opportunities to develop fluid milk with an equivalent shelf life to that of PA milk in terms of microbial and physicochemical characteristics.     

Dried milk powder containing fluoride

A milk powder was prepared from a skim milk containing 10 p.p.m. fluoride. The powder showed that the size of the powdered milk particles produced were not affected by the presence of the fluoride. The fluoride in the reconstituted milk existed in at least two reversible ionic equilibria and all of it could be recovered as 'free' fluoride after precipitation of the protein. The fluoride was evenly distributed throughout the range of milk particles, and thus mechanical handling and particle segregation would not produce unequal concentration of fluoride in a stored milk powder. The fluoride did not change its status even after 2 years storage.     

Effect of heat treatment of rennet skim milk induced coagulation on the rheological properties and molecular structure determined by synchronous fluorescence spectroscopy and turbiscan

Heat treatment applied to milk induces denaturation of whey proteins, leading to a complex mixture of whey protein and whey protein coated casein micelles. The present paper investigates the effects of heat treatment (60 and 80°C during 20min) and rennet-induced coagulation temperature (30 and 40°C) determined by rheology, synchronous fluorescence spectroscopy (SFS) and turbiscan measurements. The gelation times determined by rheology and SFS increased with the increase of heat treatment applied to milk. The rise in temperature induced a decrease in the maximum curd firming rate and an increase in the viscosity of the investigated milk samples. The principal component analysis (PCA) applied, separately, to the SF and turbiscan spectra showed a clear discrimination between: (i) raw milks and heated milks; and (ii) milks renneted at 30°C from those renneted at 40°C. The results showed the ability of SFS as a rapid and non-destructive technique for the: (i) monitoring network structure and molecular interaction during the coagulation process; and (ii) determination of gelation time of rennet-induced coagulation of studied milk samples.     

Type A and B bovine milks: Heat stability is driven by different physicochemical parameters

The value of milk hinges on its physicochemical functionality under processing conditions. We examined composition–functionality relationships with individual milks from 24 New Zealand dairy cows, sampled at 3 times over the season. Milks were classified into type A or B, according to the shape of 3-point heat coagulation time versus pH profiles. Milk type changed over the season for half of the cows in the study. Best subsets regression suggested that different factors controlled heat stability in the 2 milk types. Urea concentration was key for both types, but for type A milks, osmotic pressure and milk solids were the most important predictors of heat stability, whereas casein micelle size and ionic calcium predicted heat stability for type B milks. This study revealed that milk type is prone to change over the season, and the findings suggest that optimizing heat stability could be achieved by different means for type A versus type B milks.     

Heat damage evaluation in ultra-high pressure homogenized milk

The evaluation of heat damage in ultra-high pressure homogenized (UHPH)-treated samples has been studied by analyzing different thermal indicators such as compounds that absorb at 340 nm, hydroxymethylfurfural, free sulphydryl content, furosine, lactulose and whey protein denaturation. Milk was treated at 200 and 300 MPa with inlet temperature of 30 and 40 °C and was compared to high-pasteurized milk (PA; 90 °C, 15 s) and to commercial UHT, in bottle-sterilized and pasteurized milks. UHPH samples were differentiated from PA milk, basically by the determination of absorbance at 340 nm and β-Lactoglobulin denaturation. Both parameters indicated a reduced thermal effect in UHPH samples over PA milk. Comparison of UHPH samples with UHT and sterilized milks showed an increase of all heating indicators when severe heat process was applied. Overall, in spite of the increase of temperature during UHPH treatments, heat damage was lower compared to PA milk, suggesting the potential of this technology as an alternative to pasteurization.     

Composition and calcium status of acid whey from pasteurized, UHT-treated and in-bottle sterilized milk

Whey extracts were obtained from pasteurized, UHT-treated and in-bottle sterilized milks. After acidic precipitation of casein the concentration of protein, NPN, lactose, lipid, calcium, magnesium and potassium was determined. Among the parameters examined, protein content was significantly reduced in the whey extracts from UHT-treated and in-bottle sterilized milks compared with that from pasteurized milk, while lactose content was increased. Calcium extracted in whey was at least 80% of total calcium of the milk. The total calcium to protein ratio of whey was increased as a function of the thermal treatment of milk, while ionic calcium was about 50% of total calcium in all whey extracts. In vitro protein digestibility was found to be significantly lower in whey from UHT-treated and in-bottle sterilized milks than in that from pasteurized milk. Parallel estimation of the percentage of ionic calcium and of the solubility of proteins in the pH range 2-10 indicated that calcium was not involved in the pH-dependent solubility of proteins extracted in the whey, the extent of solubility being essentially a function of the thermal treatment of milk. The results suggest that calcium was not responsible for the formation of soluble protein macroaggregates with impaired digestibility that are present in whey from milk subjected to heat treatment of increasing intensity.     

The Role of Protein Denaturation,Extent of Proteolysis,and Storage Temperature on the Mechanism of Age Gelation in a Model System

A model system was used to investigate the effects of heat treatment, storage temperature, and proteolysis on the gelation time of both unconcentrated and concentrated (evaporated) milks. For unconcentrated milks, increasing levels of whey protein denaturation retarded the gelation process. Elevated storage temperatures accelerated the gelation process, but only in cases where milk samples had been previously proteolyzed. The actual extent of proteolysis was not related to gelation time. In evaporated milk, increasing temperature and extent of proteolysis reduced gelation time. Samples that had no induced proteolysis also gelled. Age gelation in unconcentrated and concentrated milks is controlled by two different mechanisms. In unconcentrated milk, gelation is a two-stage process where proteolysis is followed by nonenzymatic physicochemical changes. In concentrated milk, gelation occurrs by nonenzymatic physicochemical processes.     

Changes in phosphoglyceride composition during storage of ultrahigh-temperature milk, as assessed by 31P-nuclear magnetic resonance: possible involvement of thermoresistant microbial enzymes

Soluble phosphoglycerides were studied in ultrahigh-temperature (UHT) milk by 31P-nuclear magnetic resonance. It was shown that, during storage of UHT milk, manufactured from raw milk with poor microbial quality, glycerophosphocholine and glycerophosphoethanolamine disappeared in parallel with an increase in alpha-glycerophosphate (GP). Storage at 10, 20, and 30 degrees C showed a faster transformation as the temperature increased. UHT milk samples manufactured from raw milks with better microbial quality and submitted to severe heat processes did not display changes in phosphoglycerides during storage. Screening of commercial UHT milks showed variations regarding the presence of GP, while in pasteurized milk samples, the appearance of GP occurred when the commercial life had been exceeded. Inoculation of sterile milk with Pseudomonas fluorescens NCIB9046 and incubation at 10 degrees C supported that changes in phosphoglycerides could be the consequence of a phosphodiesterase activity of bacterial origin, able to survive UHT processing. A similar behavior was observed between this activity and proteolytic activity. The potential application of the detection of these compounds as spoilage predictor indices is discussed.     

The effect of high pressure-low temperature treatment on physicochemical properties in milk

This study was carried out to investigate the effect of high pressure-low temperature (HPLT) treatment on physicochemical properties and nutrients in milk. The milk was treated at 200 MPa and −4°C for 10, 20, and 30 min. Protease and lipase activities of HPLT-treated milk were highly inactivated compared with that of raw milk. Among time treatments, the 30-min treatment showed the lowest activities compared with others. Absorbance of thiobarbituric acid increased with time in HLPT-treated milks; however, no difference was observed between the raw milk and milk treated for 10 min. The concentrations of short-chain fatty acids except C₄ in HPLT-treated milks increased with time. The total free amino acids in HPLT-treated milks were greater than that of the raw milk for the 30-min treatment. l-Ascorbic acid, niacin, and riboflavin in HPLT-treated milks were significantly lower compared with concentrations in raw milk. For color, the L-value of HPLT-treated milks was significantly lower than that of the raw milk; however, there was no difference in the a-value for 10 min and in the b-value at 20 min between the raw milk and the HPLT-treated milks.     

Effects of high pressure and heat treatment on the mineral balance of goats' milk

Changes in the distribution of minerals in skim goats' milk by high pressure (400 MPa) and/or heat (85 degrees C for 30 min) treatment have been studied. Heat treatment caused reduced solubility of the calcium, magnesium and phosphorus, and this increased with the severity of heating. In contrast, high pressure released different levels of micellar elements into the soluble phase without causing appreciable changes in pH or ionic calcium concentration. The levels of soluble salts returned to their original values when the heated samples were subjected to high pressure. However, heating pressurized milk resulted in concentrations of soluble minerals that were lower than in control milks, and close to values found in heated milks. The salt balance in goats' milk was less affected by high pressure treatment at 75 degrees C than was that of cows' milk. These results are discussed in relation to the effects of high pressure and heat treatment on mineral equilibrium and micellar structure.     

Thermal inactivation of Escherichia coli in camel milk

Camels subsist and produce milk in desert pastures not utilized by other domesticated herbivores. Developing the camel milk industry can improve the economy of desert inhabitants. To comply with sanitary ordinances, camel milk is pasteurized by procedures specified for bovine milk. It is widely accepted that milk composition might affect bacterial thermal death time (TDT). Camel and bovine milks markedly differ in their chemical composition, yet data regarding TDT values of bacteria in camel milk is missing. As a first step toward developing specific heat treatments appropriate for camel milk, TDT curves of Escherichia coli in artificially contaminated camel and cow milks have been compared. Heating the milks to temperatures ranging from 58 to 65 degrees C yields similar thermal death curves and derived D- and z-values. These findings suggest that, in this temperature range, E. coli might behave similarly in bovine and camel milk. Additional TDT studies of various pathogenic species in camel milk are required before establishing pasteurization conditions of camel milk.     

基于Heracles Ⅱ超快速气相电子鼻对不同加工方式牛奶的快速鉴别

利用Heracles II超快速气相电子鼻对不同加工方式牛奶中的挥发性有机化合物进行检测。采用主成分分析、样品间区别指数分析和主成分载荷图分析对不同加工方式牛奶挥发性物质的差异性进行比较,结果表明奶粉的整体风味与其他三种不同加工方式样品差异较大,而超巴氏奶与超高温灭菌奶差异较小。结合保留指数和Aro Chem Base数据库对特异性化合物进行定性及含量分析,结果显示随着加工温度升高,丙酮、2-丁醇、辛二烯、己酸和甲硫醚的含量也随之增多。同时,甲基环己烷羧酸、丙酮、2-丁醇和水芹烯在奶粉的加工工艺中出现了大量的损失。由此得出结论,Heracles II超快速气相电子鼻能够实现对不同加工方式牛奶的快速鉴别。     

Structures and some properties of soluble protein complexes formed by the heating of reconstituted skim milk powder

The effects of heat on reconstituted skim milk powder (RSMP) at a temperature of 90 °C have been determined, with respect to the formation of soluble protein complexes. Heating at the natural pH of the RSMP forms soluble complexes in the milk serum, which can be separated by size exclusion chromatography. The diameters of the particles were found to be up to 50 nm, as shown by light scattering and by scanning electron microscopy. The formation of the soluble complexes was strongly affected by the treatment of the RSMP before heating. Treatment of the milk with small amounts of glutaraldehyde, to fix the casein micellar structure, severely inhibited the formation of the soluble complexes. The pH at which the milk was heated (in the range 6.5–7.2) also had an effect on the amounts and the sizes of the complexes, as determined by size exclusion chromatography. The amount of soluble complex present in the RSMP affected the strength of the acid gels produced from the different milks, as was shown by exchanging the sera of milks treated differently with glutaraldehyde, and by following the gelation of milks heated at different pH values. Increased amounts of soluble complexes gave stronger acid gels. However, the relationship appeared to break down if milk was heated at pH 7.2, where the soluble material appeared to change and the acid gel was much weaker.