High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h⁻¹ homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature T_in (4°C, 14°C or 24°C) on milk temperature T₂ at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The T_in values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High T_in and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d_4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at T_in=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (T_in=4°C, 14°C or 24°C) further decreased d_4.3 diameters to about 0.2 μm and narrowed the size distribution. At all T_in tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at T_in=24°C (T₂∼60°C), 250 MPa at T_in=14°C (T₂∼62°C), or 300 MPa at T_in=4°C (T₂∼65°C). At all T_in tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.     

Influence of enzymatic cross-linking on milk fat globules and emulsifying properties of milk proteins

The enzyme transglutaminase (TGase) can modify dairy protein functionality through cross-linking of proteins. This study examined the effects of TGase treatment on milk fat globules and the emulsifying properties of milk proteins. The extent of TGase-induced cross-linking of caseins increased with incubation time, with no differences between whole and skim milk. Extensive clustering of fat globules in extensively cross-linked raw whole milk occurred on homogenisation at 400 or 800 bar. Considerably less clustering of fat globules was observed when recombined milk (90 g fat L^–1) was prepared from TGase-treated skim milk and homogenised at 400 or 800 bar. TGase treatment did not affect fat globule size in cream, but prevented coalescence of fat globules therein, possibly through cross-linking of milk fat globule membrane components. TGase-induced cross-linking of milk proteins affected their emulsifying properties and may increase the stability of natural milk fat globules against coalescence.     

Isolated Milk Fat Globules as Substrate for Lipoprotein Lipase: Study of Factors Relevant to Spontaneous Lipolysis in Milk

Fat globules isolated from normal and from spontaneous milk samples were compared as substrates for purified lipoprotein lipase. Only slight differences were observed. Fat globules isolated from fresh warm milk were almost resistant to lipolysis. This included globules from milk prone to spontaneous lipolysis. Cooling made the globules accessible to rapid lipolysis even if they were from normal milk. Rewarming the fat globules did not reverse the process. Maximum rate of lipolysis (after rewarming) required fat globules be stored at 10°C or below for 5 to 10 h. Lipolysis at 4°C usually started after a lag time of 3 to 5 h, but with fat globules from spontaneous milk the lag time was shorter. Fat globules isolated from cold milk were a poor substrate at 4°C but were lipolyzed when warmed. When ¹²⁵I-labeled lipase was added to fresh warm milk, some of the lipase bound to the milk fat globules but it caused little lipolysis. Binding increased after cooling, as did lipolysis. Both binding of lipase and lipolysis were impeded by the presence of skim milk. Another way to make fat globules isolated from fresh warm milk susceptible to lipolysis was to treat them with chemicals known to remove proteins.     

The use of sedimentation field-flow fractionation in the size characterization of bovine milk fat globules as affected by heat treatment

Size distribution of fat globules affects the appearance, taste and stability of milk and milk-based products. Full-fat, semi-fat and chocolate bovine milk were subjected to heat treatment within a temperature range of 50–125 °C for 1 h. Sedimentation field-flow fractionation was employed to determine the changes in mean particle diameter of milk fat globules as affected by heat treatment. The mean particle diameter of fat droplets increased with increasing heating temperature for most samples. The particle size of fat globules increased on average 40 nm (4.65%) for full-fat and 72 nm (8.52%) for semi-fat milk following the heat treatment (50–125 °C). Chocolate milk exhibited considerable increase in particle size (104 nm, 12.53%) within a certain temperature range (50–110 °C), followed by a decrease in particle size when heated at 125 °C for 1 h. Heat-induced flocculation due to attractive interactions between hydrophobic sites on denatured protein molecules on different droplets was assumed to be mainly responsible for the increases in particle size observed in this study. Extensive heat-induced denaturation of milk proteins was also indicated by Native PAGE. Sedimentation field-flow fractionation proved to be a useful technique for adequately monitoring heat-induced changes in particle size distributions in milk.     

Modeling of particle size distribution of sonicated coconut milk emulsion: Effect of emulsifiers and sonication time

Coconut milk was extracted from grated coconut meat with addition of water by a hydraulic press. This milk was mixed with emulsifiers (Gum Acacia and maltodextrin) at different ratios of emulsifier to fat (4, 2.75, and 1.5) and different maltodextrin (MD) to Gum Acacia (GA) ratio (0.67, 1.085, and 1.5). The emulsion was sonicated for different time durations (0.5–3.0 min). Particle size analysis of the sonicated coconut milk emulsion revealed that the number fraction of fat globules below 2 × 10⁻⁶ m increased as the emulsifier to fat ratio increased. However, at the highest ratio of emulsifier to fat this trend reversed even at higher sonication times. Similar slump in the increasing trend of globules below 2 × 10⁻⁶ m were observed at the highest value of ratio of MD to GA (1.5) studied. Modeling of particle size distribution by Rosin–Rambler–Sperling–Bennet relation was found to be a good tool for prediction of uniformity of distribution n and statistical average globule diameter M. The n values varied between 0.981 and 1.318 where as M values lied between 2.11 × 10⁻⁶ and 4.12 × 10⁻⁶ m. Linear regression fitting of n and M values as function of ratio of emulsifier to fat, ratio of MD to GA and the sonication time resulted in a good fit with relative deviation of 3.96% and 10.77%, respectively. These linear regression equations provided a suitable model to predict the sonication time required to achieve certain degree of size reduction with a relative deviation of 2.33%.     

Size-based fractionation of native milk fat globules by two-stage centrifugal separation

A two-stage centrifugal separation method, at various separation temperatures and feed rates, was employed to fractionate milk and cream on the basis of fat globule size. It involved a modified and a conventional centrifugal separation in first and second stages, respectively. In the first stage, two streams of milk: one rich in larger fat globules and another rich in smaller fat globules, were obtained by fractionation in a modified cream separator. In the second stage, the two streams from the first stage were each further fractionated in a conventional cream separator. Depending on the temperature and feed rate of the first stage, this double separation method was able to create streams with mean fat globule size (D [4, 3]) as small as 1.35 μm and as large as 4.28 μm without affecting the droplet integrity. The developed method has potential for size-based fractionation of native fat globules in industrial scale.Industrial relevanceThe developed method has potential for size based fractionation of native fat globules in industrial scale.     

Proteolysis and storage stability of UHT milk produced in Turkey

The effect of raw milk quality (total and psychrotrophic bacterial and somatic cell counts, proteinase and plasmin activity) and UHT temperature (145 or 150 °C for 4 s) on proteolysis in UHT milk processed by a direct (steam-injection) system was investigated during storage at 25 °C for 180 d. High proteinase activity was measured in low-quality raw milk, which had high somatic cell count, bacterial count and plasmin activity. The levels of 12% trichloroacetic acid–soluble and pH 4.6-soluble nitrogen in all milk samples increased during storage, and samples produced from low-quality milk at the lower UHT temperature (145 °C) showed the highest values. Bitterness in UHT milk processed from low-quality milk at 145 °C increased during storage; gelation occurred in that milk after 150 d. The RP-HPLC profiles of pH 4.6-soluble fraction of the UHT milk samples produced at 150 °C showed quite small number of peaks after 180 d of storage. Sterilization at 150 °C extended the shelf-life of the UHT milk by reducing proteolysis, gelation and bitterness.     

Addition of olein from milk fat positively affects the firmness of butter

Butter is highly valued for its characteristic flavor and aroma; however, it has the disadvantage of unsatisfactory spreadability at low temperatures. The functional characteristics of butter can be modified by changing its composition or physical structure. The objective of this study was to evaluate the effect of olein on structure and composition of butter. Olein was obtained by two stage dry fractionation process of the anhydrous milk fat (AMF) and added to commercial cream prior to butter manufacture. The fractions were characterized for triacylglycerols composition, solid fat content, crystallization isotherm, and thermal behavior. Butter was manufactured using commercial cream or cream containing 50% olein. Butter samples were characterized for physicochemical composition, instrumental color, crystallization parameters, and firmness after 1 and 7 days of storage at 10 °C. The firmness of butter subjected to room temperature was also evaluated. Butter containing olein differed significantly from the control and had darker yellow color, higher crystallization time, and lower solid fat content after 120 min at 15 °C, and hence lower firmness after 1 and 7 days of refrigerated storage. Although lower firmness was observed over time for all samples at room temperature, butter containing olein exhibited lower firmness after both 1 and 7 days, thus suggesting changes in organization of solid fat crystal network in the liquid fat. The addition of olein to butter allowed obtaining a softer product, with more intense color and possible nutritional benefits due to the medium chain triglycerides and higher carotene levels.     

A Naturally Buffered Bulk Retentate Starter from Ultrafiltered Milk

Whole milk was ultrafiltered to approximately 4:1 protein concentration, heated to 85°C for 30 min, and cooled to 22°C. It was inoculated with a commercial frozen concentrated lactic starter to give approximately 10⁷ cfu/ml and incubated at 22°C for 12 h. A commercial phage inhibitory medium and 11% nonfat dry milk were used as controls. After 12 h, retentate had significantly higher colony forming units per milliliter (3.2 × 10⁹) and pH (5.21) than phage inhibitory medium (2.5 × 10⁹ and pH 5.02) and nonfat dry milk (2.4 × 10⁹ and pH 4.58). Retentate starter and phage inhibitory medium starter had equal activity in skim milk (.3% developed acidity in 4 h at 32°C) whereas nonfat dry milk starter had significantly lower activity (.26% developed acidity). After a further 8 h incubation at 22°C, retentate starter had the highest pH (4.95) compared with phage inhibitory medium (4.76) and nonfat dry milk (4.51). At this time retentate starter activity was higher (.3%) than phage inhibitory medium (.27%) and nonfat dry milk (.19%). In highly concentrated retentates (3.5:1 and 5:1), retentate starter lowered pH considerably quicker than nonfat dry milk starter.     

Acid coagulation properties and suitability for yogurt production of cows’ milk treated by high-pressure homogenisation

The effects of high-pressure homogenisation (HPH) of cows’ milk were investigated for suitability for yogurt manufacture, compared with the processes currently applied in industry. Milk at different inlet temperatures (30 °C or 40 °C) was subjected to HPH treatment at 100, 200 or 300 MPa (one stage) and 130, 230 or 330 MPa (two-stage). HPH-treated milk was compared with milk heat-treated (90 °C for 90 s) and homogenised at 15 MPa, and with milk treated under the same thermal conditions and also fortified with 3% skim milk powder. Milk treated at 300 or 200 MPa showed higher gel strengths on coagulation, higher gel firmness in texture analysis, less syneresis and lower titratable acidity compared with conventionally treated milk fortified with 3% skim milk powder.     

Monoacylglycerols in dairy recombined cream: I. The effect on milk fat crystallization

The effect of monoacylglycerols (rich in oleic acid, stearic acid or lauric acid) on milk fat crystallization in recombined cream is examined using differential scanning calorimetry to study crystallization kinetics, nuclear magnetic resonance to measure solid fat content during storage and interfacial tension analyses to analyze interfacial properties. The long-chain saturated monoacylglycerols (stearic acid) form upon cooling a two-dimensional crystal at the interface, called chain crystallization, which induces interfacial heterogeneous nucleation. Also, the crystal growth and α–β′ polymorphic transition were accelerated. On the contrary, long-chain unsaturated monoacylglycerols (oleic acid) didn't affect the crystallization behavior while mid-chain saturated monoacylglycerols (lauric acid) showed intermediate behavior. None of the monoacylglycerols influenced the solid fat content after storage for 5 days at 5 °C. The observed differences in nucleation mechanism and crystallization kinetics may influence the microstructural arrangement of the milk fat inside the fat globules and consequently the partial coalescence rate and hence the whipping properties of recombined cream.     

Microbial shelf-life of high-pressure-homogenised milk

The anti-bacterial effect of high pressure homogenisation (HPH) on milk is widely reported but the shelf-life of HPH-treated milk, as reported in this communication has not been studied thus far. Raw whole milk was homogenised at 200 or 250 MPa at 55 or 70 °C and counts of total bacteria (TBC), psychrotrophs, pseudomonads, coliforms, lactobacilli, Bacillus cereus and Staphylococcus aureus were determined throughout subsequent storage for 14 days at 4 °C. Immediately after HPH treatment, counts of all bacteria were below the level of detection but after storage for 14 days at 4 °C, TBC, psychrotroph and pseudomonad counts had reached ∼10⁸ cfu mL⁻¹ in all samples treated with HPH. The limited shelf-life obtained indicates that HPH of milk at these processing parameters it is not a suitable alternative to pasteurisation for extending the shelf-life of milk.     

Direct measurement of phase transitions in milk fat during cooling of cream—a low-field NMR approach

Low-field ¹H nuclear magnetic resonance (NMR) transverse relaxation (T₂) was measured during cooling (31–4 °C) of cream with low or high content of long-chain fatty acids (FA). Distributed analysis of the T₂ relaxation data revealed marked differences in the T₂ relaxation characteristics of the liquid fat population (10–100 ms), which was ascribed to differences in the size of the fat globules in the two types of cream. Principal component analysis (PCA) of the obtained T₂ relaxation decay data showed clear shifts in NMR relaxation behavior at 17 and 22 °C for cream with low or high content of long-chain FA, respectively. Differential scanning calorimetric measurements on the creams revealed that these shifts took place at the onset of crystallization of fat. Consequently, the present study demonstrated that low-field NMR can be used to measure phase transitions in cream.     

Long-term stability of waxy maize starch/xanthan gum mixtures prepared at a temperature within the gelatinization range

The retrogradation behavior of waxy maize starch pastes with added xanthan gum was investigated. The pastes were prepared with or without added xanthan gum at two starch concentrations (5:100 and 7:100), at two temperatures within the gelatinization range (70 °C or 72 °C) and at acid or neutral pH. The evolution of the macrostructure (texture, flow behavior and syneresis), microstructure (oscillation rheology) and molecular aggregation (pulsed-field NMR) of the pastes was followed up in an 8 week period during cold storage (7 °C). Heating temperature was found to have a profound effect on the long-term stability of the pastes. Pastes prepared at 70 °C behaved as flocculated dispersions of largely intact, swollen granules which resulted in stable structural properties during the preservation period. At 72 °C, a higher degree of gelatinization and degradation was observed, resulting in structurally unstable systems due to association and crystallization of amylopectin. Especially for these systems, xanthan gum was found to enhance the gelation process, probably due to phase separation. Although xanthan gum may restrict the granule disruption during preparation, this beneficial effect on the freshly prepared pastes did not result in an improved stability during storage.     

Microstructure and rheological properties of whipped cream as affected by heat treatment and addition of stabilizer

Whipping cream, 36% fat, was processed at 75°C for 16 s (high-temperature short time, HTST), and 140°C for 4 s, (ultra-high temperature, UHT) with and without added stabilizer. Following heat treatment and chilling for 24 h (5°C), cream samples were whipped to maximum overrun. Structural properties were assessed through low-temperature scanning electron microscopy (LTSEM) and dynamic oscillatory testing, using a controlled stress rheometer, immediately after the cream was whipped, and again after the foam was stored for 24 h at 5°C. No differences between fresh foam treatments were observed. However, aged foams exhibited changes in microstructure because of destabilization of the foam. Differences between aged foams resulted from the addition of stabilizers and the choice of heat treatment. Microstructural differences correlated well with measured changes in rheological properties.     

Temperature effects on Fossomatic cell counts in goats milk

In cows milk, analytical temperature has been identified as one of the possible factors affecting the somatic cell count (SCC). To establish the effect of the temperature used for SCC in goats milk, counts were performed on 45 goats milk samples using the Fossomatic counter. Tests were performed at two temperatures (40° and 60 °C) on preservative-free samples 3 h after collection, and on samples preserved with bronopol (BR) at 3 h, 1, 2, 3, and 4 d post-collection. The test temperature did not modify the SCC of the milk samples analyzed. Incubating samples 3 h post-collection at 60 °C failed to improve the SCC results. Similar counts were obtained for BR-preserved samples stored for 1–4 d at refrigeration temperature, suggesting the possibility of performing Fossomatic SCC in goats milk samples stored for this length of time.     

Screening for and characterization of Lactococcus lactis bacteriophages with high thermal resistance

Fifty-six Lactococcus lactis phage isolates collected from different German dairies and obtained from a starter culture manufacturer were tested for their heat resistance. About 40% of these isolates resisted treatment at 80 °C for 5 min when they were heated in milk. The most resistant phage isolate, P1532, was collected from sour cream. Plaque-formation was still detectable even after heating at 97 °C for 5 min. The second heat-resistant one, P680, showed some plaque-forming ability after heating at 95 °C for 5 min. Kinetic parameters for the thermal inactivation of these two resistant phages were determined for temperatures ranging from 70 to 97 °C. The inactivation of phage P1532 in skim milk and in buffer medium were found to follow first-order kinetics and did not exhibit tailing, whereas in the inactivation curves of phage P680 tailing was observed. The D-value of P1532 at pasteurization temperature of 72 °C was calculated as 112 min.     

Fermentation of Ultrafiltered Skim Milk Retentates with Mesophilic Lactic Cheese Starters

Acid production and its relation to pH changes by commercial, direct-set frozen concentrated lactic starters in skim milk and 2:1 skim milk retentates were studied. Retentates resisted pH change below pH 5.2 despite the production of large amounts of lactic acid by starter bacteria. Control skim milk required 6 h at 32°C to attain pH 4.6, but skim milk retentates incubated similarly could not be fermented to this pH even after 8.5 h. Doubling the starter inoculum in the retentate led to pH 4.6 in 7.5 h. Direct-set starter DS1, with more bacteria numbers than direct-set starter DS2, fermented skim milk and 2:1 skim milk retentate more rapidly.     

Effect of Milk Fat on the Stability of Vitamin A in Ultra-High Temperature Milk

The stability of vitamin A in ultra-high temperature milks with .15, 2.92, 6.16, and 9.7% fat during storage at 26°C was studied over 3 wk. Milks were fortified with synthetic retinyl palmitate to a final concentration of approximately 120 μg retinol equivalent/I 00 ml milk. The four milk samples were ultra-high temperature processed and packaged in 100 ml sterile milk dilution bottles and stored in the dark at 26°C for 3 wk. Vitamin A concentrations decreased rapidly during the first 2 wk of storage then stabilized. Degradation rates during the first 2 wk were linear and varied inversely with the fat content in the milk (the more fat, the slower the rate of degradation). Final vitamin A concentrations at the end of 3 wk of storage were higher in milk with high fat and closely corresponded to the native vitamin A concentrations present in milk prior to fortification. The results indicate a possible protective effect due to fat or a difference in stability between native and synthetic vitamin A.     

Properties of low-fat stirred yoghurts made from high-pressure-processed skim milk

Physical properties of stirred yoghurt made from reconstituted skim milk that was high-pressure (HP)-treated at 100, 250 or 400 MPa, at 25, 70 or 90 °C, for 10 min, prior to inoculation with yoghurt cultures, were studied; portions of milk HP-treated at 25 °C were also heat-treated at 90 °C for 10 min before or after pressure treatment. Control yoghurts were made from skim milk given a heat treatment at 90 °C for 10 min. Fermentation time was not affected by treatment applied to the milk. HP treatment of skim milk at 25 °C, before or after heat treatment, gave stirred yoghurts of similar viscosities to that made from conventionally heat-treated milk. Lower viscosities were obtained when stirred yoghurts were made with milk HP-treated at elevated temperatures. A model is proposed to correlate properties of yoghurt with HP/heat-induced changes in interactions and structures of protein in the milk samples.Industrial relevanceTo meet end user expectations, the dairy industry needs to diversify its product range by tailoring specific functionalities. To meet these expectations, new processing methods such as high-pressure processing are of interest for their potential to achieve specific and/or novel functionalities and/or improve efficiencies, including reduced chemical and water use. In this paper, an investigation of the use simultaneous pressurization and heating of milk before the manufacture of stirred yoghurt is presented.