The effect of hexametaphosphate addition during milk powder manufacture on the properties of reconstituted skim milk

Skim milk powders with various levels of sodium hexametaphosphate (NaHMP) were prepared. Reconstituted skim milk samples were prepared from these powders. NaHMP slightly reduced the pH, markedly reduced the serum and ionic calcium and markedly increased the serum phase orthophosphate levels of the milks. This shift in the mineral equilibrium resulted in a drastic reduction in casein micelle integrity, with a marked dissociation of casein from the micelles. κ-Casein was the predominant casein dissociated, although significant levels of α_S-casein and β-casein were also transferred to the serum phase. This dissociation of the casein micelles caused a marked decrease in size and scattering properties of the casein micelles. In addition, a small decrease in the zeta potential of the casein micelles in the milk was observed. Heat treatment of the milks with added NaHMP induced further dissociation of κ-casein, although much of the α_S-casein and β-casein re-associated with the micelles.     

Dairy cow feeding system alters the characteristics of low-heat skim milk powder and processability of reconstituted skim milk

Low-heat skim milk powder (LHSMP) was manufactured on 3 separate occasions in mid lactation (ML, July 4–20) and late lactation (LL, September 27 to October 7) from bulk milk of 3 spring-calving dairy herds on different feeding systems: grazing on perennial ryegrass (Lolium perenne L.) pasture (GRO), grazing on perennial ryegrass and white clover (Trifolium repens L.) pasture (GRC), and housed indoors and offered total mixed ration (TMR). The resultant powders (GRO-SMP, GRC-SMP, and TMR-SMP) were evaluated for composition and color and for the compositional, physicochemical, and processing characteristics of the reconstituted skim milk (RSM) prepared by dispersing the powders to 10% (wt/wt) in water. Feeding system significantly affected the contents of protein and lactose, the elemental composition, and the color of the LHSMP, as well as the rennet gelation properties of the RSM. The GRO and GRC powders had a higher protein content; lower levels of lactose, iodine, and selenium; and a more yellow-green color (lower a* and higher b* color coordinates) than TMR powder. On reconstitution, the GRO-RSM had higher concentrations of protein, casein, and ionic calcium, and lower concentrations of lactose and nonprotein nitrogen (% of total N). It also produced rennet gels with a higher storage modulus (G′) than the corresponding TMR-RSM. These effects were observed over the combined ML and LL period but varied somewhat during the separate ML and LL periods. Otherwise, feeding system had little or no effect on proportions of individual caseins, concentration of serum casein, casein micelle size, casein hydration, heat coagulation time, or ethanol stability of the RSM at pH 6.2 to 7.2, or on the water-holding capacity, viscosity, and flow behavior of stirred yogurt prepared by starter-induced acidification of RSM. The differences in the functionality of the LHSMP may be of greater or lesser importance depending on the application and the conditions applied during the processing of the RSM.     

Performance of calves fed combinations of whole milk and reconstituted skim milk powder

A 52-d study with 24 3-d-old calves compared the effect of feeding whole milk (controls) or milk diluted 1:1 or 1:3 with reconstituted skim milk powder (10%, wt/wt) on calf performance to weaning at 38 d. Skim powder diets were supplemented with vitamins and trace minerals. Liquid diet was fed at 5% body weight twice daily for 24 d, then once daily for 14 d followed by abrupt weaning. Starter was provided for ad libitum intake from 14 to 52 d. For the first 13 d, controls had the greatest weight gains, but at 38 and 52 d, gains were similar for all groups. Three-quarters of the whole milk diet could be replaced with reconstituted skim milk powder (10% wt/wt) without any reduction in calf gain to weaning because starter intake increased 23%.     

Fermentation of reconstituted milk by Streptococcus thermophilus: Effect of irradiation on skim milk powder

The growth of Streptococcus thermophilus ST12 (ST12) in milk reconstituted from non-irradiated and irradiated at 10 kGy low-heat skim milk powders (RSM and irrRSM, respectively) at 40 °C was monitored by microcalorimetry. Statistically significant differences of the growth patterns of ST12 in RSM and irrRSM were found. Distinctively diauxic growth curves in RSM were replaced by one-stage growth curves in irrRSM. The final pH in RSM was 5.56 while in irrRSM samples it was 4.41. The time of initiation of gel formation was about 36 min shorter; however, the gels were considerably weaker in irrRSM than in RSM. The front-face fluorescence spectra were also used to characterise the differences in acidification processes. The microcalorimetric data together with the concentrations of metabolites determined during fermentation, rheological and fluorescence measurements indicated the substantial changes in the growth of ST12 in irrRSM in comparison with RSM.     

Fermentation of reconstituted skim milk supplemented with soy protein isolate by probiotic organisms

ABSTRACT:  Utilization of lactose and production of organic acid were determined in reconstituted skim milk (RSM) and RSM supplemented with soy protein isolate (SPI) (RSMS) by 6 probiotic organisms, including L. acidophilus 4461, L. acidophilus 4962 , L. casei 290, L. casei 2607, B. animalis subsp. lactis bb12, and B. longum 20099. The viable counts of probiotic organisms of RSM and RSMS were enumerated and pH measured during fermentation. Our results showed that 3% to 10% more lactose was utilized by all the 6 probiotic microorganisms from RSMS than RSM. All 6 probiotic organisms produced significantly more acetic acid in RSMS than RSM. However, the viable microbial populations in RSMS were lower than those in RSM due to lower pH of the former. It appears that addition of SPI enhanced lactose utilization and acetic acid production but slightly reduced the lactic acid production and the growth of probiotic microorganisms.     

Effects of pH,calcium-complexing agents and milk solids concentration on formation of soluble protein aggregates in heated reconstituted skim milk

The proportion of protein present in the supernatant after centrifugation and the formation of soluble protein aggregates in heated (90 °C for 10 min) reconstituted skim milk were investigated as a function of (a) pH, (b) milk concentration 9–21%, w/w (milk solids non-fat, MSNF) and (c) addition of calcium-complexing agents (orthophosphate or EDTA). Compositional changes in milk, resulting from pH adjustment or salt addition, altered the distribution of caseins between the serum and micellar phases of milk prior to heating, and influenced the amount and composition of soluble aggregates formed during heat treatment. Although milk pH was a good predictor of the aggregation behaviour of the proteins during heating of milk of different solids concentrations, neither the proportion of protein in the supernatant prior to heating nor the pH alone could predict the aggregation behaviour when the composition was adjusted with mineral salts.     

Use of activated carbon for removing phenylalanine from reconstituted skim milk powder hydrolysates

With the aim of preparing dietary supplements for phenylketonurics, the activated carbon was used in this work to remove phenylalanine (Phe) from skim milk powder enzymatic hydrolysates. Six hydrolysates were prepared, using a protease from Aspergillus oryzae (AO), isolated or in association with papain (PA). Different conditions were tested for removing Phe, and the best one showed to be the use of a activated carbon:casein ratio of 118 (g) and a temperature of 25 °C, which produced 96–99% of Phe removal. Among the hydrolytic conditions employed, the association of AO with PA (1 h, 1 g of enzyme/100 g of substrate and 4 h, 2 g of enzyme/100 g of substrate, respectively) led to the lowest absolute value for the final Phe concentration (0.060×10⁻⁴ mg/100 mg of protein).     

Effect of pH at heating on the acid-induced aggregation of casein micelles in reconstituted skim milk

Reconstituted skim milk samples at pH between 6.5 and 7.1 (heating pH) were heated at 80°C, 90°C or 100°C for 30 min (heating temperature). The particle size of the casein micelles was measured at pH 4.75-7.1 (measurement pH) and at temperatures of 10°C, 20°C and 30°C (measurement temperature) using photon correlation spectroscopy. The particle size of the casein micelles, at a measurement pH of 6.7 and a measurement temperature of 20°C, was dependent on the heating pH and heating temperature to which the milk was subjected. The casein micelle size in unheated milk was about 215 nm. At a heating pH of 6.5, the casein micelle size increased by about 15, 30 and 40 nm when the milk was heated at 80°C, 90°C or 100°C, respectively. As the heating pH of the milk was increased, the size of the casein micelles decreased so that, at pH 7.1, the casein micelles were ∼20 nm smaller than those from unheated milk. Larger effects were observed as the heating temperature was increased from 80°C to 100°C. The size differences as a consequence of the heating pH were maintained at all measurement temperatures and at all measurement pH down to the pH at which aggregation of the micelles was observed. For all samples, size measurements at 10°C showed no aggregation at all measurement pH. Aggregation occurred at progressively higher pH as the measurement temperature was increased. Aggregation also occurred at a progressively higher measurement pH as the heating pH was increased. The particle size changes on heating and the aggregation on subsequent acidification may be related to the pH dependence of the association of whey proteins with, and the dissociation of κ-casein from the casein micelles as milk is heated.     

Impact of source and level of calcium fortification on the heat stability of reconstituted skim milk powder

Calcium enrichment of food and dairy products has gained interest with the increased awareness about the importance of higher calcium intake. Calcium plays many important roles in the human body. Dairy products are an excellent source of dietary calcium, which can be further fortified with calcium salts to achieve higher calcium intake per serving. However, the addition of calcium salts can destabilize food systems unless conditions are carefully controlled. The effect of calcium fortification on the heat stability of reconstituted skim milk was evaluated, using reconstituted skim milks with 2 protein levels: 1.75 and 3.5% (wt/wt) prepared using low and high heat powders. Calcium carbonate, phosphate, lactate, and citrate were used for fortification at 0.15, 0.18, and 0.24% (wt/wt). Each sample was analyzed for solubility, heat stability, and pH. The addition of phosphate and lactate salts lowered the pH of milk, citrate did not have any major effect, and carbonate for the 1.75% protein samples increased the pH. In general, changes in solubility and heat stability were associated with changes in pH. Calcium addition decreased the solubility and heat stability. However, interestingly, the presence of carbonate salt greatly increased the heat stability for 1.75% protein samples. This is due to the neutralizing effect of calcium carbonate when it goes into solution. The results suggested that the heat stability of milk can be affected by the type of calcium salt used. This may be applied to the development of milk-based calcium enriched beverages.     

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。     

Effect of beta-lactoglobulin A and B whey protein variants on the rennet-induced gelation of skim milk gels in a model reconstituted skim milk system

The effect of fortifying reconstituted skim milk with increasing levels of the β-lactoglobulin (β-LG) genetic variants A, B, and an A-B mixture on rennet-induced gelation was studied by small deformation-sensitive rheology. Free-zone capillary electrophoresis and high-sensitivity oscillatory rheology were used to elucidate the role of potential heterotypic associative interactions between whey proteins and casein in a mixed colloidal system, subjected to moderate heating (65°C for 30 min) prior to renneting, on the gelling properties of the system. Increasing levels of added whey protein, in the concentration range of 0.225 to 1.35% of added protein, led to a concomitant progressive increase in the equilibrium shear storage modulus, G′ (recorded after ∼10,800 s), in the order β-LG B > β-LG A and β-LG A-B, as the general expected consequence of the setup of denser casein gel networks. The preferential effect of β-LG B over β-LG A on the mechanical strength of the gels may be due to the formation of cross-links and aggregates involving whey proteins and rennet hydrolysis products or an increase in the size of the casein micelle caused by the grafting of β-LG B to its surface, or both. The results of free-zone capillary electrophoresis were consistent with the notion that β-LG B (and not β-LG A) binds to the casein micelle under an optimal stoichiometry of 1:0.045 (mg/mg), even in the absence of heat treatment. The liquid-like character of the gel networks formed, tan δ, was a parameter sensitive to the level of addition of β-LG A in particular. At low concentrations (up to 0.45%) of β-LG A, tan δ increased by almost twice as much, which was interpreted as a result of the increase in the loss modulus, G″, of the sol fraction because of the presence of unbound β-LG A. At greater incremental concentrations of β-LG (>0.45%), the formation of smaller whey protein aggregates confined to the sol fraction may have led to a progressive decrease in tan δ. The critical gel time, t_gel, was also affected by the concentration of added whey protein and described 3 zones of behavior, irrespective of the type of whey protein variant. The critical gel time was slightly shorter for β-LG B than for β-LG A at 0.45% of added whey protein, but this difference became larger at 0.67%. Even when only β-LG B was found to associate with casein prior to renneting, both β-LG A and β-LG B, either alone or mixed, had a profound influence on the mechanical strength and coagulation kinetics of the rennet-induced casein gels. This knowledge is expected to be useful to exert better control and optimize processing conditions during the manufacturing of cheese and cheese analogs.     

Comparison of casein micelles in raw and reconstituted skim milk

During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spray-drying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40°C.     

Heat stability of reconstituted, protein-standardized skim milk powders

We determined the effects of standardization material, protein content, and pH on the heat stability of reconstituted milk made from low-heat (LH) and medium-heat (MH) nonfat dry milk (NDM). Low-heat and MH NDM were standardized downward from 35.5% to 34, 32, and 30% protein by adding either edible lactose powder (ELP) or permeate powder (PP) from skim milk ultrafiltration. These powders were called standardized skim milk powders (SSMP). The LH and MH NDM and SSMP were reconstituted to 9% total solids. Furthermore, subsamples of reconstituted NDM and SSMP samples were set aside to measure heat stability at native (unadjusted) pH, and the rest were adjusted to pH 6.3 to 7.0. Heat stability is defined as heat coagulation time at 140°C of the reconstituted LH or MH NDM and SSMP samples. The entire experiment was replicated 3 times at unadjusted pH values and 2 times at adjusted pH values. At an unadjusted pH, powder type, standardization material, and protein content influenced the heat stability of the samples. Heat stability for reconstituted LH NDM and SSMP was higher than reconstituted MH NDM and SSMP. Generally, decreased heat stability was observed in reconstituted LH or MH SSMP as protein content was decreased by standardization. However, adding ELP to MH SSMP did not significantly change its heat stability. When pH was adjusted to values between 6.3 and 7.0, powder type, standardization material, and pH had a significant effect on heat stability, whereas protein content did not. Maximum heat stability was noted at pH 6.7 for both reconstituted LH NDM and SSMP samples, and at pH 6.6 for both reconstituted MH NDM and SSMP samples. Furthermore, for samples with adjusted pH, higher heat stability was observed for reconstituted LH SSMP containing PP compared with reconstituted milk from LH SSMP containing ELP. However, no statistical difference was observed in the heat stability of reconstituted milk from MH NDM and MH SSMP samples. We conclude that powder type (LH or MH) and effect of standardization material (ELP or PP) can help explain differences in heat stability. The difference in the heat stability of powder type may be associated with the difference in the pH of maximum heat stability and compositional differences in the standardization material (ELP or PP).     

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.     

Comparison between reconstituted and fresh skim milk chemical and electrochemical acidifications

The aim of this study was to compare the electrochemical and chemical acidification of reconstituted and fresh skim milk in terms of electrodialytic parameters, precipitation kinetics, chemical composition and physicochemical and functional properties of isolates produced by bipolar membrane electro acidification (BMEA). The electrodialytic parameters were not influenced by the type of milk when both milks were compared at a similar protein and salt content. The difference in precipitation kinetics observed between the two milks, whatever the acidification procedure, can be explained mainly by a difference in salt content. Isolates produced by BMEA showed similar physicochemical and functional properties (except for foaming capacity) to isolates produced chemically. The main factor affecting the composition and the physicochemical and functional properties was the pre‐treatment of milk prior to acidification: it had a higher impact on the functional properties than the acidification treatment itself. Copyright © 2002 Crown in the right of Canada. Published for SCI by John Wiley & Sons, Ltd     

Improvement of rheological properties of firm acid gels by skim milk heating is conserved after stirring

The enhancement of the strength of set acid gels by heating milk was related to rheological parameters (water retention capacity, storage modulus) of corresponding stirred gels. To obtain accurate rheological data from stirred gel it was necessary to maintain a constant granulometry of gel particles and to recognize time after stirring as a contributing factor. Two hours after stirring, the gel exhibited a higher storage modulus when milk was heated above 80 degrees C. A measurement of viscosity of just-stirred yoghurt was sufficient to predict correctly the quality of a stirred gel analysed by viscoelastic measurements. Increased resistance to syneresis of just-stirred gels was related to higher viscosity. The quantity of beta-lactoglobulin (beta-Ig) bound to casein micelles explains the improvement of these gel qualities. We have considered that the structure of the initial firm gel (mesostructure level) was conserved in fragments within the stirred gel. Consequently, the explanation given by various authors for the effect of heating milk on the properties of set gels can also be applied to stirred gels. The same mechanism, described in literature for structure formation of set gels from acidified milk is purposed to explain the role of heating milk on the recovery of gel structure after stirring. The beta-Ig association with casein micelles during heating favoured micelle connections during the acidification. It also favoured the association of gel fragments after stirring during the recovery in gel structure.     

Water sorption and glass transition properties of spray dried lactose hydrolysed skim milk powder

The moisture sorption behaviour and glass transition temperature of spray dried skim milk powder with hydrolysed lactose (SMPHL) were determined. Spray drying of skim milk with hydrolysed lactose resulted in very low cyclone recovery of 25% and a large amount of powder remained stuck inside the spray dryer. The equilibrium moisture content of SMPHL was lower than that of lactose for each range of water activity when humidified for 21 days at 23 °C using saturated salt solutions. Unlike lactose, SMPHL did not lose water when the water activity exceeded 0.432 and no crystallization was noticed at water activity ?0.753. The sorption isotherm data for SMPHL fitted well with the BET and GAB models with monolayer moisture contents of 7.55 and 8.27 g/100 g, respectively. The glass transition temperature of anhydrous SMPHL was 49 °C. The critical water activity and moisture content for SMPHL were 0.15 and 2.4 g/100 g dry solid, respectively. The low critical values indicated hydrolysis of lactose necessities maintenance of very low moisture of powder for its long-term stability.     

Effect of transglutaminase and acidification temperature on the gelation of reconstituted skim milk

Effects of transglutaminase (TG), acidification temperature and total milk solids level on the acid gelation of skim milk were investigated. Despite similar acidification kinetics, TG-treated milk acidified at ≥35 °C showed differences in elastic modulus (Gʹ) with acidification time, particularly by inhibiting the formation of the peculiar shoulder (Gʹ_shoulder) observed at an early stage of gelation in the control milk. Regardless of the milk solids content, the G'_shoulder was absent in both types of milk at 30 °C. However, control milk above 2.5% (w/w) milk solids showed the G'_shoulder at 45 °C. G'_shoulder is proposed as the transition from the first increase in Gʹ, due to aggregation of soluble protein complexes at the early stage of acidification, to the second, due to further aggregation of casein micelles (and aggregated soluble complexes) as acidification progresses. The G'_shoulder was absent in acidified TG-treated milk due to the lack of soluble protein complexes containing caseins.     

Effect of milk solids concentration on the gels formed by the acidification of heated pH-adjusted skim milk

Reconstituted skim milk of 10–25% total solids was adjusted to pH values between about 6.2 and 7.1 and heated at 80 °C for 30 min. Gels were formed from the heated milks by slow acidification to pH 4.2 and the gelation process and final gels were analyzed for their rheological properties. At each milk concentration, the final acid gel firmness (final G′) and breaking stress could be changed markedly by manipulation of the pH during heating. The final gel firmness and breaking stress could also be modified by changing the concentration of the milk solids prior to heating and acidification. The results indicated that similar gel firmness and breaking stress could be achieved over a range of milk concentrations by control of the pH of the milk during heating. When expressed as a percentage change in final G′ or breaking stress relative to that obtained at the natural pH, plots of the change in final G′ or breaking stress versus pH fell close to a single curve, indicating that the same mechanism may influence the gelation properties at all milk concentrations. The final G′ and breaking stress were related to the denaturation and interaction of the whey proteins with the casein micelles, and the formation of non-sedimentable casein when the milk was heated.