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.     

Effect of preheating and other process parameters on whey protein reactions during skim milk powder manufacture

Skim milk powder was manufactured in a milk powder plant using different preheating temperatures, concentrate heating temperatures and spray drying temperatures. Varying the preheating conditions from 70 °C for 52 s to 120 °C for 52 s had a marked effect on the denaturation of $\beta$-lactoglobulin A, $\beta$-lactoglobulin B, $\alpha$-lactalbumin, bovine serum albumin (BSA), and immunoglobulin G. In contrast, varying concentrate heating temperature (65–74 °C) and inlet/outlet air dryer temperature (200/101 °C–160/89 °C) had a minimal effect on whey protein denaturation. Most of the whey protein denaturation and association with the casein micelle occurred in the preheating section of the powder plant. Aggregation of β-lactoglobulin (β-lg) and BSA predominantly involved disulphide bonds. Although, greater than 90% of the β-lg and BSA was denatured after preheating at 120 °C for 52 s, the extent of association with the casein micelle was lower, 50% for β-lg and 75% for BSA.     

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.     

Changes in concentration of aflatoxin M1 during manufacture and storage of skim milk powder

In this study, skim milk powder was produced from cow's milk contaminated artificially with aflatoxin M1 (AFM1) at two different levels, 1.5 and 3.5 microg/liter (ppb), and the effects of process stages on the AFM1 contents were investigated. Pasteurization, concentration, and spray drying caused losses of about 16, 40, and 68%, respectively, in AFM1 content of the milk contaminated with 1.5 microg/liter AFM1, and losses of 12, 35, and 59%, respectively, in the milk contaminated with 3.5 microg/liter AFM1. These losses were found to be statisticially significant at the level of P < 0.01. After 3- and 6-month storage periods, AFM1 content of the skim milk powder produced from milk with 1.5 microg/liter AFM1 decreased by 2 and 5%, respectively, whereas these rates were 2 and 4%, respectively, for the skim milk powders made from milk with 3.5 microg/liter AFM1 (after adjustment for sample weight). Changes in AFM1 content of milk powder samples were found statistically insignificant (P > 0.05 and P > 0.01) for 3- and 6-month storage periods.     

Lactosylation of milk proteins during the manufacture and storage of skim milk powders

Extensive lactosylation of milk proteins in standard skim milk powder dried against air between 185 and 90°C (inlet and outlet temperatures of the air) was detected by capillary electrophoresis. Optimisation of the drying conditions included keeping the outlet temperature low (preferably <80°C), since this was the parameter which most affected the extent of lactosylation of milk proteins. The inlet temperature was set in order to obtain the best compromise between a low extent of lactosylation and a high drying rate (170–175°C). These conditions allowed the manufacture of low-lactosylated skim milk powder. Storage of freeze-dried and control low-lactosylated skim milk powder at different temperatures showed that both temperature and moisture content affected the progress of lactosylation during storage. Further drying to less than 2.5% moisture content (w/w) and storage at low temperatures were required to prevent the development of lactosylation in the low-lactosylated skim milk powder.     

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).     

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。     

The formation of thermophilic spores during the manufacture of whole milk powder

A survey was undertaken at a whole milk powder manufacturing plant to determine the origin and the rate of spore formation of thermophilic bacteria. Spores were generally not detected (< 10 cfu/mL) in either the pasteurization process or the pasteurized milk directed into the powder plant. The predominant sites of spore formation were the preheater plate heat exchanger and the evaporator. Spores began to be detected approximately 9 h into an 18-h manufacturing run. Spore isolates were identified as Anoxybacillus flavithermus and Geobacillus species. A. flavithermus predominated in the preheat section, whereas a mix of both organisms was present in the later manufacturing stages.     

Some properties of set yoghurt made from caprine milk and bovine–caprine milk mixtures fortified by ultrafiltration or the addition of skim milk powder

Set yoghurt was produced from caprine milk (A), 70% caprine-30% bovine (B) and 50% caprine–50% bovine milk (C) mixtures, and stored for 14 days at ±4°C. Two methods of fortification, namely ultrafiltration (UF) and the addition of bovine skim milk powder (SMP), were applied to the milk mixtures. Some chemical, physical, microbiological and sensory properties of the six samples were analysed on the 1st, 7th and 14th days of storage. The effects of milk type, concentration method and storage period on the physicochemical and microbiological properties of the samples were investigated statistically.     

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).     

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.     

Fortification of reconstituted skim milk powder with different calcium salts: Impact of physicochemical changes on stability to processing

Reconstituted skim milk powder (RSMP) was fortified with 12.5 mM/L calcium (Ca) using soluble [Ca chloride (CChlor), Ca gluconate (CGluc) or Ca hydroxide (CHyd)] or insoluble [Ca carbonate (CCarb), Ca phosphate (CPhos) or Ca citrate (CCit)] salts. CPhos and CCit decreased heat stability moderately at 140 °C, while CCarb had no effect. Soluble salts had a pronounced destabilising effect at 140 °C due to increased ionic Ca levels. After a laboratory‐scale high‐temperature short‐time heating process, CHyd‐fortified RSMP had a lower viscosity than all other samples. CChlor and CGluc increased sedimentation during accelerated physical stability testing, with CHyd causing greater sedimentation than CChlor or CGluc.     

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.     

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.     

The effect of free Ca2+ on the heat stability and other characteristics of low-heat skim milk powder

Low-heat skim milk powder (SMP), reconstituted to 25% total solids, was found to have poor heat stability. This could be improved by reducing the free Ca²⁺ concentration to 1.14 mm, or lower, by the addition of either Amberlite IR-120 ion-exchange resin in its sodium form or tri-sodium citrate in skim milk prior to evaporation and spray drying. Reduction in Ca²⁺ concentration was accompanied by increases in pH, particle size, and kinematic viscosity, and by a reduction in ζ-potential and changes in colour. In-container sterilisation of the reconstituted powder increased particle size, ζ-potential, kinematic viscosity and a* and b* values. However, Ca²⁺ concentration, pH and whiteness decreased. This study elucidated the importance of Ca²⁺ concentration and pH on heat stability of low-heat SMP, suggesting that Ca²⁺ concentration and pH in bulk milk are useful indicators for ensuring that spray dried milk powder has good heat stability.     

Stickiness of skim milk powder using the particle gun technique

The particle gun method for investigating the initiation of stickiness of dairy powders was examined using skim milk powder (SMP). The point at which significant stickiness occurred was at a constant temperature above the glass transition temperature (T_g) of amorphous lactose regardless of the temperature and relative humidity (RH) conditions of the air in the particle gun. This point has been called (T–T_g)_critical. The initial water activity of SMP did not significantly affect (T–T_g)_critical, confirming that stickiness is a surface phenomenon. Changing the particle feed rate did not effect (T–T_g)_critical or the rate at which the powder accumulated on the target plate. The errors in measuring the value of (T–T_g)_critical and the rate of stickiness development [slope of the deposition line versus (T–T_g)] for SMP were found to be 33.6 ± 0.8 °C and 3.1 ± 0.7% deposition °C^?1, respectively, after standardisation of the ambient air conditions around the powder feeder and the initial water activity of the SMP.