Physicochemical properties of skim milk powders prepared with the addition of mineral chelators.

The objective of this study was to determine the effect of mineral chelator addition during skim milk powder (SMP) manufacture on the solubility, turbidity, soluble protein, and heat stability (HS). Three chelators (sodium citrate dihydrate, sodium polyphosphate, and disodium EDTA) at 3 different concentrations (5, 15, and 25mM) were added to skim milk concentrate (30% total solids), and the pH was adjusted to 6.65 before spray drying to produce SMP. Spray-dried SMP samples were tested for solubility index (SI). Additionally, samples were reconstituted to contain 9% total solids, adjusted to pH 7.0, and tested for turbidity, protein content from supernatants of ultracentrifuged samples, and HS. Lower SI values were observed for samples treated with 5mM disodium EDTA and sodium polyphosphate than control samples or samples with 5mM sodium citrate dihydrate. Furthermore, lower SI values were observed with an increased level of chelating agents regardless of chelator type. A decreased turbidity value was found with increasing levels of mineral chelating salt treatment. Low turbidity with increasing levels of added chelators may be associated with the dissociation of caseins from micelles. Furthermore, higher protein content was observed in supernatants of ultracentrifuged samples treated with increased level of chelators as compared with the control sample. Higher HS was observed in samples treated with 5mM compared with samples treated with 25mM mineral chelator. The results suggest improved solubility and HS upon addition of mineral chelators to SMP during its manufacture.

Short communication: Effect of storage temperature on the solubility of milk protein concentrate 80 (MPC80) treated with NaCl or KCl.

A previous study in our laboratory showed that addition of 150 mM NaCl or KCl into diafiltration water improved the solubility of freshly made milk protein concentrate 80 (MPC80). In the present study, the objectives were (1) to evaluate the solubility of NaCl- or KCl-treated MPC80 samples kept at varying temperatures and then stored for extensive periods at room temperature (21 °C ± 1 °C); and (2) to determine if MPC80 samples stored at different temperatures and protein conformation can be grouped or categorized together. Freshly manufactured MPC80 samples were untreated (control), processed with NaCl, or processed with KCl. One set of sample bags was stored at 4 °C; second and third sets of bags were kept at 25 °C and 55 °C for 1 mo (31 d) and then transferred to room temperature (21 °C ± 1 °C) storage conditions for 1 yr (365 d). Samples were tested for nitrogen solubility index (NSI) and for protein changes by Fourier-transform infrared (FTIR) spectroscopy. Analysis of variance results for NSI showed 2 significantly different groupings of MPC80 samples. The more soluble group contained samples treated with NaCl or KCl and stored at either 4 °C or 25 °C. These samples had mean NSI >97.5%. The less soluble groups contained all control samples, regardless of storage temperature, and NaCl- or KCl-treated samples stored at 55 °C. These samples had mean NSI from 39.5 to 58%. Within each of these groups (more soluble and less soluble), no significant differences in solubility were detected. Pattern recognition analysis by soft independent modeling of class analogy (SIMCA) was used to assess protein changes during storage by monitoring the amide I and amide II (1,700(-1) to 1,300 cm(-1)) regions. Dominant bands were observed at 1,385 cm(-1) for control, 1,551 cm(-1) for KCl-treated samples, and 1,694 cm(-1) for NaCl-treated samples. Moreover, SIMCA clustered the MPC80 samples stored at 4 °C separately from samples stored at 25 °C and 55 °C. This study demonstrates that (1) the addition of NaCl or KCl during MPC80 manufacture reduces the deleterious changes in solubility upon prolonged storage at 4 °C or 25 °C, and (2) the solubility of samples stored at 55 °C is poor irrespective of salt treatment.

Partial calcium depletion during membrane filtration affects gelation of reconstituted milk protein concentrates.

Milk protein concentrate powders (MPC) with improved rehydration properties are often manufactured using processing steps, such as acidification and high-pressure processing, and with addition of other ingredients, such as sodium chloride, during their production. These steps are known to increase the amount of serum caseins or modify the mineral equilibrium, hence improving solubility of the retentates. The processing functionality of the micelles may be affected. The aim of this study was to investigate the effects of partial acidification by adding glucono-δ-lactone (GDL) to skim milk during membrane filtration on the structural changes of the casein micelles by observing their chymosin-induced coagulation behavior, as such coagulation is affected by both the supramolecular structure of the caseins and calcium equilibrium. Milk protein concentrates were prepared by preacidification with GDL to pH 6 using ultrafiltration (UF) and diafiltration (DF) followed by spray-drying. Reconstituted UF and DF samples (3.2% protein) treated with GDL showed significantly increased amounts of soluble calcium and nonsedimentable caseins compared with their respective controls, as measured by ion chromatography and sodium dodecyl sulfate-PAGE electrophoresis, respectively. The primary phase of chymosin-induced gelation was not significantly different between treatments as measured by the amount of caseino-macropeptide released. The rheological properties of the reconstituted MPC powders were determined immediately after addition of chymosin, both before and after dialysis against skim milk, to ensure similar serum composition for all samples. Reconstituted samples before dialysis showed no gelation (defined as tan δ=1), and after re-equilibration only control UF and DF samples showed gelation. The gelation properties of reconstituted MPC powders were negatively affected by the presence of soluble casein, and positively affected by the amount of both soluble and insoluble calcium present after reconstitution. This work, testing the chymosin-induced gelation behavior of various reconstituted MPC samples, clearly demonstrated that a decrease in pH to 6.0 during membrane filtration affects the integrity of the casein micelles supramolecular structure with important consequences to their processing functionality.

Changes in the physical properties, solubility, and heat stability of milk protein concentrates prepared from partially acidified milk.

A limiting factor in using milk protein concentrates (MPC) as a high-quality protein source for different food applications is their poor reconstitutability. Solubilization of colloidal calcium phosphate (CCP) from casein micelles during membrane filtration (e.g., through acidification) may affect the structural organization of these protein particles and consequently the rehydration and functional properties of the resulting MPC powder. The main objective of this study was to investigate the effects of acidification of milk by glucono-δ-lactone (GDL) before ultrafiltration (UF) on the composition, physical properties, solubility, and thermal stability (after reconstitution) of MPC powders. The MPC samples were manufactured in duplicate, either by UF (65% protein, MPC65) or by UF followed by diafiltration (80% protein, MPC80), using pasteurized skim milk, at either the native milk pH (~pH 6.6) or at pH 6.0 after addition of GDL, followed by spray drying. Samples of different treatments were reconstituted at 5% (wt/wt) protein to compare their solubility and thermal stability. Powders were tested in duplicate for basic composition, calcium content, reconstitutability, particle size, particle density, and microstructure. Acidification of milk did not have any significant effect on the proximate composition, particle size, particle density, or surface morphology of the MPC powders; however, the total calcium content of MPC80 decreased significantly with acidification (from 1.84 ± 0.03 to 1.59 ± 0.03 g/100 g of powder). Calcium-depleted MPC80 powders were also more soluble than the control powders. Diafiltered dispersions were significantly less heat stable (at 120°C) than UF samples when dissolved at 5% solids. The present work contributes to a better understanding of the differences in MPC commonly observed during processing.

Effect of NaCl addition during diafiltration on the solubility, hydrophobicity, and disulfide bonds of 80% milk protein concentrate powder.

We investigated the surface hydrophobicity index based on different fluorescence probes [1-anilinonaphthalene-8-sulfonic acid (ANS) and 6-propionyl-2-(N,N-dimethylamino)-naphthalene (PRODAN)], free sulfhydryl and disulfide bond contents, and particle size of 80% milk protein concentrate (MPC80) powders prepared by adding various amounts of NaCl (0, 50, 100, and 150 mM) during the diafiltration process. The solubility of MPC80 powder was not strictly related to surface hydrophobicity. The MPC80 powder obtained by addition of 150 mM NaCl during diafiltration had the highest solubility but also the highest ANS-based surface hydrophobicity, the lowest PRODAN-based surface hydrophobicity, and the least aggregate formation. Intermolecular disulfide bonds caused by sulfhydryl-disulfide interchange reactions and hydrophobic interactions may be responsible for the lower solubility of the control MPC80 powder. The enhanced solubility of MPC80 powder with addition of NaCl during diafiltration may result from the modified surface hydrophobicity, the reduced intermolecular disulfide bonds, and the associated decrease in mean particle size. Addition of NaCl during the diafiltration process can modify the strength of hydrophobic interactions and sulfhydryl-disulfide interchange reactions and thereby affect protein aggregation and the solubility of MPC powders.

Evaluating the Effectiveness of Digital Content Marketing Under Mixed Reality Training Platform on the Online Purchase Intention.

The purpose of this research is to investigate the effectiveness of Digital Content Marketing (DCM) on a Mixed Reality (MR) training platform environment with the consideration of online purchase intention (OPI) through social media. E-commerce today encounters several common issues that cause customers to have reservations to purchase online. With the absence of physical contact points, customers often perceive more risks when making purchase decisions. Furthermore, online retailers often find it hard to engage customers and develop long-term relationships. In this research, a Structural Equation Model (SEM) is proposed to examine the efficacy of DCM from both immediate and long-term OPI. The results examine whether adopting DCM on an MR training platform environment through social media brings positive results in OPI. Empirical research was carried out through online questionnaires collected in 2021 and 2022. A total of 374 questionnaires were qualified for data analysis in this study, conducted with IBM SPSS and AMOS. The results imply that DCM is critical to stimulating both immediate and long-term OPI. The immediate OPI is positively affected by increasing perceived value through MR in DCM. Regarding the long-term OPI, increased customer engagement with DCM under MR environment can cultivate brand trust and significantly affect the long-term OPI.

Zinc-binding capacity of yak casein hydrolysate and the zinc-releasing characteristics of casein hydrolysate-zinc complexes.

Many factors affect the bioavailability of dietary Zn, which leads to its low availability in some food systems and Zn nutrient deficiency. However, some proteins or peptides can form complexes with Zn and increase its absorption and bioavailability in intestinal conditions. The purpose of this work was to determine the Zn-binding activity of yak casein hydrolysate (YCH) and examine its stability, solubility, and dialyzability in a simulated intestinal environment. The Zn-binding activity of YCH, prepared using alcalase, pepsin, trypsin, Flavozyme (Novo Nordisk Biochem Inc., Franklinton, NC), or papain, was investigated. Evidence for the formation of complexes between Zn and YCH also were detected by UV-visible spectroscopy and Fourier transform infrared spectroscopy. Results were that YCH prepared with alcalase and trypsin possessed the highest Zn-binding capacity compared with YCH prepared with pepsin, Flavozyme, or papain. The 6-h YCH obtained with alcalase showed the highest Zn-binding capacity. Compared with native yak casein, the Zn-binding activity of YCH was significantly lower, but its solubility and dialyzability were markedly higher under intestinal basic pH ranges. This is important because high solubility and dialyzability is associated with better bioavailability. Both UV-visible spectroscopy and Fourier transform infrared spectroscopy spectra indicated that some sites of YCH can bind with Zn ions and form complexes that make Zn more soluble and dialyzable under simulated intestinal conditions. Therefore, YCH-Zn complexes may have potential to improve Zn bioavailability.

Solubility of commercial milk protein concentrates and milk protein isolates.

High-protein milk protein concentrate (MPC) and milk protein isolate (MPI) powders may have lower solubility than low-protein MPC powders, but information is limited on MPC solubility. Our objectives in this study were to (1) characterize the solubility of commercially available powder types with differing protein contents such as MPC40, MPC80, and MPI obtained from various manufacturers (sources), and (2) determine if such differences could be associated with differences in mineral, protein composition, and conformational changes of the powders. To examine possible predictors of solubility as measured by percent suspension stability (%SS), mineral analysis, Fourier transform infrared (FTIR) spectroscopy, and quantitative protein analysis by HPLC was performed. After accounting for overall differences between powder types, %SS was found to be strongly associated with the calcium, magnesium, phosphorus, and sodium content of the powders. The FTIR score plots were in agreement with %SS results. A principal component analysis of FTIR spectra clustered the highly soluble MPC40 separately from the rest of samples. Furthermore, 2 highly soluble MPI samples were clustered separately from the rest of the MPC80 and MPI samples. We found that the 900 to 1,200 cm⁻¹ region exhibited the highest discriminating power, with dominant bands at 1,173 and 968 cm⁻¹, associated with phosphate vibrations. The 2 highly soluble MPI powders were observed to have lower κ-casein and α-(S1)-casein contents and slightly higher whey protein contents than the other powders. The differences in the solubility of MPC and MPI were associated with a difference in mineral composition, which may be attributed to differences in processing conditions. Additional studies on the role of minerals composition on MPC80 solubility are warranted. Such a study would provide a greater understanding of factors associated with differences in solubility and can provide insight on methods to improve solubility of high-protein milk protein concentrates.

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).

Evaluation of rice flour for use in vanilla ice cream.

The effects of varying concentrations (2, 4, and 6%) of 2 types of rice flours (RF 1 and RF 2) on the physicochemical properties and sensory characteristics of vanilla ice cream samples were assessed at different fat levels (0, 4, and 10%) and storage conditions (control vs. heat-shocked). Fat and total solids were measured as well as hardness, viscosity, and melting rate. Eight trained panelists conducted descriptive sensory analyses of the samples at 0 and 7 wk. The 2% rice flour level and to a certain extent the 4% usage level generally improved texture while affecting to a lesser extent the flavor characteristics of the samples compared with the control. The RF 2 generally had a more significant effect than RF 1, especially on the texture attributes. Although the rice flour reduced the negative impact of temperature abuse on textural properties, the samples still deteriorated in textural properties (more icy) under temperature abuse conditions. In addition, rice starch does lower perceived sweetness and can have a "flour flavor" at high usage levels. The use of rice flour appears to be most advantageous for low fat ice cream samples.

Thermal stability of reconstituted milk protein concentrates: Effect of partial calcium depletion during membrane filtration.

Milk protein concentrate (MPC) powders are increasingly utilized in manufacturing of protein fortified beverages. Thermal stability of the protein dispersions is of significant importance in such applications. It is known that a decrease in pH can induce partial dissociation of casein micelles and modify the natural equilibrium of calcium and phosphate between the micelles and the serum phase. The presence of soluble casein may improve the rehydration properties of MPC powders, and may impact their thermal stability. The objective of this work was to investigate the effects of partial acidification of milk prior to ultrafiltration on the heat stability of reconstituted MPC dispersions. Milk protein concentrate powders were prepared from skim milk acidified to pH6.0 by addition of glucono-δ-lactone, and then concentrated using ultrafiltration (UF) and diafiltration (DF). The heat stability of the reconstituted MPC dispersions was studied, by determining heat coagulation time, particle size, turbidity, viscosity, soluble and colloidal calcium and phosphate, and non-sedimentable casein both before and after heating at 120°C. Reconstituted MPC powders made with partially acidified skim milk contained lower soluble calcium and phosphate and exhibited very poor thermal stability compared to MPC powders made with skim milk at its natural pH. The thermal stability of the acidified MPC dispersions was not only recovered by restoration of pH and the serum composition through dialysis against skim milk, but it was improved compared to control MPC dispersions. All dialyzed samples had comparable pH, protein content and calcium and phosphate concentration, but the structure of the casein micelles was altered, causing differences in the type of soluble aggregates. It was concluded that the integrity of the casein micelles and the amount of dissociated, non-sedimentable caseins play a major role in determining the thermal stability of the MPC dispersions.

Epidermal keratinocyte growth: changes in protein composition and synthesis of keratins in differentiating cultures.

The epidermal skin layer undergoes extensive changes in all areas of metabolism as the lowermost basal cell differentiates into a dead structure that forms the upper, protective stratum corneum. Epidermal keratinocyte differentiation was studied in vitro using primary epidermal keratinocyte cultures that grow from a basal cell monolayer to a differentiating and proliferating, multilayered keratinocyte structure. Specific groups of proteins that form complex structures as keratinocyte differentiation occurs were studied. The quantities and synthesis of the proteins forming the tonofilamentous bundles, keratohyaline granules, and cornified cell envelopes in these cultures were determined. The selective solubilities of these proteins in a series of buffers were exploited to separate the proteins into 6 fractions. Protein assays, [3H]-amino acid pulse labeling, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and fluorography were used to quantitate and characterize the proteins extracted from basal monolayers and from stratifying and fully differentiated keratinocyte cultures. The results showed that multilayered, more differentiated cultures accumulated the greatest amount of keratin, keratohyaline granule, and cell envelope proteins, although there was no apparent increase in the synthesis of these proteins in the more differentiated cultures. These cultures showed extensive disulfide cross-linking of the keratins. Covalent keratin bonding occurred at least 6 hr after the synthesis and rapid noncovalent bonding of the polypeptides into keratins; thus various stages of keratin formation were identified. The differentiation of the epidermis appeared to be a complex, orderly, and regulated process that can be studied in vitro using this epidermal cell culture system.

Differential effects of retinoids on DNA synthesis in calcium-regulated murine epidermal keratinocyte cultures.

To study the possibility that the state of proliferation of epidermal keratinocytes can influence the action of retinoids, the rate of proliferation of murine epidermal keratinocytes was manipulated by growing the cells in media containing high or low concentrations of Ca++. In contrast to what other investigators have reported, keratinocytes cultured in medium containing 1.4 mM Ca++ proliferate faster, instead of slower, than cells cultured in medium with 0.09 mM Ca++. Other experiments showed that Ca++ was stimulatory to keratinocytes in medium containing a low level of growth factors, and inhibitory in medium containing a high level of growth factors, suggesting that the discrepancy could be due to a difference in the sera used. The high Ca++ cells prominently expressed the 48kD/56kD pair of keratin, showing that they were in a hyperproliferative state. Exposure of the faster growing high Ca++ cells to all-trans retinoic acid, 13-cis retinoic acid, etretinate, etretin, and arotinoid ethyl ester caused dose-dependent inhibition of DNA synthesis. In contrast, exposure of the slower growing low Ca++ cells to these retinoids resulted in dose-dependent stimulation of DNA synthesis. In addition, all-trans retinoic acid caused dose-related increases in cell number in the low Ca++ cultures. These findings correlate with the reported differential effects of retinoids on normal and hyperproliferative epidermis, and suggest that Ca++ and low growth factor-regulated keratinocyte cultures are useful for studying the mechanism of hyperproliferation and retinoid actions.

Trans retinoic acid enhances the growth response of epidermal keratinocytes to epidermal growth factor and transforming growth factor beta.

Retinoids have been shown to either stimulate or inhibit epidermal keratinocyte proliferation. We have observed that in serum and growth factor free medium (basal medium), epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) stimulated DNA synthesis in mouse epidermal keratinocyte cultures (mKC) in a time- and dose-dependent manner. Incubation with all-trans retinoic acid (RA) greatly enhanced the stimulatory effect of EGF. Transforming growth factor beta (TGF beta) inhibited the EGF-induced DNA synthesis in a dose-dependent manner, and the inhibition was greatly enhanced by a low dose of RA. Treatment of growth-factor deprived human keratinocyte cultures (hKC) with RA before incubation in basal medium containing EGF or a mixture of EGF, bovine pituitary extract (BPE), and insulin caused a dose-related increase in DNA synthesis and cell growth (cell number), respectively. A low concentration of RA also enhanced the inhibitory effect of TGF beta on growth-factor-induced DNA synthesis and cell growth in hKC. These findings suggest that the differential effects of retinoids on epidermal keratinocyte proliferation are in part due to an enhancement of the response of keratinocytes to positive and negative peptide growth factors.

The impact of antioxidant addition on flavor of cheddar and mozzarella whey and cheddar whey protein concentrate.

Lipid oxidation products are primary contributors to whey ingredient off-flavors. The objectives of this study were to evaluate the impact of antioxidant addition in prevention of flavor deterioration of fluid whey and spray-dried whey protein. Cheddar and Mozzarella cheeses were manufactured in triplicate. Fresh whey was collected, pasteurized, and defatted by centrifugal separation. Subsequently, 0.05% (w/w) ascorbic acid or 0.5% (w/w) whey protein hydrolysate (WPH) were added to the pasteurized whey. A control with no antioxidant addition was also evaluated. Wheys were stored at 3 degrees C and evaluated after 0, 2, 4, 6, and 8 d. In a subsequent experiment, selected treatments were then incorporated into liquid Cheddar whey and processed into whey protein concentrate (WPC). Whey and WPC flavors were documented by descriptive sensory analysis, and volatile components were evaluated by solid phase micro-extraction with gas chromatography mass spectrometry. Cardboard flavors increased in fluid wheys with storage. Liquid wheys with ascorbic acid or WPH had lower cardboard flavor across storage compared to control whey. Lipid oxidation products, hexanal, heptanal, octanal, and nonanal increased in liquid whey during storage, but liquid whey with added ascorbic acid or WPH had lower concentrations of these products compared to untreated controls. Mozzarella liquid whey had lower flavor intensities than Cheddar whey initially and after refrigerated storage. WPC with added ascorbic acid or WPH had lower cardboard flavor and lower concentrations of pentanal, heptanal, and nonanal compared to control WPC. These results suggest that addition of an antioxidant to liquid whey prior to further processing may be beneficial to flavor of spray-dried whey protein. Practical Application: Lipid oxidation products are primary contributors to whey ingredient off-flavors. Flavor plays a critical and limiting role in widespread use of dried whey ingredients, and enhanced understanding of flavor and flavor formation as well as methods to control or minimize flavor formation during processing are industrially relevant. The results from this study suggest that addition of an antioxidant to liquid whey prior to further processing may be beneficial to minimize flavor of spray-dried whey 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.

Process for calcium retention during skim milk ultrafiltration.

Lactose is separated from milk or other fluid dairy products for a variety of reasons. Ultrafiltration is a known process of removing lactose from these products. However, during ultrafiltration, valuable minerals, such as calcium in soluble form, are also lost into permeate. In this study, a process was developed in which first the lactose reduction in skim milk was achieved by ultrafiltration (4x volumetric concentration) using a 10-kDa membrane. Then, the calcium present in permeate was precipitated using one of three methods: 1) heat treatment, 2) pH adjustment, or 3) a combination of pH adjustment and heat treatment to permeate, then recovered by refiltering permeate. The process was first developed at laboratory scale, and then its applicability was tested at the pilot scale. Skim milk, retentates, permeates, and the treated permeates were analyzed for total solids, ash, protein, or total nitrogen, calcium, and lactose content. About 76% of the total lactose and about 16% of the calcium present in skim milk permeated through the membrane during ultrafiltration. The three treatments applied produced white precipitates and turned the clear permeates turbid. On refiltering the treated permeates approximately 42, approximately 50, and approximately 70% of the total calcium present could be recovered from 1) heat-treated, 2) pH-adjusted, and 3) pH-adjusted and heat-treated permeates, respectively. There was no marked change in the lactose content due to any of the three treatments and subsequent refiltering of the treated permeates.

Augmentation of keratinocyte differentiation by the epidermal mitogen, 8-bromo-cAMP.

The effect of the epidermal mitogen, 8-bromo-cAMP, on keratinocyte differentiation was studied. A 3 X 10(-4) M dose of 8-bromo-cAMP was added to primary neonatal mouse epidermal keratinocyte cultures that slowly proliferate, stratify and differentiate over 2-3 weeks time. [3H]Thymidine autoradiography coupled with an NH4Cl plus reducing agent technic which separates basal and differentiating keratinocytes was used to determine the target cell for the 8-bromo-cAMP mitogenic effect. A histologic stain and a four buffer protein extraction protocol, in conjunction with PAGE and fluorographic technics, were used to assess the differentiation of the cultures. The data indicated that 8-bromo-cAMP primarily stimulated the proliferation of the basal cell monolayer. Simultaneous with the mitogenic effect was an increase in the production of keratohyalin granule, keratin and cell envelope proteins, which are specific markers of epidermal differentiation. The results indicate that keratinocytes stimulated by the epidermal mitogen 8-bromo-cAMP simultaneously express differentiation-related processes.

Ripening of cheddar cheese with added attenuated adjunct cultures of lactobacilli.

We made Milled curd Cheddar cheese with Lactococcus starter and an adjunct culture of Lactobacillus helveticus I or Lactobacillus casei T subjected to different attenuation treatments: freeze shocking (FS), heat shocking (HS), or spray drying (SD). Proteolysis during cheese ripening (0 to 6 mo), measured by urea-PAGE and water-soluble nitrogen, indicated only minor differences between control and most adjunct-treated cheeses. However, there were significant differences in the effect of Lactobacillus adjuncts on the level of free amino nitrogen in cheese. Cheeses made with FS or HS Lb. helveticus adjunct exhibited significantly greatest rates of free amino group formation. Lipolysis as measured by total free fatty acids was consistently highest in adjunct-treated cheeses, and FS Lb. casei-treated cheeses showed the highest rate of free fatty acid formation followed by FS Lb. helveticus treated cheeses. Mean flavor and aroma scores were significantly higher for cheeses made with Lb. helveticus strain. Freeze-shocked Lb. helveticus-treated cheeses obtained the highest flavor and aroma scores. Sensory evaluation indicated that most of the adjunct-treated cheeses promoted better texture and body quality.

Characterization of particles in cream cheese.

Cream cheese is used as a spread and as an ingredient in many food applications. A gritty or grainy mouthfeel is an undesirable textural defect that occurs in cream cheese. However, the factors that cause the textural defect are not well understood. The objectives of this study were to isolate and characterize particles from cream cheese and to study the effect of particles on cheese texture. Particles were isolated by washing cream cheese with water first at 25 degrees C and then at 50 degrees C repeatedly 4 to 5 times. The size of these particles was determined using a particle size analyzer. The particles as well as the original cheeses were analyzed for moisture, fat, protein, ash, and lactose. The particle size ranged of 0.04 to 850 microm. It was found that isolated particles were significantly higher in protein content as compared with the whole cheese. To study the effect on the cheese texture, particles were added at 5, 15, and 25% (wt/wt) levels to smooth cream cheese, and a sensory ranking test was done on the samples. Isolated particles were further separated into 2 size classes of 2.5 to 150 microm and > or =150 microm. These particles were then mixed with smooth cream cheese at 16 and 29% (wt/wt), and a sensory test was conducted on these samples. Smooth cream cheese with only 5% (wt/wt) added particles was perceived as significantly grittier than the control sample. This experiment also revealed that the perceived grittiness increased with increase in amount and size of particles.