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.     

Microbiological analysis and starter culture growth in retentates.

Pasteurized skim milk was concentrated by UF to 2-, 4-, and 5-fold. The retentates were evaluated for microbiological quality, heat treatments to inactivate microorganisms, and lactic acid bacterial starter culture activity. Aerobic mesophilic bacterial counts in raw milk decreased from an initial 1.4 x 10(6) to 3.9 x 10(2) cfu/ml after pasteurization. During UF, counts increased from 3.9 x 10(2) cfu/ml UF, counts increased from 3.9 x 10(2) cfu/ml in pasteurized milk to 1.4 x 10(3), 1.4 x 10(4), and 1.8 x 10(4) cfu/ml in 2-, 4- and 5-fold retentates, respectively. Psychrotrophic bacterial counts decreased from 9.9 x 10(5) cfu/ml in raw milk to 3.7 x 10(1) cfu/ml in pasteurized milk and gradually increased to 1.0 x 10(2), 2.5 x 10(2), and 1.4 x 10(3) cfu/ml in 2-, 4-, and 5-fold retentates, respectively. Thermophilic bacterial counts remained less than 10 cfu/ml in all samples. Skim milk and retentates inoculated with five starter cultures at 1% failed to decrease the pH below 4.6 in (2-, 4- and 5-fold). The 4- and 5-fold retentates inoculated with Lactococcus lactis spp. cremoris or Lactococcus lactis spp. lactis cultures were partially coagulated with pH greater than 5.6. In general, the pH of retentates remained higher than that of skim milk. Clotting of uninoculated samples was observed, and a spore-forming contaminant, tentatively characterized as Bacillus cereus and capable of clotting milk at a pH greater than 6, was isolated from the clotted samples.(ABSTRACT TRUNCATED AT 250 WORDS)     

THE INFLUENCE OF LACTIC CULTURES ON GROUND BEEF QUALITY

SUMMARY– The effects of adding 5 and 10% lactic culture (Streptococcus lactis and Leuconostoc citrovorum) grown in skim milk; 2.5, 5, 10, and 20% lactic cultures grown in 20% milk solids; 1 and 2% frozen concentrated lactic cultures; lactic acid to pH 5.0 and 4.5; and 2.5, 5, and 10% skim milk to ground beef stored at refrigeration temperature (7°C) were studied in the 6 preliminary trials. Results obtained were used to formulate the principal investigation. The influence of 10% lactic culture and 10% lactic culture plus 450 ppm of ascorbic acid were tested in 5 replicate trials for the principal investigation. Cultures used in the replicate trials were grown in 20% milk solids. CVT (crystal violet tetrazolium) counts for gram-negative bacteria, pH, VNC (volatile nitrogen content), and organoleptic observations were evaluated. Data from the preliminary study indicated a profound inhibitory action of the lactic cultures on the growth of the inherent gram-negative bacteria in ground beef. The addition of 10% lactic culture grown in 20% milk solids was effective in preventing aerobic growth. The addition of pure lactic acid inhibited microbial growth, but caused an undesirable color and aroma. Frozen concentrated cultures required 1% lactose to inhibit the growth. The CVT count in the uncultured meat significantly increased (P < .01) as the storage time progressed. Cultured meat did not exhibit a significant increase in CVT count until 7 days of storage. The pH of the cultured meat significantly declined (P <.01) whereas the uncultured meat indicated a significant increase in pH (P < .01) during storage. VNC in the uncultured meat was significantly higher (P <.01) than in the cultured meat. Cultured meat with 450 ppm of ascorbic acid was consistently preferred for flavor, aroma, and when color was compared with the uncultured meat and the meat with culture alone.     

STEADY SHEAR VISCOSITY OF ULTRAFILTERED WHOLE AND SKIM MILK RETENTATES BEFORE AND DURING FERMENTATION

Steady shear viscosities are reported for skim and whole milk retentates of different volume concentrations after ultrafiltration, as well as during fermentation for the most concentrated retentates with and without salt addition. Retentates up to 3X exhibit Newtonian behaviour, while the most concentrated retentates (5X and 5.3X whole and skim) fit the power law model. The fermentation follows a three-phase model, during which the pH decreases from the natural pH of 6.0–6.5 to around 5.0–5.1. The viscosities of the fermented retentates vary significantly with the change of pH, depending on the fat and salt contents.     

Physico-chemical properties of skim milk retentates from microfiltration.

Physico-chemical properties of retentates obtained from selective concentration of skim milk up to 8 times its original weight using a microfiltration system were studied. The effects of process variables, namely concentration (8.6 to 27 wt.%), temperature (20 to 50 degrees C) and pH (6.0, 6.3, and 6.5) on density (rho), apparent viscosity (mu(a)), consistency coefficient (K), flow behavior index (n), and activation energy (Ea) of the retentates were examined. Depending on pH, retentates showed significant increase in apparent viscosity, deviated from classical Newtonian behavior and exhibited shear-thinning between 11 to 17% solids concentration. No yield stress was detected in the range of concentration studied. The power law parameters (n and K) followed a similar trend. An Arrhenius-type equation described well the effect of temperature on apparent viscosity. Although activation energy increased 120 to 130% for a threefold increase in solids concentration, it was not significantly different from that of other types of concentrated milk at approximately the same concentration. Increasing solids were responsible for change in flow properties with concentration, while the effect of pH was attributed to differential protein (primarily casein) retention and the change in solvation properties and voluminosity of casein micelles. Models relating concentration, temperature, and pH to retentate apparent viscosity and consistency coefficient were identified. Skim milk microfiltration with in-process pH adjustment produces retentates depleted in whey proteins and calcium with significantly altered properties.     

The sol–gel transition temperature of skim milk concentrated by microfiltration as affected by pH and protein content

This study aimed at determining sol–gel transition temperatures of microfiltered skim milk retentates for different protein levels (6, 8 and 10% (w/w)) and a wide pH range (native pH to 4.6) by means of small‐amplitude oscillatory shear rheology. For a pH of 5.4 to 5.0, the sol–gel transition temperatures decreased significantly with increasing protein content, which did not differ for pH 4.8 and 4.6. The sol–gel transition temperatures of retentates with 10% (w/w) protein at pH 5.4 and 4.6 were 58.4 °C and 10.9 °C, respectively.     

Production of quarg by ultrafiltration

The increased protein concentration in UF concentrate caused some problems in achieving the desired pH for quarg making when yogurt and mixed lactic cultures were used. Yogurt culture could ferment concentrated milk to a lower pH than the mixed culture. With the increasing concentration during UF, levels of total ash, calcium and phosphorus in the concentrate increased, but these increases were much lower at pH 4.6. Quarg obtained from UF concentrated sour milk was rated close to conventional quarg and had no bitter taste. A high heat treatment of milk before lactic fermentation and subsequent UF concentration resulted in a quarg with a smoother texture. Diafiltration of UF concentrated milk did not result in significant elimination of excessive calcium. The quality of the quarg was also poor with respect to taste, body and texture. Thus diafiltration would be of little use in quarg making.     

Antagonism between Listeria monocytogenes and lactococci during fermentation of products from ultrafiltered skim milk.

Tyndallized samples of unfiltered skim milk and retentate (concentrated fivefold or twofold by volume) and permeate from UF skim milk were inoculated with 5.5 x 10(3) to 1.5 x 10(5) cfu/ml of Listeria monocytogenes strains California or V7 together with 4 x 10(7) to 2.3 x 10(8) cfu/ml of mesophilic lactic acid bacteria. Numbers of L. monocytogenes (McBride Listeria agar) and lactic acid bacteria (all purpose Tween agar) were determined after 0, 6, 12, 24, 30, and 36 h of incubation at 30 degrees C. Lactic acid bacteria significantly inhibited or inactivated L. monocytogenes in all three products. Inactivation was greater in permeate (6.77 orders of magnitude) than in unfiltered skim milk (3.67 orders of magnitude) or in retentate (4.21 orders of magnitude). Degree of inactivation in retentate was related to the extent of concentration. Inactivation was not complete, and L. monocytogenes survived in these products during fermentation for up to 36 h. When fermented products were refrigerated (4 degrees C), L. monocytogenes survived for 4 to 6 wk in skim milk, 3 to 5 wk in retentate, and 1 wk in permeate. At refrigeration temperature, length of survival was dependent on type of product and strain of the pathogen.     

Vatless manufacturing of low-moisture part-skim mozzarella cheese from highly concentrated skim milk microfiltration retentates

Low-moisture, part-skim (LMPS) Mozzarella cheeses were made from concentration factor (CF) 6, 7, 8, and 9, pH 6.0 skim milk microfiltration (MF) retentates using a vatless cheese-making process. The compositional and proteolytic effects of cheese made from 4 CF retentates were evaluated as well as their functional properties (meltability and stretchability). Pasteurized skim milk was microfiltered using a 0.1-microm ceramic membrane at 50 degrees C to a retentate CF of 6, 7, 8, and 9. An appropriate amount of cream was added to achieve a constant casein:fat ratio in the 4 cheesemilks. The ratio of rennet to casein was also kept constant in the 4 cheesemilks. The compositional characteristics of the cheeses made from MF retentates did not vary with retentate CF and were within the legal range for LMPS Mozzarella cheese. The observed reduction in whey drained was greater than 90% in the cheese making from the 4 CF retentates studied. The development of proteolytic and functional characteristics was slower in the MF cheeses than in the commercial samples that were used for comparison due to the absence of starter culture, the lower level of rennet used, and the inhibition of cheese proteolysis due to the inhibitory effect of residual whey proteins retained in the MF retentates, particularly high molecular weight fractions.     

Growth and activity of some lactic streptococci cultivated on soymilk medium infected with bacteriophage

Pure cultures of S. lactis, S. cremoris, S. diacetilactis and S. thermophilus were cultivated on soymilk medium containing 3% lactose and skim milk medium which were infected or not infected with bacteriophage. The cultures were incubated for 24 h at the optimum temperature of each organism. The growth, lactic acid production and activity of each culture were periodically examined. The growth and the acidity development of the cultivated cultures on soymilk medium infected with bacteriophage increased gradually during the incubation period, whereas the cultures grown on skim milk infected with bacteriophage showed less growth and acid production. Moreover, the growth and the rate of lactic acid production of the cultures cultivated on soymilk medium were higher than those of the cultures grown on skim milk medium (control). Therefore, soymilk medium containing 3% lactose could be recommended as a protective medium against bacteriophage infection for lactic acid starter.     

Influence of milk ultrafiltration on bacteriophages of lactic acid bacteria

Bacteriophages added to whole milk were partially concentrated during ultrafiltration. At 4:1 retentate, phage had concentrated 2.4:1. Thermal destruction at 54 degrees C followed first order kinetics up to 6% protein, whereafter it deviated. When allowed to grow in retentate in the presence of appropriate host, 3.5 generations of phage appeared after 12 h at 22 degrees C compared with four generations in skim milk. In the presence of phage, lactic acid bacteria population increased to only 10(7) cfu/ml compared with 3 X 10(9) in their absence. Retentate starter prepared in the presence of phage was as active as skim milk starter prepared in the presence of phage.     

Influence of partial acidification of skim milk on ultrafiltration performance and physicochemical properties of the produced streams

The influence of partial acidification of skim milk (SM) using glucono-δ-lactone (GDL), citric acid (CA) or lactic acid (LA), on physicochemical properties (e.g. viscosity and calcium balance) of ultrafiltration (UF) retentates produced at 10 and 55°C was investigated. Ultrafiltration retentates produced using CA showed a significantly lower amount (P < 0.05) of ionic calcium and higher apparent viscosity than GDL and LA. Regardless of the acid used, total calcium concentration and apparent viscosity of streams were modified compared with SM, which impaired UF overall performance. Specifically, CA at 10°C and GDL at 55°C, both reduced the permeate flux when compared to the other acids.     

Comparative evaluation of bacterial cellulose (nata) as a cryoprotectant and carrier support during the freeze drying process of probiotic lactic acid bacteria

Nata or bacterial cellulose produced by Acetobacter xylinum was compared for its cryoprotective and carrier support potential for probiotic lactic acid bacteria against other established cryoprotectants like 10% skim milk, calcium alginate encapsulation or 0.85% physiological saline and distilled water. Individual lactic acid bacteria were grown in MRS broth in the presence of nata cubes or the bacterial suspension mixed with either powdered bacterial cellulose (PBC), 10% skim milk, saline or distilled water and freeze dried. These freeze dried cells were stored at temperatures of either 30 °C or 4 °C and periodically checked for viability. The freeze dried cells on carrier supports were directly used to prepare fermented milks to establish the activity of these cultures. Scanning electron microscopy was employed to visualize the support matrix with the attached lactic acid bacteria. The freeze drying process resulted in a 3.0 log cycle reduction in the colony forming units as compared to the original cell suspension in the case of all the lactic acid bacteria. The growth of lactic acid bacteria in the presence of bacterial cellulose (nata) offers a convenient and easy method to preserve bacteria for short durations and use it as a support to carry out other fermentation processes.     

Standardization of milk using cold ultrafiltration retentates for the manufacture of parmesan cheese

The effects of using cold ultrafiltered (UF) retentates (both whole and skim milk) on the coagulation, yield, composition, and ripening of Parmesan cheese were investigated. Milks for cheese making were made by blending cold UF retentates with partially skimmed milk to obtain blends with 14.2% solids and a casein:fat ratio of 1.1. Cutting times, as selected by the cheese-maker, were approximately 15 and approximately 20 min for experimental and control milks, respectively. Storage modulus values at cutting were similar, but yield stress values were significantly higher in UF retentate standardized milks. Cheese yields were significantly higher in UF retentate standardized milks (approximately 12%) compared with control milk (cream removed) (approximately 7 to 8%). Significantly higher protein recoveries were obtained in cheeses manufactured using cold UF retentates. There were no differences in the pH and moisture contents of the cheeses prior to brining, and there was no residual lactose or galactose left in the cheeses. Using UF retentates resulted in a significant reduction in whey volume as well as a higher proportion of protein in the solids of the whey. Proteolysis, free fatty acids, and sensory properties of the cheeses were similar. The use of milk concentrated by cold UF is a promising way of improving the yield of Parmesan cheese without compromising cheese quality. The question remaining to be answered by the cheesemaker is whether it is economical to do so.     

Effects of Ultrafiltration on Retention of Minerals and Other Components of Milk

Retentions of milk components were determined following laboratory-scale ultrafiltration using membranes of different composition and molecular weight cut-offs, milks at different stages of processing, and concentration factors of 2.0x and 1.5x. Component retentions were not affected by membrane composition, molecular weight cut-off, pasteurization or homogenization. Whole milk showed a higher percent recovery of total solids than skim milk. Pasteurized, homogenized whole milk concentrated 1.5x retained more total solids than when concentrated 2.0x, although mineral retentions were not appreciably affected. There was virtually complete retention of protein (93–100%) in all milks, while the transfer of lactose through the membranes was similar to that of water. Mineral retentions varied little from milk to milk and 2.0x retentates had individual recoveries of 16 different minerals ranging from approximately 50% to over 90%.     

Membrane Fractionation Processes for Removing 90% to 95% of the Lactose and Sodium from Skim Milk and for Preparing Lactose and Sodium‐Reduced Skim Milk

ABSTRACT: Pilot‐scale microfiltration (MF), microfiltration‐diafiltration (MDF), ultrafiltration (UF), ultrafiltration‐diafiltration (UDF), and nanofilration (NF) membrane fractionation processes were designed and evaluated for removing 90% to 95% of the lactose and sodium from skim milk. The study was designed to evaluate several membrane fractionation schemes as a function of: (1) membrane types with and without diafiltration; (2) fractionation process temperatures ranging from 17 to 45 °C; (3) sources of commercial drinking water used as diafiltrant; and (4) final mass concentration ratios (MCR) ranging from about 2 to 5. MF and MDF membranes provided highest flux values, but were unsatisfactory because they failed to retain all of the whey proteins. UDF fractionation processes removed more than 90% to 95% of the lactose and sodium from skim milk. NF permeate prepared from UDF cumulative permeate contained sodium and other mineral concentrations that would make them unsuitable for use as a diafiltrant for UDF applications. A method was devised for preparing simulated milk permeate (SMP) formulated with calcium, magnesium, and potassium hydroxides, and phosphoric and citric acids for use as UDF diafiltrant or for preparing lactose and sodium reduced skim milk (L‐RSM). MF retentates with MCR values of 4.7 to 5.0 exhibited extremely poor frozen storage stabilities of less than 1 wk at ?20 °C, whereas MCR 1.77 to 2.95 MDF and UDF retentates and skim milk control exhibited frozen storage stabilities of more than 16 wk. L‐RSM exhibited a whiter appearance and a lower viscosity than skim milk, lacked natural milk flavor, and exhibited a metallic off‐flavor.     

Cheese pH, protein concentration, and formation of calcium lactate crystals

The occurrence of calcium lactate crystals (CLC) in hard cheeses is a continual expense to the cheese industry, as consumers fail to purchase cheeses with this quality defect. This research investigates the effects of the protein concentration of cheese milk and the pH of cheese on the occurrence of CLC. Atomic absorption spectroscopy was used to determine total and soluble calcium concentrations in skim milk (SM1, 8.7% total solids), and skim milk supplemented with nonfat dry milk (CSM1, 13.5% total solids). Calcium, phosphorus, lactic acid, and citrate were determined in cheeses made with skim milk (SM2, 3.14% protein), skim milk supplemented with ultrafiltered milk (CSM2, 6.80% protein), and nonfat dry milk (CSM3, 6.80% protein). Supplementation with nonfat dry milk increased the initial total calcium in CSM1 (210 mg/100 g of milk) by 52% compared with the total calcium in SM1 (138 mg/100 g of milk). At pH 5.4, soluble calcium concentrations in CSM1 were 68% greater than soluble calcium in SM1. In cheeses made from CSM2 and CSM3, total calcium was 26% greater than in cheeses made from SM2. As the pH of cheeses made from SM2 decreased from 5.4 to 5.1, the concentration of soluble calcium increased by 61.6%. In cheeses made from CSM2 and CSM3, the concentrations of soluble calcium increased by 41.4 and 45.5%, respectively. Calcium lactate crystals were observed in cheeses made from SM2 at and below pH 5.1, whereas CLC were observed in cheeses from CSM2 and CSM3 at and below pH 5.3. The increased presence of soluble calcium can potentially cause CLC to occur in cheese manufactured with increased concentrations of milk solids, particularly at and below pH 5.1.     

Micellar casein concentrate production with a 3X, 3-stage, uniform transmembrane pressure ceramic membrane process at 50°C

The production of serum protein (SP) and micellar casein from skim milk can be accomplished using microfiltration (MF). Potential commercial applications exist for both SP and micellar casein. Our research objective was to determine the total SP removal and SP removal for each stage, and the composition of retentates and permeates, for a 3×, continuous bleed-and-feed, 3-stage, uniform transmembrane pressure (UTP) system with 0.1-μm ceramic membranes, when processing pasteurized skim milk at 50°C with 2 stages of water diafiltration. For each of 4 replicates, about 1,100 kg of skim milk was pasteurized (72°C, 16s) and processed at 3× through the UTP MF system. Retentate from stage 1 was cooled to <4°C and stored until the next processing day, when it was diluted with reverse osmosis water back to a 1× concentration and again processed through the MF system (stage 2) to a 3× concentration. The retentate from stage 2 was stored at <4°C, and, on the next processing day, was diluted with reverse osmosis water back to a 1× concentration, before running through the MF system at 3× for a total of 3 stages. The retentate and permeate from each stage were analyzed for total nitrogen, noncasein nitrogen, and nonprotein nitrogen using Kjeldahl methods; sodium dodecyl sulfate-PAGE analysis was also performed on the retentates from each stage. Theoretically, a 3-stage, 3× MF process could remove 97% of the SP from skim milk, with a cumulative SP removal of 68 and 90% after the first and second stages, respectively. The cumulative SP removal using a 3-stage, 3× MF process with a UTP system with 0.01-μm ceramic membranes in this experiment was 64.8 ± 0.8, 87.8 ± 1.6, and 98.3 ± 2.3% for the first, second, and third stages, respectively, when calculated using the mass of SP removed in the permeate of each stage. Various methods of calculation of SP removal were evaluated. Given the analytical limitations in the various methods for measuring SP removal, calculation of SP removal based on the mass of SP in the skim milk (determined by Kjeldahl) and the mass SP present in all of the permeate produced by the process (determined by Kjeldahl) provided the best estimate of SP removal for an MF process.     

Proteins recovered from acid whey/skim milk mixtures heated at alkaline pH values

Skim milk and mixtures prepared by combining acid whey with skim milk at volume ratios of 2:1, 1:1, 1:2, 1:3 and 1:4 were adjusted to pH 7.5 and heated at 90°C × 15 min. Protein was isolated from these heated samples by precipitation at pH 4.6 and it was found that 65% of the whey protein was recovered in each case. Non-recovered proteins included the proteose peptones and small quantities of β-lactoglobulin, α-lactalbumin and bovine serum albumin. The solubility of these isolates, which contained from 10–25% whey protein, decreased to > 95% when the whey protein exceeded ˜16%. Further characterization of the isolate, prepared from the 1:1 volume ratio of acid whey and skim milk, showed that ˜50% of the whey protein was insoluble, bound to casein and non-functional while the other ˜50% was complexed with casein and was soluble. The addition of a reducing agent suggests that sulphydryl bonding alone is not responsible for complex formation.     

Acid adaptation affects the viability of Salmonella typhimurium during the lactic fermentation of skim milk and product storage

In this study, Salmonella typhimurium was acid adapted at pH 5.5 for 4 h. The viability of the acid-adapted and non-adapted cells of S. typhimurium was investigated both during the lactic fermentation of skim milk with Streptococcus thermophilus or Lactobacillus bulgaricus, and during the storage of lactic fermented milk products at 5 degrees C. It was found that the viable population of S. typhimurium, regardless of acid adaptation, increased in skim milk during the initial 24 h of lactic fermentation and then declined. However, the viable population of acid-adapted S. typhimurium was significantly higher (P<.05) than that of non-adapted cell at the end of 48 h of fermentation. Acid-adapted cells of S. typhimurium were also found to have survived better than non-adapted cells in the S. thermophilus-prepared fermented milk and two commercial lactic fermented milk products. The viability of the acid-adapted and non-adapted S. typhimurium at 5 and 37 degrees C in cell-free fermented milks that had their pHs adjusted to 6.4 and skim milk (pH 6.4) was further investigated. Results revealed that acid adaptation, in addition to enhancing acid tolerance, reduced the susceptibility of S. typhimurium to refrigerated temperature and other detrimental factors which might be present in lactic fermented milk products. These responses all contribute to the enhanced survival of acid-adapted S. typhimurium in the lactic fermented milk products observed in the present study.