Influence of protein concentration and coagulation temperature on rennet-induced gelation characteristics and curd microstructure

peer-reviewedThis study characterized the coagulation properties and defined the cutting window (CW; time between storage modulus values of 35 and 70 Pa) using rheometry for milk standardized to 4, 5, or 6% protein and set at 28, 32, or 36°C. Milks were standardized to a protein-to-fat ratio of approximately 1 by blending ultrafiltration retentate, skim milk, and whole milk. The internal curd microstructure for selected curd samples was analyzed with transmission electron microscopy and scanning electron microscopy. Lowering the coagulation temperature caused longer rennet coagulation time and time to reach storage modulus of 35 Pa, translating into a wider CW. It also led to a lower maximum curd-firming rate (MCFR) with lower firmness at 40 min at a given protein level. Increasing protein levels resulted in the opposite effect, although without an effect on rennet coagulation time at a given temperature. On coagulation at 28°C, milk with 5% protein resulted in a similar MCFR (∼4 Pa/min) and CW (∼8.25 min) compared with milk with 4% protein at 32°C, which reflects more standard conditions, whereas increasing milk to 6% protein resulted in more than doubling of the curd-firming rate (MCFR = 9.20 Pa/min) and a shorter CW (4.60 min). Gels set at 28°C had lower levels of rearrangement of protein network after 40 min compared with those set at 36°C. Protein levels, on the other hand, had no influence on the levels of protein network rearrangement, as indicated by loss tangent values. The internal structure of curd particles, as investigated by both scanning electron microscopy and transmission electron microscopy, appeared to have less cross-linking and smaller casein aggregates when coagulated at 28°C compared with 36°C, whereas varying protein levels did not show a marked effect on aggregate formation. Overall, this study showed a marked interactive effect between coagulation temperature and protein standardization of milk on coagulation properties, which subsequently requires adjustment of the CW during cheesemaking. Lowering of the coagulation temperature greatly altered the curd microstructure, with a tendency for less syneresis during cutting. Further research is required to quantify the changes in syneresis and in fat and protein losses to whey due to changes in the microstructure of curd particles arising from the different coagulation conditions applied to the protein-fortified milk

Gas-Forming Nonstarter Lactorbacilli

An obligatory heterofermentative lactic acid bacterium Lactobacillus wasatchii sp. nov. isolated from gassy Cheddar cheese was studied for growth, gas formation, salt tolerance and survival against pasteurization treatments at 63°C and 72°C. Initially, Lb. wasatchii was thought to only use ribose as a sugar source and we were interested in whether it could utilize galactose. Experiments to determine rate and extent of growth and gas production in carbohydrate restricted (CR) de Man, Rogosa, and Sharpe (MRS) medium under anaerobic conditions with various combinations of ribose and galactose at 12, 23, and 37°C were conducted with 23°C being the more optimum growth temperature of Lb. wasatchii. When grown on ribose (0.1%, 0.5%, and 1%), maximum specific growth rates (μmax) within each temperature were similar. When galactose was the only sugar, μmax was 2 to 4 times lower than with ribose. At all temperatures, highest final cell densities (OD640) of Lb. wasatchii were achieved in CR-MRS plus 1% ribose, 0.5% ribose and 0.5% galactose, or 1% ribose combined with 1% galactose. Similar μmaxvalues and final cell densities were achieved when 50% of ribose in CR-MRS was substituted with galactose. Such enhanced utilization of galactose in the presence of ribose to support bacterial growth has not previously been reported. It appears that Lb. wasatchii co-metabolizes ribose and galactose, utilizing ribose for energy and galactose for other functions such as cell wall biosynthesis. Co-utilization of both sugars could be an adaptation mechanism of Lb. wasatchii to the cheese environment to efficiently ferment available sugars for maximizing metabolism and growth. As expected, gas formation by the heterofermenter was observed only when galactose was present in the media. Growth experiments with MRS plus 1.5% ribose at pH 5.2 or 6.5, with 0, 1, 2, 3, 4, or 5% NaCl revealed that Lb. wasatchii is able to grow under salt and pH conditions typical of Cheddar cheese (4 to 5% salt-in-moisture, ~pH 5.2). Finally, we found Lb. wasatchii cannot survive LTLT pasteurization but survives HTST lab pasteurization with 4.5 log reduction occurred. The ability of Lb. wasatchii to survive HTST pasteurization and grow under cheese ripening conditions implies that the presence of this nonstarter lactic acid bacteria can be a serious contributor to gas formation and textural defects in Cheddar cheese

Effects of Phosphate and Citrate on the Gelation Properties of Casein Micelles in Renneted Ultra-High Temperature (UHT) Sterilized Concentrated Milk

Milk was concentrated to 3X (volume reduction) by ultrafiltration. Disodium phosphate and sodium citrate were added, and the milk concemrates were homogenized. The concentrates were then heated at 135°C for 50 s in a labora· tory ultra-high temperature (UHT) heating system. Rennet gels were made from heated and unheated milk concentrates and their curd firmness measured using a Fonnagraph. Gel microstructures were ex:amined by electron microscopy. When rennet was added to unhomogenized milk concentrate before UHT heating, the resultant gel consisted of a strong protein network that encapsulated the fat globules. Pockets of milk serum were associated with the fat. Homogenization caused the fat droplets to be coated with casein micelles and become tied into the protein network as an integral pan of the gel structure. The microstructure of UHT milk concentrate gels was different from gels made from unheated milk. Gelation of UHT milk proceeded more slowly and the gels were weaker. Much of the casein in such samples had lost their micellar identity and was present as a homogeneous mass around the fat droplets. Large areas in the gel lacked protein network, which weakened the UHT milk gels. Samples with disodium phosphate added did not gel after UHT treatment, even if high concentrations of ren net were added. Samples with sodium citrate added formed only a weak rennet gel after UHT treatment

Investigating the Filled Gel Model in Cheddar Cheese Through Use of Sephadex Beads

Cheese can be modeled as a filled gel whereby milkfat globules are dispersed in a casein gel network. We determined the filler effects using Sephadex beads (GE Healthcare Life Sciences, Pittsburgh, PA) as a model filler particle. Ideally, such a model could be used to test novel filler particles to replace milkfat in low-fat cheese. Low-filler (6% particles), reduced-filler (16%), and full-filler (33%) cheeses were produced using either Sephadex beads of varying sizes (20 to 150 μm diameter) or milkfat. Small- and large-strain rheological tests were run on each treatment at 8, 12, and 18 wk after cheese manufacturing. Differences in rheological properties were caused primarily by the main effects of filler volume and type (milkfat vs. Sephadex), whereas filler size had no obvious effect. All treatments showed a decrease in deformability and an increase in firmness as filler volume increased above 25%, although the beads exhibited a greater reinforcing effect and greater energy recovery than milkfat

Enhanced Nutty Flavor Formation in Cheddar Cheese Made with Malty Lactococcus Lactis Adjunct Culture

Nutty flavor in Cheddar cheese is desirable, and recent research demonstrated that 2- and 3-methyl butanal and 2-methyl propanal were primary sources of nutty flavors in Cheddar. Because malty strains of Lac-tococcus lactis (formerly Streptococcus lactis var. malti-genes) are characterized by the efficient production of these and other Strecker aldehydes during growth, this study investigated the influence of a malty L. lactis adjunct culture on nutty flavor development in Cheddar cheese. Cheeses made with different adjunct levels (0, 10(4) cfu/mL, and 10(5) cfu/mL) were ripened at 5 or 13 degrees C and analyzed after 1 wk, 4 mo, and 8 mo by a combination of instrumental and sensory methods to characterize nutty flavor development. Cheeses ripened at 13 degrees C developed aged flavors (brothy, sulfur, and nutty flavors) more rapidly than cheeses held at 5 degrees C. Additionally, cheeses made with the adjunct culture showed more rapid and more intense nutty flavor development than control cheeses. Cheeses that had higher intensities of nutty flavors also had a higher concentration of 2/3-methyl butanal and 2-methyl propanal compared with control cheeses, which again confirmed that these compounds are a source of nutty flavor in Cheddar cheese. Results from this study provide a simple methodology for cheese manufacturers to obtain consistent nutty flavor in Cheddar cheese

Growth and gas production of a novel obligatory heterofermentative Cheddar cheese nonstarter lactobacilli species on ribose and galactose

An obligatory heterofermentative lactic acid bacterium, Lactobacillus wasatchii sp. nov., isolated from gassy Cheddar cheese was studied for growth, gas formation, salt tolerance, and survival against pasteurization treatments at 63°C and 72°C. Initially, Lb. wasatchii was thought to use only ribose as a sugar source and we were interested in whether it could also utilize galactose. We conducted experiments to determine the rate and extent of growth and gas production in carbohydraterestricted (CR) de Man, Rogosa, and Sharpe (MRS) medium under anaerobic conditions with various combinations of ribose and galactose at 12, 23, and 37°C, with 23°C being the optimum growth temperature of Lb. wasatchii among the three temperatures studied. When Lb. wasatchii was grown on ribose (0.1, 0.5, and 1%), maximum specific growth rates (μmax) within each temperature were similar. When galactose was the only sugar, compared with ribose, μmax was 2 to 4 times lower. At all temperatures, the highest final cell densities (optical density at 640 nm) of Lb. wasatchii were achieved in CR-MRS plus 1% ribose, 0.5% ribose and 0.5% galactose, or 1% ribose and 1% galactose. Similar μmax values and final cell densities were achieved when 50% of the ribose in CR-MRS was substituted with galactose. Such enhanced utilization of galactose in the presence of ribose to support bacterial growth has not previously been reported. It appears that Lb. wasatchii co-metabolizes ribose and galactose, utilizing ribose for energy and galactose for other functions such as cell wall biosynthesis. Co-utilization of both sugars could be an adaptation mechanism of Lb. wasatchii to the cheese environment to efficiently ferment available sugars for maximizing metabolism and growth. As expected, gas formation by the heterofermenter was observed only when galactose was present in the medium. Growth experiments with MRS plus 1.5% ribose at pH 5.2 or 6.5 with 0, 1, 2, 3, 4, or 5% NaCl revealed that Lb. wasatchii is able to grow under salt and pH conditions typical of Cheddar cheese (4 to 5% salt-in-moisture, pH ~5.2). Finally, we found that Lb. wasatchii cannot survive low-temperature, long-time pasteurization but survives high-temperature, short-time (HTST) laboratory pasteurization, under which a 4.5 log reduction occurred. The ability of Lb. wasatchii to survive HTST pasteurization and grow under cheese ripening conditions implies that the presence of this nonstarter lactic acid bacterium can be a serious contributor to gas formation and textural defects in Cheddar cheese

Growth and Gas Formation by Lactobacillus wasatchensis, a Novel Obligatory Heterofermentative Nonstarter Lactic Acid Bacterium, in Cheddar-style Cheese Made Using a Streptococcus thermophilus Starter

A novel slow-growing, obligatory heterofermentative, nonstarter lactic acid bacterium (NSLAB) Lactobacillus wasatchensis WDC04 was studied for growth and gas production in Cheddar-style cheese made using Streptococcus thermophilus as the starter culture. Cheesemaking trials were conducted using St. thermophilus alone or in combination with Lb. wasatchensis deliberately added to cheese milk at a level of ~104 cfu/ml. Resulting cheeses were ripened at 6 or 12°C. At d 1, starter streptococcal numbers were similar in both cheeses (~109 cfu/g) and fast-growing NSLAB lactobacilli counts were below detectable levels (\u3c102 cfu/g). As expected, Lactobacillus wasatchensis counts were 3 x 105 cfu/g in cheeses inoculated with this bacterium and below enumeration limits in the control cheese. Starter streptococci decreased over time at both storage temperatures but declined more rapidly at 12°C, especially in cheese also containing Lb. wasatchensis. Populations of fast-growing NSLAB and the slow-growing Lb. wasatchensis reached 5 x 107 and 2 x 108 cfu/g, respectively, after 16 wk of storage at 12°C. Growth of NSLAB coincided with a reduction in galactose concentration in the cheese from 0.6% to 0.1%. Levels of galactose at 6°C had similar decrease. Gas formation and textural defects were only observed in cheese with added Lb. wasatchensis ripened at 12°C. Use of St. thermophilus as starter culture resulted in galactose accumulation that Lb. wasatchensis can utilize to produce CO2, which contributes to late gas blowing in Cheddar-style cheeses, especially when the cheese is ripened at elevated temperature

The X-ray Properties of z>4 Quasars

We report on a search for X-ray emission from quasars with redshifts greater than four using the ROSAT public database. Our search has doubled the number of z>4 quasars detected in X-rays from 6 to 12. Most of those known prior to this work were radio-loud and X-ray selected sources; our study increases the number of X-ray detected, optically selected z>4 quasars from one to seven. We present their basic X-ray properties and compare these to those of lower redshift quasars. We do not find any evidence for strong broad-band spectral differences between optically selected z>4 quasars and those at lower redshifts.Comment: 7 pages, 4 figures included, LaTeX emulateapj.sty, accepted for publication in the Astronomical Journa