Graduation date: 1972

Note:

The effect of reducing sodium chloride in Cheddar cheese was studied. Milled curd from a split-lot was salted at selected NaCl concentrations and analyzed through aging by sensory and rheological tests. Estimation of differences in compositional analysis, lactic acid populations, degree of proteolysis, water activity, and pH were also determined. Consumer panel analysis of the cheese on a hedonic scale over seven months aging showed no significant differences in overall desirability between cheese containing 1.75 - 0.88% NaCl. At 0.75% NaCl in cheese, ratings were lower, but flavor and texture scores were considered acceptable. Addition of even a small amount of salt (0.38 - 0.44%) significantly improved sensory characteristics relative to an unsalted cheese. Regression analysis of trained panel evaluations to predict consumer response showed that "firmness" and "adhesiveness" were the most important attributes for favorable consumer ratings of texture. Trained panel determination of "Cheddar intensity" and "unpleasant aftertaste" were found to be the two most important factors for determining consumer panel flavor scores. These four attributes, as rated by the trained panel, correlated with salt concentration in cheese. Instron evaluation of reduced sodium Cheddar cheese showed a decrease in firmness, chewiness, and gumminess and an increase in determinations showed an increase in proteolysis and water activity and a decrease in pH of the finished cheese. Lactic acid bacterial populations were significantly higher in the cheese containing reduced NaCl concentrations with the highest population differences observed through one month of age. / Graduation date: 1984

A sensory method was developed to determine cheese texture by hand evaluation. Cheese sensory evaluation was conducted by panelists (n=8) on four commercial samples in duplicates. Standards, descriptors, methods of each attribute evaluation, sample size, and ballot were developed based on panelists' consensus. Fifteen total attributes, divided into five groups, were tested. Crumbliness was defined as the ease of the sample to break apart during manipulation using the thumb and two fingers for five times. Using Principal Component Analysis (PCA), four components were extracted with the first two explaining most of the variability (60.4%). PCA showed that moistness, crumbliness, color, cohesiveness, irregularity, and oiliness were the main attributes describing the samples. Irregularity and cohesiveness had 83.6% and -88.1% correlations with crumbliness, respectively. Panelists' performances were not significantly different (p≤0.05) and each subject used the method consistently for crumbliness. This method was then applied to evaluate and compare the sensory attributes of Queso Fresco. Three types of Queso Fresco cheese were made: raw cheese (RC), High Hydrostatic Pressure (HHP) treated raw cheese (HP), and cheese made from HHP treated milk (HPM). Sensory attributes, compositions, microstructures and protein profile were compared. Sensory attributes were examined by ten trained panelists using hand evaluation method developed and instrumental methods (Texture Profile Analysis (TPA) and 80% compression test). Protein, fat, and moisture contents were valuated by Micro Kjeldahl, Babcock, and Forced air draft oven respectively. Microstructure was examined by light microscopy using Acid Fuschin protein staining, while native and SDS PAGE were carried out to show the protein profile. One and eight days storage times were studied. HHP treatment of cheese or cheese milk (400 MPa, 20 min, ambient temperature) were shown to reduce microbial loads. HP and RC had similar microstructure, compositional (p-value≤0.05), and sensory attributes, except color (p-value≤0.05). HP and RC had distinct protein network, while HPM had a very diffuse network. HPM was different from both RC and HP. HPM was the least firm, least crumbly, most sticky and oily. HPM day one was firmer, less oily, less springy than day eight. HPM had higher moisture and yield, due to incorporation of denatured whey proteins, than RC and HP cheese. The hand evaluation method developed was proven to be able to differentiate cheese textural attributes. Overall, HHP treatment of Queso Fresco produced cheese with similar characteristics as traditionally made Queso Fresco, while HHP treatment of cheese milk created cheese with weak texture characteristics. HHP treatment of cheese might be an alternative way to produce Queso Fresco with acceptable attributes and reduced microbial load. / Graduation date: 2003

The objectives of this study were (l) to screen out active proline-specific peptidases from Lactobacillus casei subspecies, (2) to study growth kinetic and enzyme production from enriched medium (MRS) and cheese whey medium, (3) to purify and characterize two active proline-specific enzymes, and (4) to investigate the action of purified enzyme on bitter tryptic digests of $ beta$-casein as well as bitter enzyme-modified cheese. Lactobacillus casei subsp. casei LLG and Lactobacillus casei subsp. rhamnosus S93 were examined for extra- and intra-cellular proline-specific peptidase activities. Both strains showed strong activity for x-prolyl dipeptidyl peptidase and proline iminopeptidase but had weak activities for prolidase, prolinase, and post proline endopeptidase. Histochemical staining of crude enzyme extract from Lactobacillus casei ssp. casei LLG with different substrates revealed a distinct protein band for x-prolyl dipeptidyl peptidase as well as for proline iminopeptidase. The growth kinetics showed that the intracellular proline-specific peptidases increased gradually at the beginning of the exponential phase and reached a maximum at the beginning of stationary phase. / Storage stability of x-prolyl dipeptidyl peptidase and proline iminopeptidase in crude extract, with and without stabilizers showed no significant loss in activity of these two enzymes at 4$ sp circ$C for 9 days without adding any stabilizers. The levels of x-prolyl dipeptidyl peptidase, proline iminopeptidase, and post proline endopeptidase activities of cells grown in whey did not vary markedly from cells grown in MRS broth. X-prolyl dipeptidyl peptidase and proline iminopeptidase were purified from crude cell-free extract of Lactobacillus casei ssp. casei LLG by Fast Protein Liquid Chromatography (FPLC) equipped with ion-exchange and gel-filtration columns. X-prolyl dipeptidyl peptidase was found to be a serine-dependent enzyme with molecular mass of 79 kDa. The pH and the temperature optima by the purified enzyme were 7.0 and 50$ sp circ$C, respectively. Proline iminopeptidase was sulfhydryl enzyme with molecular mass of 46 kDa. The maximum enzyme activity was observed at pH 7.5 and 40$ sp circ$C. This is the first report describing the purification and characterization of x-prolyl dipeptidyl peptidase and proline iminopeptidase from Lactobacillus casei to homogeneity. / The debittering of tryptic digests from $ beta$-casein by x-prolyl dipeptidyl peptidase was studied by reversed phase high performance liquid chromatography (RP-HPLC) and liquid chromatography/mass spectrometry. The results showed that two bitter peptides (f53-97 and f03-209) containing X-Pro-Y-Pro in their amino acid residues were completely hydrolyzed and many other peptides with high hydrophobicity were decreased in peak area. The addition of purified x-prolyl dipeptidyl peptidase on bitter enzyme-modified cheese (EMC) also showed that at least one bitter peptide with X-Pro-Y derived from $ alpha$-casein hydrolysis was removed.

No description available.

Numerous attempts have been made to identify the flavor compounds in Blue cheese, however, duplication of Blue cheese flavor has not yet been accomplished. Therefore, it was desirable to make a qualitative and quantitative investigation of Blue cheese flavor compounds and to study the effect of certain microorganisms on Blue cheese flavor. The aroma fraction of Blue cheese was isolated by centrifugation of the cheese and molecular distillation of the recovered fat. The volatiles were separated by gas chromatography on packed columns containing polar and nonpolar phases and by temperature programmed capillary column gas chromatography. Relative retention time data and fast scan mass spectral analysis of the capillary column effluent were used to identify compounds in the aroma fraction. Compounds positively identified were as follows: 2-pentanone, 2-hexanone, 2-heptanone, 2-octanone, 2-nonanone, 2-decanone, 2-undecanone, 2-tridecanone, 2-propanol, 2-pentanol, 2-heptanol, 2-octanol, 2-nonanol, methyl butanoate, methyl hexanote, methyl octanoate, methyl decanoate, methyl dodecanoate, ethyl formate, ethyl acetate, ethyl butanoate, ethyl hexanoate, ethyl octanoate, ethyl decanoate, ethanal, 3-methyl butanal, 2-methyl butanol, 3-methyl butanol, 1-pentanol, benzene, and toluene. Tentatively identified compounds included acetone, delta-octalactone, delta-decalactone, methyl acetate, isopropyl hexanoate, 3-methylbutyl butanoate, pentyl hexanoate, ethyl-2-methylnonanoate, isopropyl decanoate, furfural, 2-methyl propanal, methanol, ethanol, 2-phenylethanol, cresyl methyl ether, dimethylcyclohexane, diacetyl, methyl mercaptan, and hydrogen sulfide. A combination of liquid-liquid column chromatography and gas-liquid chromatography was utilized to quantitate the major free fatty acids in Blue and Roquefort cheese samples. The average concentration (mg acid/kg cheese) in three Blue cheese samples was as follows: 2:0, 826; 4:0, 1, 448; 6:0, 909; 8:0, 771; 10:0, 1,318; 12:0, 1,588; 14:0, 5,856; 16:0, 12,789; 18:0, 4,243; 18:1, 12,455; 18:2, 1,072; 18:3, 987. Roquefort cheese was found to be proportionately more abundant in 8:0 and 10:0 acids and low in 4:0 acid compared to Blue cheese. No formic, propionic, or isovaleric acid was detected in any of the cheeses tested. A quantitative procedure involving adsorption chromatography, liquid-liquid chromatography and absorption spectrophotometry was used to isolate and measure the concentration of the C₃, C₅, C₇, C₉, and C₁₁ methyl ketones in the fat of Blue and Roquefort cheese. The average methyl ketone concentration (micromoles ketone/10 g cheese fat) of five Blue cheese samples was as follows: acetone, 1.7; 2-pentanone, 5.9; 2-heptanone, 11.2; 2-nonanone, 9.3; 2-undecanone, 2. 4. Considerable variation in ketone concentration was noted between samples, but no consistent differences were observed between Blue and Roquefort cheese. One Roquefort sample contained no acetone. The annount of ketone formed during cheese curing does not depend directly on the amount of available fatty acid precursor. There appears to be a selective conversion of the 8:0, and to a lesser extent the 6:0 and 10:0, fatty acids to methyl ketones by the Penicillium roqueforti spores. The concentration of the C₅, C₇, and C₉ secondary alcohols was determined in the same cheeses used for ketone analysis. The previously measured ketones acted as internal standards and facilitated a semi-quantitative calculation of alcohol concentrations from peak areas of gas chrorriatograms. The average alcohol concentration (micromoles alcohol/10 g cheese fat) in five Blue cheese samples was as follows: 2-pentanol, 0. 3; 2-heptanol, 2. 1; 2-nonanol, 0. 8. The alcohols were present in approximately the same ratios as their methyl ketone analogs, but at much lower concentrations. A synthetic Blue cheese flavor was prepared using a blend of butterfat, dry curd cottage cheese, cream, and salt as a base. The most typical flavor was obtained using the following' compounds: the 2:0, 4:0, 6:0, and 8:0 fatty acids at two-thirds the average concentration found in cheese; twice the average concentration of the C₃, C₅, C₇, C₉, and C₁₁ methyl ketones and C₅, C₇, and C₉ secondary alcohols found in cheese: 2.0 mg/kg of base of 2-phenylethanol; 1.5 mg/kg of base of ethyl butanoate; 6.0 mg/kg of base of both methyl hexanoate and methyl octanoate. Incorporation of higher acids caused a soapy flavor. The presence of 2-phenylethanol and the esters was judged as very important in duplicating Blue cheese flavor. The mycelia of Penicillium roqueforti appear to be more active in the reduction of methyl ketones to secondary alcohols than the spores. Yeasts associated with Blue cheese are capable of reducing methyl ketones to secondary alcohols. Yeasts also may play a role in Blue cheese flavor by producing ethanol and other alcohols and certain esters. / Graduation date: 1966

No description available.

Carboxypeptidase Y (E.C. 3.4.12.1) was produced from Kluyveromyces fragilis ATCC 28244. The maximum growth and enzyme production were obtained during 24 hr of growth at the late logarithmic phase with optimized conditions (25$ sp circ $C, 300 rpm, pH 5) using YPD (1% yeast extract, 2% peptone, 2% dextrose, w/v) broth medium. A Fast Protein Liquid Chromatography (FPLC) was used for the enzyme purification. The enzyme was purified to 216 fold over the crude extract with a recovery of 18%. The apparent molecular weight of the purified enzyme was estimated to be 120 kDa on Native-PAGE and 56 kDa on SDS-PAGE suggesting that carboxypeptidase Y from Kluyveromyces fragilis consists of two subunits. The pH and temperature optima of the enzyme were pH 6.0 and 35$ sp circ$C, respectively. The enzyme activity was strongly inhibited by diisopropylphosphofluoridate (DIPF) and phenylmethylsulfonylfluoride (PMSF), and caused an average 50% loss of activity when incubated with various metal cations. / The apparent K$ sb{ rm m}$ and V$ sb{ rm max}$ values obtained for n-benzoyl-L-tyrosine-p-nitroanilide (BTPNA) and carboxybenzoxyphenylalanylalanine (Cbz-Phe-Ala) were 5.1 mM and 13.4 $ mu$mole/min/mg and 2.98 mM and 22.58 $ mu$mole/min/mg, respectively. Carboxypeptidase Y hydrolysis on the tryptic digests of $ alpha sb{ rm s1}$- and $ beta$-casein showed that the enzyme randomly removed five and three hydrophobic peptides, respectively and greatly reduced the size and heights of the other peptides analysed on Reversed Phase-High Performance Liquid Chromatography (RP-HPLC).

A total of 853 milk samples with different phenotypes of $ kappa$-casein ($ kappa$-CN) and $ beta$-lactoglobulin ($ beta$-LG) and different preheating temperatures of 30, 70, 75 and 80$ sp circ$C were used for the making of individual laboratory scale Cheddar type cheese and for the determination of coagulating properties. Data obtained from milk input, cheese output and chemical analyses were used to calculate actual, 37% moisture adjusted and Van Slyke's theoretical yields and yield efficiency. Least squares analyses of data indicated that higher 37% moisture adjusted yields and yield efficiencies were obtained with milk types VII, VIII and IX, which have the B gene for $ kappa$-CN when compared to milk types I, II and III, which have the A gene for $ kappa$-CN irrespective of preheating temperatures. Moisture adjusted yield, 10.49%, was the highest when milk type VII containing $ kappa$-CN BB and $ beta$-LG AA phenotypes was preheated at 30$ sp circ$C, whereas milk type IX, which has phenotype BB for $ kappa$-CN and BB for $ beta$-LG, had the highest adjusted yields with values of 11.36, 11.91 and 11.99% when preheated at 70, 75 and 80$ sp circ$C, respectively. When cheese was made from milk preheated at 30$ sp circ$C, total solids (64.19%), fat (35.31%) and protein (25.82%) were highest in cheese obtained from milk types IX, VII and IX, respectively, all of which have the B gene for $ kappa$-CN. These three components (61.54%, 30.85% and 24.47%) were lowest in cheese made from milk types III, III and I, respectively, all of which have the A gene for $ kappa$-CN. Cheese with moisture content close to 39% were produced by milk types I and II preheated at 30$ sp circ$C. by milk types III, IV, V, VI, VII and VIII preheated at below 70$ sp circ$C and by milk type IX preheated at below 75$ sp circ$C. (Abstract shorted by UMI.)