Effects of Aging Time and Temperature on Proteolysisof Commercial Goat Milk Cheeses Producedin the United States

YONG K. JINl and YOUNG W. PARK2Cooperative Agricultural Research Center

Prairie View A&M UniversityTexas A&M University System

Prairie View n446-4079

ABSTRACT

We evaluated the effects of agingtime and temperature on proteolytic in­dices of freshly prepared commercialcheeses from goat milk. Five varieties ofcheeses from goat milk (plain soft, pep­per soft, Caciotta, Monterey Jack, andraw milk Cheddar) and two from cowmilk (one fresh and one 2-mo-old Ched­dar) were collected from four differentmanufacturers located in Texas and Wis­consin. Two replicates of the seven vari­eties of the cheeses were assigned tothree temperatures (4, 13, and 22°C) andsix storage periods (0, 2, 4, 8, 16, and 24wk) in a 7 x 3 x 6 factorial arrangement.For all samples, indices of proteolysiswere determined: the ratio of water­soluble N to total N and the ratio of9.6% TCA-soluble N to total N. Hydrol­ysis and degradation of casein moietiesof the cheeses during the 6-mo ripeningwere determined by PAGE; pH andADV were also determined. Percentagesof water-soluble N in all cheese varietieswere doubled after 2 wk of aging at 4°C.Soluble N (water-soluble N and TCA­soluble N) contents steadily increased asaging time and temperature increased forall cheeses, with some variations. pH,water-soluble N, and TCA-soluble N ofsoft cheeses were lower than those ofhard varieties throughout the experimen­tal period. The acid degree values weresynergistically elevated by aging timeand temperature, and those of the pepper

Received October II. 1994.Accepted August 7. 1995.1Present address: Korea Cold Storage Co.. Ud.

19-16 Noryangjin-Dong. Dongjak-Ku. Seoul. Korea.2To whom correspondence should be addressed.

soft cheese displayed extremely highvalues at 6 mo of aging. Effects of allthree main factors (variety, time, andtemperature) and their two-way andthree-~ay interactions were significantfor all proteolytic parameters tested.Correlations were positive among levelsof water-soluble N, TCA-soluble N, andpH for all three temperature treatments.Electrophoretic degradation patterns ofthe protein samples exhibited a generalpositive relationship to levels of water­soluble N and TCA-soluble N in thecorresponding cheeses at various agingtreatments.(Key words: proteolysis, goat cheese,aging time, temperature)

Abbreviation key: ADV = acid degree value,TCA·SN = TCA-soluble N, WSN = water­soluble N.

INTRODUCTION

Proteolysis is a key process governing therate of flavor and texture development of mostcheese varieties. Proteolysis and lipolysis aretwo major processes in the multifacetedphenomenon of cheese aging, which involvesthe occurrence of a variety of chemical, physi­cal, and microbiological changes under con­trolled environmental conditions (1, 2, 5, 11,13, 27).

Several approaches have been adopted forquantitative measurement of proteolysis ofcheese during ripening: 1) solubility of pep­tides and amino acids in various solvents orprecipitants, 2) liberation of reactive functionalgroups, 3) chromatography, and 4) electropho­resis (6).

Nitrogen solubility under various conditionshas been measured by many workers: the solu­bility was assayed in 5% NaCI (6); sodiumacetate buffers, pH 4.6 (3, 18); citrate buffers,pH 4.4 (4, 31); 2, 2.5,4, 10, or 12% TCA (4,8,

1995 J Dairy Sci 78:2598--2608 2598

PROTEOLYSIS OF GOAT MILK CHEESES 2599

18, 20, 21, 22); 50% ethanol (24); 70% ethanol(8); 80% ethanol plus 75% acetone (21); 5%phosphotungstic acid (22); and .85% picric acid(23). Percentages of soluble N in cheeses werecompared in a selection of these solvents, butamounts of soluble peptides in each systemwere not compared (23). The water-soluble N(WSN) in water or salt solutions at pH 4.6contains proteins, peptides, or amino acids, andTCA-soluble N (fCA-SN) or alcohol-solubleextracts contains peptides or amino acids (11).These indices of soluble N have a generalapplication in the routine assessment of cheeseage, but organoleptic methods remain essentialfor the assessment of flavor quality (11).

Changes in pH are also closely related tocheese aging. The utilization of lactic acid bythe Penicillium sp. and release of ammonia byits proteolytic activities cause the increase insurface pH and changes in pH gradient in softcheeses (17). The proteolysis of Cheddarcheese is dependent on pH; the higher pH ofwashed-curd cheese permits a softer proteinmatrix as proteolysis progresses, and the lowerpH cheese tends to become more crumbly asinterstitial water is bound by ionic groups (2).The increase in pH is vital for the softening ofCamembert cheese and encourages the activityof enzymes with neutral or alkaline pH optima.

Cheese maturation during storage is af­fected by the interplay of several factors. suchas proteolytic and lipolytic enzymes, storagetemperature and period, salting, pH of thecurd, and humidity (11, 13, 24). The enzymesystem requires a pH and temperature optimafor normal or accelerated cheese ripening.Although many studies have been conductedon proteolysis during ripening of cheeses fromcow milk, few data are available for quantita­tive measurements of proteolytic changes incommercial goat milk cheeses.

The objectives of this study were 1) todetermine amounts ofWSN, TCA-SN, pH, andacid degree value (ADV) in commerciallymanufactured fresh goat milk cheeses for 6 moof aging; 2) to compare effects of variety,storage temperature, and time on concentra­tions of WSN, TCA-SN, pH, and ADV in thegoat cheeses relative to fresh and ripened cowCheddar cheeses; and 3) to evaluate the proteo­lytic patterns of cheese proteins by electropho­resis with reference to changes in soluble N inthe experimental cheeses.

MATERIALS AND METHODS

Preparation of Cheese Samples

Five varieties of freshly manufactured softand hard types of commercial cheeses fromgoat milk and one fresh and one aged Cheddarcheese from cow milk were collected fromfour different producers located in Texas andWisconsin. The experimental varieties of goatcheese included plain soft, pepper soft,Caciotta (hard), Monterey Jack (hard frompasteurized goat milk), and C.he.ddar (hard fr~m

raw goat milk). Different vaneties of goat mIlkcheeses were made by three manufacturers us­ing procedures described by Le Jaouen (14).One fresh Cheddar from cow milk was in­cluded as a reference control, and another2-mo-old Cheddar from cow milk and aged atTC was also added as a treatment group tocompare chemical indices of proteolysis withall fresh goat cheeses during the 6 mo of thetrial.

Experimental Design

Extents of proteolysis of the seven ex­perimental cheese varieties during 6 mo .ofripening were evaluated in a 7 x 3 x 6 factonalexperiment. Five varieties of goat and two cowCheddar cheeses in two replicates were as­signed to three temperatures (4, 13, and 22°C)and six storage periods (0, 2, 4, 8, 16, and 24wk). Two lots of each variety of the ex­perimental cheeses were freshly made by themanufacturers. Samples of 56.7 g (2 oz) werecut from each lot of each cheese variety, in­dividually vacuum-packaged, and transportedin dry ice via an overnight delivery beforebeing placed in the designated aging treat­ments. After each storage period, the individu­ally packed cheese samples were opened,finely ground, and thoroughly mixed beforechemical analysis. Proteolytic indices meas­ured in the study were moisture, total protein,WSN, TCA-SN, pH, and ADV. The ADy,although not a direct parameter of proteolySIS,is included as an indirect index of cheesematuration in this study.

Chemical Analysis

Moisture. Moisture contents of all cheesesamples were determined by drying in a

Journal of Dairy Science Vol. 78, No. 12, 1995

2600 JIN AND PARK

laboratory oven for 24 h at 105°C with2-g samples.

pH. pH of the experimental cheeses weredetermined using a pH meter (Accumet modelnumber 910; Fisher Scientific, Pittsburgh, PA)according to the method of Vangtal and Ham­mond (32). A 10-g cheese sample and 20 ml ofdouble-deionized water were homogenized in aWaring blender (Waring Products, Inc., NewHartford, en for 2 min, and then the pH wasmeasured from the prepared sample.

Total Protein. Total N was assayed bymicro-Kjeldahl procedure (25), using a micro­Kjeldahl analyzer (Kjeltee Auto 1030; Tecator,Hoganas, Sweden). Total protein was calcu­lated by percentage of total N x 6.38.

WSN. The percentage of WSN was deter­mined by a modified procedure of Kuchrooand Fox (9). A 10-g cheese sample and 40 mlof double-deionized water were homogenizedin a Waring blender. The blended cheese slurrywas transferred into a SO-ml centrifuge tubeand incubated in a 40°C water bath for 1 h toensure complete solubility of WSN in water.After incubation, the blended homogenateswere centrifuged at 3000 x g for 30 min. Aftercentrifugation, the supernatant was filtered(Whatman number I filter paper; Whatman,Clifton, NY), and 10 ml of the supernatantwere decanted into a micro-Kjeldahl tube for Nanalysis. The percentage of WSN was calcu­lated as the ratio of WSN to total N.

TCA-SN. The concentration of TCA-SNwas assayed by a modified procedure ofRowland (26). A 10-g cheese sample and 40 mlof double-deionized water were poured in aWaring blender cup and homogenized for 2min. A 1O-ml aliquot of the homogenate wastransferred into a 2.5- x 19.5-cm test tube;then, 40 ml of 12% TCA were added to thehomogenate, which resulted in the final con­centration of 9.6% TCA. The tube was shakenevery 10 min for 30 min to extract TCA-SNcompletely and then was centrifuged at 600 xg for 30 min using a Babcock centrifuge(model number 424; Graver Inc., Union City,IN). The supernatant was treated in the sameway as the WSN sample to determine TCA-SNcontent.

ADV. The ADV of the cheese samples wasanalyzed (25) as follows. Approximately 10 gof sample were shredded, ground, and placedinto a Babcock cheese bottle for fat extraction;1 ml of the final extracted fat was titrated

Journal of Dairy Science Vol. 78, No. \2, \995

against the standard alcoholic .02N KOH solu­tion. Calculation of ADV was performed usingthe following formula: ADV =the millilitersof KOH for sample minus the milliliters ofKOH for blank x N x 100, divided by weightof fat, where N =normality of alcoholic KOHsolution in methanol.

Electrophoresis. Protein samples for elec­trophoresis were prepared according to Shalabiand Fox (29). Approximately .6 g of cheesesample was dissolved in 25 ml of 8 M urea(pH 8.0) and homogenized for 2 min. Thecheese extract was incubated in a water bath at37°C for 2 h for complete solubilization ofcasein fractions. The extracted solution wascentrifuged at 10,000 x g for 30 min at 4°Cand filtered with Whatman number 1 filterpaper to remove fat and other insoluble solidsfrom the solution. The filtered extract wasdialyzed to 60% of original volume using aSpectrum® membrane (6000 to 8000 molecularweight cutoff, SpectraIPor; Fisher Scientific).Dialysis was performed at room temperature(22°C) overnight. The final extract containing atotal of 40 J.Lg of protein was pipetted into a1.S-ml polypropylene microcentrifuge tube(Fisher Scientific) and freeze-dried to preparethe final injection volume for electrophoresis.

Proteolytic patterns of hydrolyzed anddegraded cheese proteins were evaluated by15% SDS-PAGE using Laemmli discontinuousbuffer system (10). Protein standards obtainedfrom Sigma Chemical Co. (St. Louis, MO)included BSA, as\-casein, ~-casein, K-casein,~-lactoglobulin, and a-lactalbumin. Caseinfractions of unknown samples were identifiedby comparing relative mobilities of their pro­teins (ratio of distance migrated by the proteinto the distance migrated by the colorant front)with those of standard proteins.

Statistical Analysis

The experimental data were statisticallyanalyzed for analysis of variance, correlationamong parameters, and Duncan's multiplecomparison among treatments using leastsquares means as described by Steel and Tome(30). Data were also analyzed by the generallinear models procedure of SAS (28).

RESULTS AND DISCUSSION

The initial chemical parameters of the ex­perimental cheeses showed that significant (P< .05 or P < .01) differences existed among

PROTEOLYSIS OF GOAT MILK CHEESES 2601

TABLE I. Initial mean moisture, protein, pH, and ADV of seven varieties of commercial cheeses from goat and cowmilk1

Cheese variety Moisture Protein pH ADV

(%)

X SD X SD X SD X SD

GoatPlain soft 65.3a .31 12.7e .15 4.19d .01 .084c .003Pepper soft 63.8a .81 13.4e .02 4.22d .04 .I46b .041Caciotta 43.5b .24 24.l>ab .02 5.48a .II .110b .019Monterey Jack 41.9b .02 22.6b .56 4.68e .01 .02Qd .001Cheddar 36.3c .II 25.4a .06 5.03b .01 .221" .010

CowCheddar 38.1e .27 23.2b .33 5.06b .01 .092e .013Ripened Cheddar1 36.8e .53 24.3ab .09 4.97b .01 .228a .027

a.b.e.dMeans without common superscripts within the same column differ (P < .05).

1Four observations for each variety.

2Ripened for 2 mo before the experiment.

cheese varieties for all variables tested (TableI). The initial pH of Caciotta cheese was thehighest, followed by Cheddar cheese from goator cow milk; those of soft cheese varietieswere lower (P < .01) . Total protein contents ofthe goat cheese varieties tested in this study

were somewhat lower than those of cor­responding varieties reported by Park (19). Theinitial ADV showed some degree of variationamong varieties, but all values were <.4, whichis regarded as normal, and no lipolysis orspoilage existed in the initial products.

6 -

5.5

:g, 4.5 i

3.5 ~

24222018161412108642

3-'-~~~-~~~-~-~~'--~~-

oStorage Time (wk)

Figure 1. The pH changes of the commercial soft and hard goat cheeses relative to Cheddar cheese from cow milkduring 6 mo (24 wk) of ripening at 4'C for plain soft (~), pepper soft (tI), Caciotta e-). Monterey Jack (®), Cheddar fromgoat milk (0), fresh Cheddar from cow milk (+), and ripened Cheddar from cow milk (*).

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2602 JIN AND PARK

Aging time steadily elevated the pH of allvarieties at 4'C, except Caciotta and peppersoft cheese at 4 wk of ripening (Figure 1).Although pH of soft cheeses increased as ag­ing time increased, the pH of soft cheeses werelower (P < .05) than those of hard varietiesthroughout the experiment (Figure 1). The slowcoagulation and natural draining without press­ing for the manufacturing processes of softgoat cheeses left greater amounts of lactic acidin the cheese curds than the processes for hardcheese (19), which resulted in lower (P < .01)pH of the soft cheeses. The soft cheeses at22'C displayed lower (P < .01) pH than thoseat 4 and 13'C, indicating further accelerationof the acid production and catabolism of pro­tein and other nutrients in the cheeses at 22'C.However, the opposite trend occurred for pHchanges in hard varieties at 22'C as the ripen­ing time progressed.

Protein degradation in this study wasclearly demonstrated by the definitive in­creases in WSN concentrations of all testedcheeses (Table 2), which increased (P < .01) asaging time and temperature increased. As ex­pected, the ripened Cheddar from cow milkcontained two- to threefold higher contents ofWSN at the beginning of the aging experimentthan did the fresh cheeses. All three varietiesof hard goat cheeses had greater WSN contents(P < .01) than the two Cheddar cheeses fromcow milk. However, the two soft goat cheesescontained less WSN (P < .01) than the Cheddarfrom cow milk, which was apparently at­tributable to the acid coagulation with smallamount of rennet applied to the manufacture ofsoft cheeses. The soft goat cheeses also con­tained lower initial protein (P < .01) than thehard ones (Table I).

TABLE 2. Comparison of mean water soluble N (WSN) contents (as a percentage of total N) among seven experimentalcheeses aged at 4, 13, and 22'C for 6 roo (24 wk.).l

Tem- Aging timeCheese pera-variety ture 02 wk. 2 wk. 4 wk 8 wk 16 wk. 24 wk

('C) X SO X SO X SD X SO X SO X SO

GoatPlain 4 3.16 .24 6.74 .83 7.83 .25 8.15 .37 8.97 .12 10.17 .35soft 13 3.16 .24 7.95 .12 7.55 .48 9.83 .33 12.26 .DO 12.46 ,DO

22 3.16 .24 9.49 .36 9.67 .12 12.01 .12 14.75 .50 17.41 ,12

Pepper 4 3.13 .18 6.83 .23 7.71 .11 7.56 .11 7.98 .05 10.96 .23soft 13 3.13 .18 8.25 .23 7.79 .23 8.56 .05 10.14 .45 10.06 .48

22 3.13 .18 10.12 .11 13.06 .04 10.60 .14 12.66 .11 15.24 .12

Caciotta 4 4.61 .37 16.03 .13 16.51 .06 21.90 .18 29.39 .36 33.29 2.7213 4.61 .37 17.16 .37 22.75 .01 35.40 .62 42.61 .59 37.31 .2422 4.61 .37 24.93 .15 25.70 .25 38.47 .29 38.38 .25 43.41 .86

Monterey 4 5.97 .12 15.12 2.19 16.79 .07 19.30 .60 25.89 .67 28.04 .27Jack 13 5.97 .12 17.58 .07 21.13 .13 23.00 .13 27.99 .40 32.86 1.27

22 5.97 .12 19.79 .13 21.68 .13 28.02 .21 37.18 .13 40.06 .11Cheddar 4 5.53 .62 10.43 .06 12.35 .06 19.89 1.20 23.51 .12 26.48 .59

13 5.53 .62 12.62 .18 15.95 .13 22.99 .71 27.61 1.01 31.86 .3622 5.53 .62 17.39 .12 23.89 .06 30.99 .12 37.38 .06 36.73 .24

CowCheddar 4 2.70 .06 6.880 .13 8.17 .39 9.92 1.30 12.90 .12 15.94 .26

13 2.70 .06 8.650 .06 10.70 .07 13.89 .01 17.70 .27 22.75 .2622 2.70 .06 12.46 .06 16.40 .06 21.72 .19 27.96 .06 31.56 .06

Ripened 4 6.74 .25 14.45 .25 13.58 .50 13,98 .56 17.33 .06 19.07 .DOCheddar 13 6.74 .25 14.15 .05 16.11 .06 18.23 .87 21.84 .12 21.55 .26

22 6.74 .25 17.89 .19 19.64 .06 23.99 .18 27.36 .DO 35.78 .38

IAll among least squares means between and within variety, aging time, and temperature differed (P < .01).

2Means of the six initial samples of each variety before aging treatments were taken for WSN values at 0 wk, becauseno differences in WSN contents were expected in the original samples.

Journal of Dairy Science Vol. 78, No. 12. 1995

PROTEOLYSIS OF GOAT MILK CHEESES 2603

Elevations of TCA-SN concentrations in theseven experimental cheeses aged at 4°C for the6-mo period revealed a trend similar to thoseof WSN with a few exceptions at 4 wk ofaging (Figure 2). The reasons are not knownfor the decreases in NPN contents at 4 wk fora few varieties, including plain soft, MontereyJack, and ripened Cheddar cheeses from cowmilk. In contrast, no cheese aged at 13 or 22°Cshowed any decline in TCA-SN at 4 wk, andTCA-SN steadily increased toward the end ofthe experiment.

When the three important varieties in thestudy (plain soft and Cheddar from goat milkand Cheddar from cow milk) were selectivelycompared for WSN and TCA-SN at the begin­ning and end of the aging experiment (Figure3), several interesting results were found: 1)overall concentrations of WSN and TCA-SNin the soft goat milk cheese were lower (P <.01) than those of Cheddar cheeses from goatand cow milks; 2) after 6 mo of aging, WSNand TCA-SN increased substantially in all var­ieties; 3) effect of temperature on elevations ofWSN and TCA-SN was additive and distinc-

tive for all three varieties for the three temper­atures (4, 13, and 22°C); 4) WSN contents weregreater (P < .01) than TCA-SN contents for allthree cheeses at all stages of aging; and 5)Cheddar from goat milk contained higherWSN and TCA-SN (P < .01) than cow Ched­dar, suggesting that more proteolysis occurredin the Cheddar from goat milk.

The ADV of the experimental cheeses alsosteadily increased for all treated samples as theaging advanced (Figure 4). The ripened Ched­dar from cow milk displayed the highest ADVthroughout the experiment, as expected.However, the ADV of the pepper soft goatcheese were extremely high and exceededthose of the ripened Cheddar from cow milk at6 mo of aging. An ADV of ~1.5 is consideredto indicate that cheese is extremely lipolyzedor rancid (25); ADV of all cheese varietiesaged at 22°C beyond 16 wk were extremelyhigh and much higher than the standard ac­ceptable value, especially in the pepper softand ripened Cheddar cheeses from cow milk.

The statistical analysis of the overall pooleddata of changes in WSN, TCA-SN, pH, and

IH ~

16 ~

14 -

12-

~10 -

:z:rI;

8 -«u...6 -

4 -,-

2 -

0

0 4 H 10 12 14 16 111 20 22 24

Storage Time (wk)

Figure 2. Concentrations of 9.6% TCA-soluble N (as percentages of total N) during 6 mo (24 wk) of ripening at 4'Cfor plain soft (~), pepper soft lP). Caciotta <-), Monterey Jack (0), Cheddar from goat milk (0). fresh Cheddar from cowmilk (.). and ripened Cheddar from cow milk (.).

Journal of Dairy Science Vol. 78, No. 12. 1995

2604 JIN AND PARK

ADV during the 6 mo of ripening revealed thatall parameters increased (P < .01) as the agingadvanced, regardless of variety and tempera­ture, with a few exceptions in NPN and pH at4 wk. The data also indicated that higher tem­perature accelerated the ripening process. Theprolonged aging time showed an additive ef­fect on the elevation of the proteolyticparameters; the combination of higher temper­ature and longer ripening time synergisticallyincreased the WSN, TCA-SN, and ADV of allvarieties tested. Bertola et aI. (1) reported thatthe Tybo Argentino cheese (a semihard cheesefrom cow milk) ripened at WOC for 70 dreached 32% WSN and 9% NPN. The goat andcow Cheddar at 13°C in the present study hadsimilar WSN and TCA-SN, and the elevatedconcentrations were considerably higher forthe cheese groups aged at 22°C.

Fox (5) reported that proteolysis contributesto cheese ripening in at least four ways: 1) adirect contribution to flavor with breakdownproducts (peptides and amino acids), some of

~n -

which may cause off-flavors such as bitterness(15), or direct catabolism of amino acids toamine, acids, thiols, and thioesters (7, 12); 2)more release of sapid compounds duringmastication (16); 3) changes in pH throughformation of NH3; 4) changes in texture fromthe breakdown of the protein network, in­creased pH, and higher water binding by thenewly formed amino and carboxyl groups (13).The pH of hard cheeses with higher tempera­ture and longer aging time were greater in thepresent study, which would be attributable tothe higher catabolism of the cheese proteinsand greater formation of NH3, as indicated inprevious reports (7, 12, 15, 16).

Effects of variety, time, and temperatureand their interactions on the measured proteo­lytic parameters showed that all three mainfactors influenced (P < .01) WSN, TCA-SN,pH, and ADV. All two-way and three-wayinteractions, such as variety x time, variety xtemperature, time x temperature, and variety xtime x temperature, affected (P < .01) all pro-

Plain Soft Goat Cheddar Cow Cheddar

Cheese Varieties

Figure 3. Means of water-soluble N (WSN) and 9.6% TCA-soluble N (TCA-SN) concentrations (both as percentagesof total N) of goat plain soft. goat Cheddar, and cow Cheddar cheeses at 6 mo (24 wk) of ripening aged at three differenttemperatures (4, 13, and 22'C). The treated three cheeses shown are WSN, 4"C (checked bar); WSN, 13'C (light bar);WSN, 22"C (gray bar); TCA-SN. 4'C (horizontally striped bar); TCA-SN. 13'C (black bar); and TCA-SN. 22'C (verticallystriped bar).

Journal of Dairy Science Vol. 78. No. 12. 1995

PROTEOLYSIS OF GOAT MILK CHEESES 2605

TABLE 3. Correlation coefficients among levels of meas­ured proteolytic indices in cheeses for 6 mo (24 wk) ofaging. l

lCorrelation coefficients for the first, second, third,and fourth rows represent the initial, 4"C, B"C, and 22"Ctreatment groups, respectively, across all variety andstorage periods (n = 84).

2TCA-SN = TCS-soluble N, ADV = acid degree value,and WSN = water-soluble N.

.p < .05.

**P < .01.

WSN

TCA-SN

pH

TCA-SN2

.831**

.782**

.987**

.984**

pH

.366

.840 ..

.899 ..

.971 **

.291

.6fJ7"

.932 ..

.946 ..

ADV

.354-.444-.556"-.492

.384-.068-.522-.398

.218-.690"·-.322-.423

teolytic indices tested for the 6-mo period. Forall parameters, comparisons by the Duncan'smultiple range test among varieties, times,temperatures, and their interactions differed (P< .01).

Levels of WSN and TCA-SN correlated (P< .01) across all varieties and temperaturetreatments (Table 3). This result was due to theparallel increase of the two indices as theaging time advanced (Table 2; Figures 2 and3), and the same phenomenon was reported ina previous study (1). The correlations betweenpH and WSN and between pH and TCA-SNwere similar for the three temperature treat­ment groups, suggesting that the WSN, TCA­SN, and pH had positive (P < .05 or .01)relationships as the cheese matured. The ADVvalues were generally negative and had poorcorrelations with the other three parameters.

Degradation patterns of cheese proteins andcasein moieties by electrophoresis analysisrevealed that similarities existed in proteolyticpatterns of the proteins in the four selected

7.: r6.5 -

6 1

5.S IS I

4.S

.. 4Q-( 3.S

3

2.S

2 -

1.S -

18 20 22 24

Storage Time (wk)

Figure 4. Mean acid degree values (ADV) during 6 mo (24 wk) of ripening at 4"C for five gOal cheese varieties andcow Cheddar: plain soft (t:.), pepper soft (0), Caciotta <-), Monterey Jack (0), Cheddar from goat milk (0), fresh Cheddarfrom cow milk (.), and ripened Cheddar from cow milk (").

Journal of Dairy Science Vol. 78, No. 12, 1995

2606 lIN AND PARK

vanetIes, such as plain soft and pepper softfrom goat milk and Cheddars from goat or cowmilk (Figures 5 and 6). The PAGE characteris­tics of casein moieties of plain soft and peppersoft cheese from goat milk at 4·C (Figure 5)showed that the initial intensity of the ,s-caseinband was much greater than that of as I-casein,and both casein bands gradually becameweaker as the aging progressed. The intensityof the 'Yl-casein band greatly increased at 8 wkof ripening, and then decreased thereafter.

The disappearance of large molecularcaseins in the electrophoretic patterns clearlyindicated increases in protein degradationproducts, such as peptides and amino acids,which in tum caused the elevation of WSNand TCA-SN contents of the correspondingcheeses.

Figure 6 illustrates the differences inhydrolysis and degradation of casein moietiesin Cheddar cheeses from goat and cow milks

aged at 4·C. The initial cheeses (0 wk) showedthat as I-casein was the major protein band inCheddar cheese from cow milk and that 13­casein was the major protein band in Cheddarfrom goat milk, as previously reported (1, 29).The asl-casein gradually disappeared up to 16wk for the Cheddar from cow milk at 4·Ctreatment. The intensities of 'Yl-casein andsmall peptide bands increased up to 16 wk andthen became faint at 24 wk. Proteolytic pat­terns of a sl- and ,s-caseins of the other cheesesappeared to be similar to the two Cheddarcheeses, for which smaller molecular weightpeptide bands were greater for the samplestreated at 13 and 22°C than for those treated at4°C. The disappearance of the asl- and 13­caseins during aging in this study also agreedwith results of a previous report (1) in which90% of the asl-casein and 70% of the ,s-caseinwere hydrolyzed during 70 d of ripening.

1

BSA- '"

,s-LG- •

ll'-LA- _

2 3 4 5 6 7 8 9 10

.,

Figure 5. The SDS-PAGE patterns of casein and whey proteins in plain soft and pepper soft cheese from goat milk at4'C during 6 mo (24 wk) of ripening. Lanes I and 10 represent protein standards: BSA. CtsI-casein, l3-casein. K-casein,'Yl-casein. 13-lactoglobulin, and Ct-lactalbumin. Lanes 2. 3. 4. and 5 represent degradation patterns of protein samples fromplain soft goat cheeses aged for O. 8. 16. and 24 wk. respectively. Lanes 6, 7, 8, and 9 represent degradation patterns ofprotein samples from pepper soft goat cheese aged for O. 8. 16, and 24 wk. respectively.

Journal of Dairy Science Vol. 78. No. 12, 1995

PROTEOLYSIS OF GOAT MILK CHEESES 2607

2

BSA-

1.-

3 4 5 6 7 8 9 10

is-LG- •_1iIIIIIII1II

cx-LA-

Figure 6. The SDS-PAGE patterns of casein and whey proteins in Cheddar from goat and cow milks at 4'C during 6mo (24 wk) of ripening. Lanes I and 10 represent proteins standards: BSA, asI -casein, ~-casein, K-casein, 'Yt-casein, ~­

lactoglobulin, and a-lactalbumin. Lanes 2, 3, 4, and 5 represent degradation patterns of protein samples in Cheddar fromgoat milk aged for 0, 8, 16, and 24 wk, respectively. Lanes 6, 7, 8, and 9 represent degradation patterns of proteinsamples in Cheddar from cow milk aged for 0, 8, 16, and 24 wlc, respectively.

CONCLUSIONS

Results of the present study confmned thata strong positive correlation existed betweenelevation of WSN and TCA-SN and stages ofproteolysis. The hard goat cheeses maintainedgreater (P < .01) WSN and TCA-SN contentsthan did the Cheddar from cow milk as agingadvanced. Aging temperature and time had asynergistic effect on elevation of WSN, TCA­SN, and ADV in the experimental cheeses.Soft goat cheeses maintained lower (P < .01)pH, WSN, and TCA-SN than all hard cheesesthroughout the 6 mo of aging. Electrophoreticpatterns of degraded proteins of all cheeses hada strong positive correlation with the elevationof soluble N and disappearance of largemolecular caseins.

ACKNOWLEDGMENTS

The authors thank Ronald Richter, KentSalmon, Bud Larsen, and Paula Lambert for

their donations of cheese samples; Gary New­ton for the use of his laboratory for electropho­resis analysis; and Adela Walker for typing themanuscript.

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