Food Structure
Volume 4 Number 1 Article 11
1985
Development of Microstructure in a Cream Cheese Based on Queso Blanco Cheese
Miloslav Kalab
H. Wayne Modler
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Recommended Citation Kalab, Miloslav and Modler, H. Wayne (1985) "Development of Microstructure in a Cream Cheese Based on Queso Blanco Cheese," Food Structure: Vol. 4 : No. 1 , Article 11. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol4/iss1/11
This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Food Structure by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. FOOD MICROSTRUCfURE, Vol. 4 (1985), pp. 89-98 SEM Inc., AMF O'Hare (Chi cago), IL 60666-0507 U.S .A.
DEVELOPMENT OF MICROSTRUCTURE IN A CREAM CHEESE BASED ON QUESO BLANCO CHEESE
Miloslav Kalab and H. Wayne Modler
Food Research Institute, Research Branch, Agriculture Canada Ottawa, Ontario, Canada KlA OC6
Abstract Introduction
A Cream cheese was made by m~x~ng cultured high The development of microstructure in Cream cheese fat cream (5~% fat) with Queso Blanco curd and by has been the subject of several studies (4 , 12, l3, homogenizing the mix at 70°C. The final product con 1':1). In general, there are four types of Cream cheese tained 30. 01. to 33.)% fat and 54.1 to 54. 5% moisture. manufactured. In the traditional system of manufacture, The Queso Blanco curd was obtained by precipitating the a c r eam mix is ripened with a lactic bacteria culture, casein and denatured whey protein from heated whole the curd thus formed is heated, and the whey is drained milk (b2.~ to SI8.0°C) by bringing the pH to 5. 3-5. 5 (D). In the so-called newly formulate d method of manu with citric acid, using a continuous process. The ex facturing Cream cheese-type products (U), the concen tent of denaturation of the whey protein portion varied tration of the total solids in the cream mix is in from 5 to 100%, depending on the heat treatment of the creased to correspond to the final desired composition milk. Scanning and transmission electron microscopy of the cheese. This makes draining of the curd un revealed the development of microstructure at indivi necessary. The third method employs ultrafiltration of dual manufacturing stages. milk to increase the fat/solids content in the reten A characteristic core-and-lining ultrastructure of tate to the approximate composition of the Cream cheese the casein matrix was observed in the Queso Blanco (6). Recently, a manufacturing method was developed curd, the nature of which depended on the temperature (l6, 17), which consists of mixing hig h-fat cultured of coagulation. Nixing of the curd with the cultured cream lt>U-7U% milkfat) with coagulated whey protein cream in a Polytron blender dispersed the curd into such as Ricotta cheese, or with milk curd such as Queso particles approximately 50 j.J m in diameter. Su bsequent Blanco cheese (Latin American White cheese), which con homogenization at l7.2J MPa \i.e., 2,50U psi or 170 tains both casein and denatured whey proteins (l2). atm) further reduced the dimensions of both the protein The Canadian standards for Cream cheese (SI) speci (<7 j.J m in diameter) and fat (<) J.J m in diameter) parti fy a minimum of 30% milkfat while the moisture must not cles and produced a unifonn microstructure. exceed 55%. Cream cheese spread produc ts must contain a minimum of 51 % of Cream cheese , no more than t>U % mois ture, and a minimum of 24 % milkfat (LO). The development of microstructure in a Cream cheese spread based on acid-coagulated whey protein curd (Ricotta cheese) was described earlier (12), where it was noted that Ricotta and Queso Blanco cheeses Initial paper received March 7, 1985. significantly differed in microstructure. The objective Final manuscript received May 26, 1985. of this study was to describe the development of micro Direct inquiries to M. Kalab. structure in the Cream cheese made from the latter Telephone number: (613) 995-3700 Ext. 272. starting mate rial.
Materials and Methods
Preparation of milk curd. Pooled whole milk obtained from the Central Exper imental Farm herd in Ottawa (3. 5% fat , 3.44% protein) KEY WORDS: Casein; Core-and-lining structure; Cream was pasteurized at bl .~ °C for 3U min. One part of this c heese, Fat globules ; Latin American White c heese ; milk was heated to temperatures between 62. 8 and Queso Blanco. ~2 . 2°C, and coagulat ed with a l .51. citric acid solution (w/v) t o produce final pH between 5.3 and 5.5 (measured at 25 °C). Ano the r part of the milk was heated to 96- 980C, held for 10 min, and coagu l ated with the curd being pr oduced by the same procedure. The temperature regimens (temperatures, holding times, and sampling) are shown in Fig. 1.
89 M. KALAB and H.W . MODLER
oc I I I I I I I I the Kjeldahl method (3) and was converted to protein us ing a factor of 6.38. pH was measured using a Model 100- D - 26 Radiometer pH-meter equipped with a combination Jena Thalamid electrode. r--t Scanning e l ectron microscopy (SEM ). 90 - - Drained curd was cut into 1x1x10 IJIJl samples and the samples were fixed in a 3. 5% glutaraldehyde solu tion at 6°C for 24 h, dehydrated in a graded ethanol c series, defatted in ch loroform, placed in absolute 80 t- a l coho l, frozen in liquid Freon 12 at -1 50°C, and B,... fractured under liquid nitrogen. The fragments were criti cal point-dried from carbon dioxide, mounted on t t t t SEM stubs, sputter-coated with go ld (:=e2Q nm), and ex 70 - t - amined in a Cambridge Stereoscan Mark II scanning elec tron microscope operated at 20 kV. A Cultured high- fat cream was aspirated into agar gel tubes, 1.2 mm in inner diameter, and the tubes were 60 r-- - sealed with agar gel at both ends (2). The column of I I t I I I I I cream was 12-15 mm long. The samples were f i xed in a 0 10 20 30 40 50 60 70 80 90 3. 5% glutaraldehyde solution at 6°C for 2 h and divided into two parts. Samples in one part were dehydrat ed and min processed for SEM in the same way as the curd, and Fig. 1. Coagulation of milk with citric acid to final samp l es in the other part were postfixed with an imid pH of 5.3-5.5 at varying temperatures and times. azole-buffered (pH 7.4) 2% osmium tetroxide solution at Arrows indicate the temperature (° C) of the milk and 6°C for 24 h to retain the fat (1). The buffer was the time (min) at which citric acid was added. All the prepared by mixing a 0.2 M imidazole solution (adjusted curds thus obtained were examined by electron micrcr to pH 7.4 with 1 N HCl) with a 0.05 M veronal-acetate scopy. Letters A, B, C, and D mark the samples de buffer (pH 7.4) at a ratio of 1:1 (v/v). Postfixation scribed in this study. with imidazole-buffered Os04 was aimed at r etaining the fat globules in the sample during subsequent dehydra The coagulum was drained t hrough a cheese cloth. tion in a graded ethano l seri es, f reeze-fracturing, and The drained curd of the low temperature heat treatment critical point-drying from carbon dioxide. The dried (62.1> to ll2 .2°C) contained , on an average, 47 .6/o total fragment s were sputter-coated with gold and examined as solids , 2l. ll'1o fat, and 21. 8% t otal protein (20) , and described above. its pH was 5. 3b. Composition of the curd obtained by In addition, the cream was also examined by co ld the high temperature heat treatment is presented i n stage SEM. The samp l es were placed in hollow SEM sample Tab l e 1. holders (11) in such a way as to protrude approximately Table 1. 1 mm from them, and were immediately frozen in Freon 12 at -150 °C. The prot ruding parts of the samples were PROTEIN , FAT , AND TOTAL SOLIDS CONTENTS IN 11-IE NILK , fractured off under liquid nitrogen and the freeze QUESO BLANCO CHEESE BASE, AND THE FINISHED C~1 CrlliESE f ractured samples were coated with go ld by vacuum evap OF 11-IE HIGH TD1PERA1'URE TREATMENT oration using Ka toh's procedure ( 11, 14 ). Go ld-coated samples were examined a t -110 to -150°C in the Cam bridge St ereoscan l1ark II e l ectron microscope equipped Compone nt (%) : Nilk: Queso Bl anco : Cream cheese: with a home-made cold s t age. The microscope was operat ed at 10 kV . Cream cheese samples at various stages of manufac Total protein: 3.44±().12 20 . 3Q±.O . Uil 8. 22±() . 03 ture were prepared for SEM using procedures similar to Fa t: 3. ) ljy. 20 15.1':1±.1. 75 33 . 54jy.24 the ones used for the curd samples. Total solids: 1L.06±0. 17 45.01jjJ. 61 45 . 85±_0 .13 Transmission electron microscopy (TEM). Small particles (<1111ID3) of the ingredients (cream, curd) and the Cream cheese at various stages of manu Manufacture of the Cream cheese. facture were fixed in a 3.5% glutaraldehyde solution High-fat cultured cream (58% fat, 0. 7ts% acidity) and postfixed wi th imidazole-buffered or veronal was blended with the Queso Blanco curd t o yield a acetate-buffered osmium tetroxide solutions. Only curd product with approximately 4()-45% solids and L7-3L% samples were fixed unprotected; cream and Cream cheese milkfat. The ingredients were f irst mixed in a Groen samples which had been placed on the tip of a needle processing kettle (Nadel TDC /TA- 2USP) and heated to 71 - were coated with agar gel by dipping them for a frac 730C. Additional fat dispersion was achieved by 20 s of tion of a second in a 2% agar sol (approx. 35 °C warm), mixing in a Polytron blender (Model 45TE) followed by withdrawing the needle from the sample, sealing the homogenization in a single- stage Hanton-Gaulin homogen opening in the sample left by the needle with agar gel, i zer (Hodel 7694341) at 17. 23 MPa (i.e., 2, 500 psi or and placing the srunple thus encapsulated into fixative. 170 atm). The product was packaged directly from the Dehydration in a graded ethanol series, embedding in a homogenizer in 450-mL plastic tubs while still hot (68- Spurr's low-viscosity medium (21), sectioning, stain 700C ). i ng , and electron microscopy in a Philips EM-300 elec Analyses. tron microscope operated at 60 kV were routine steps Fa t was determined by the pr ocedure of Mo jonnier (5, 7). and Troy (18). Total so lids were determined by the Structure dimensions were measured in enlarged vacuum oven procedure (3). Nitrogen was determined by micrographs using a Zeiss MOP-3 digital image analyzer.
90 CREAM CHEESE BASED ON QUESO BLANCO
Fig. 2. SEM of Queso Blanco c urd p r epared b y coagula t extraction of f at f r om the sampl es, Light arrows point ing heat ed mil k with c itric aci d to a f inal pH 5. 3- 5.5. t o grains in the prote in matrix (p). Dark arrows in:ii Th e mil k was h eat ed t o 62. 8 ° C ( A ), 76. 5°C (B), B2. 7 ° C cat e loosely aggr egat ed pr o tein. ( C), and 96- 98°C (D). Voi d spaces ( v ) r esulted f r om the
Results and Discussion heated at 62.8 °C for 30 min (Fig. 1, sample A), appear ed to consist of fibres when broken with a spatula, and The Queso Blanco cheese base for the Cream cheese tended to be stringy. It became separated into smaller was made by coagulating milk with citric acid at 63- particles in the mouth when subjected to sensory evalu 980C to a f inal pH of 5.3 to 5.5. In this respect, the ation. The curd obtained by acidulating milk heated at procedure and the curds differed considerably from the 82.2°C and held at that temperature for 10 min (sample manufacture of renneted curd. The amount of whey pro C) as well as the curd obtained at 96-98°C (sample D) tein denatured increased with the increasing heat produced the feeling of smoothness and elasticity. All treatment. In curds A and B (obtained at 62.8 and the whey protein had been denatured in both samples. 76.7"C, respectively, Fig. 1), 5 and 33 %, respectively, The microstructure of the Queso Blanco curd was of the whey proteins were denatured. In curds C and D related to the temperature of coagulation and the dura (obtained at 82.2 and 96-98 °C, respectively), a total tion of heating (Figs. 2-4). SEM micrographs (Fig. 2) (100%) denaturation of the whey proteins had been show the granularity of the freeze-fractured samples. achieved, as was determined by the Rowland's procedure The grains, which had diameters of approximately 0.5 11m (20). in curds B and C and less than 5 11 m in curd D, were There was a trend toward an increased compactness apparently unrelated to the mouthfeel of the samples. of the curd as the temperature and duration of heating Acid coagulation of heated milk gave the resulting were increased. Smoothness of the curd as a sensory curd a microstructure significantly different from attribute increased with the increased compactness of renneted curd. This is shown particularly well in the the samples. The curd obtained by acidulating milk TEM micrographs. Renneted curd consists of casein
97 M . KAL A B a n d /I . W . MODLER
Fig. 3. TEM of Queso Blanco curd prepared by coagulat r esid ues. In mic r ogr aphs B t o D, dark arrows point t o ing heated milk with citric acid to a final pH 5.3- 5.5. the core-and- lining s truc tura The milk was heated to 62. 8°C (A), 76.5°C (B), 82. 7° C a = 92 CREAM CHEESE BASED ON QUESO BLANCO Fi g. 4. Detail of the core-and- 1 ining s tructure in the point to the lining on the sur face of coagulated pro Queso Blanco curd. tein. c =minut e particles are compacted to varying In micrographs A to C, 1 ight arrows point to minute extent; p = loosely aggregated pro t e in; r = protein in protein pa rticles. In micrographs B to D, dark arrows lobular form; s = protein grains. 1 )Jm long) , compact particles bearing sig ns of the thickness of the sections (approximately 90 nm) examin core-and-lining structure. Prot ein in the lobular form ed; the linings and the annular void spaces between t he was covered with an envel ope (lining). In the curd made linings and the cores are well developed. a t 82. 7°C , the occurrence of loosely aggregated protein The presence of fat in the whole-milk curd is particles and of lobular particles was lower (Fig. 3C) noticeable in samples which had been defatted for SEM. a nd the matrix was fonned mostly by particles in which Spherical void spaces in the protein matrix indicate the core-and-lining structure was well developed. In the distribution and dimensions of the fat g lobules the curd made at 96-98 °C , the protein was in only two initially present in the curd (Fig. 2). forms. Compact particles, 0. 5- 5 )J m in diameter, pre The microstructure of cultured hig h-fat cream was dominated. They revealed a well developed core- and examined by: (a) cold- stage SEM (Fig. 5); (b) conven lining s tructure with regions of different density. tional SEM following postfixation with imidazole-buf Loosely aggregated protein was also present. These fered osmium tetroxide to retain fat (Fig. 6); (c) TEH structures are shown at greater detail in Fig. 4: In after conventional postfixation (Fig. 7); and (d) TEI1 curd A, the larger poorly distinguishable fused parti after postfixation with Os04 in the presence of imidaz cles appear to be composed of globules, approximately ole (Fig. 8). In spite of the limitations in the con O. l )J m in diame t e r, i.e. similar to casein micelle di trol of the extent of freeze-etching using the Katoh' s mensions. In the curds B and C, however, considerably (14) procedure for metal coating of f r eeze- fractured smaller particles are seen to form the lobular as well samples for SEM, the micrographs provide s ufficient as the compact protein regions. In curd 0, the cores of information concerning the dimensions and the distribu the protein particles appear to be uniform at the tion of fat g lobules in the cream (Fig. 5). 93 M. KALAB and H.W. MODLER Figs. 5, 6, 7, and B. Microstructure of cultured higlr with imidazol~bufferai osa4• fat cream. Light arrows in Figs. 5 and 6 point t o fa t globule 5 = Cold-stage SEM; 6 = Conventional SEM of a cream membranes, dark arrows point t o fractured fat globules. a aqueous phase; b bacteria; f fat; p protein. sample postfixed with imidazol~buffere:i osa4; 7 = TEM = = = = of con ventionally fixed cream; 8 = TEM of c ream fixed ~1ost fat globules are 3 to 6 ~ m in diameter, as Experiments on Cottage cheese revealed that the frac assessed from the fractures. In samples destined for ture plane ran preferably between the corpuscl es (ca conventional SEM, the fat had been fixed with osmium sein micelles and lactic acid bac teria) in samples tetroxide in the presence of imidazole (1) and the impregnated with ethanol. In samples with the aqueous aqueous phase had been remo ved by dehydration and crit phase present, the fracture plane ran across such cor ical-point drying. This step exposed the fat globules puscles. The micrographs in Figs. 5 and 6 apparently and made it possible to examine the sample in some provide support for this phenomenon occurring also in depth (Fig. 6). The fat globules appear to be smaller high-fat cream. when viewed by this latter technique than when examined TEM micrographs of cream fixed conventionally by cold-stage SEM. There could be two reasons for this (Fig. 7) and cream postfixed with imidazole-buffered apparent difference. The samples destined for conven osmium tetroxide (Fig. 8) show the fat globules and, in tional SEM were first dehydrated in ethanol and freeze particular, the fat globule membranes, with different fractured while impregnated with this organic solvent. degrees of contrast. The latter technique makes it It is probable that the dehydration caused the cream easier to distinguish between fat and the aqueous matrix to shrink to some extent. However, no measure phase. Small fat globules ( <1 ~ m), such as those seen ments were carried out to support this assumption. to surround the central fat globule in Fig. 8, were Another reason can be differences in fracturing of the rare in the high-fat cream used. samples with the aqueous phase present or replaced with Mixing of the Queso Blanco curd with cultured ethanol, as was reported with other produc ts (11). cream in the Polytron blender resulted in an 94 CREAM CHEESE BASED ON QUESO B LA NCO Fig& 9 - 13. Microstructure of a mixture prepared in a Pol ytron blender from high-fat c ream and a h i gh-temper ature Queso Blanco cur d. 9 = Sample, from which fat had been extracted before SEM. Large void spaces (dark arrows) indicate fat glob ule cluster s from the added c r eam. Compact protein granules ( c , 1 ight arrows) are part of the initial Queso Blanco curd; small void spaces in the granules developed due to the extraction of the fat p r esent in the whole- milk Queso Blanco curd. 10 = Void spaces {v) in t he granular protein matrix {p) developed as the result of fat extracti on during sample preparation; fat globule membrane r esidues have been retained in the void spaces. 11 = SEM of a sample with the fat retained by fixation with imidazole- buffered oso • Dark arrows point t o 4 ind i vidual fat globules. f = fat; p = protein. 12 = Detail of fat globules {f) with fat r etained in the protein matrix whi ch has compact {c ) and loose {p) areas. 13 = TEM shows fat globules and large fat {f) p3rticles in a protein matrix consisting of gr ains having a core clusters of 1 oosel y aggregated protein particles ( p). and- lining s tructure (dar k arr ows) and of chains and b = bacteria. 95 M . KALAB and H.W. MODLER Figs. 14 - 18. Microstructure of the Cream cheese pre pared by ho£OCJgenization of a mixture of high-tempera ture Queso Blanco c urd and high-fat cream. 14 = Conventional SEM sh::Ming the reduction in size of the compact protein particles (c) as well as of the fat globules as evident from the void spaces (v) in the finished product. 15 = Detail of the protein matrix consisting partly of compact protein particles (c) and partly of finely dis persed protein (p); void spaces (v) indicate the pres ence of small fat globules. 16 = SEM of a sample with the fat retained. The image appears flat and distinction between the protein (dark arrow) and fat components ( light arrow) is difficult. 17 = TEM showing the reduction in size and uniform distribution of fat globules (f) and protein grains ( p). Protein is a 1 so in the form of short chains ce menting the fat globules. 18 = Detail of protein particles cementing minute fat globules (f). Large dark arrows point to the lining arri light arrows point to minute particles in the protein cores (p). Short protein chains (small dark arrows) are also evident. 96 CREAM CHEESE BASED ON QUESO BLANCO intermediate product, which was slightly gritty when Blanco curd, mostly composed of casein, may be the main tasted, irrespective of the temperature at which the contributing factor. milk proteins had been coagulated. This product con The emphasis of the manufacture of the Cream sisted of relatively large protein and fat particles cheese was on the simplicity of the procedure and its (Figs. 9-13). The dimensions of the protein particles rapidity. For this reason, the relatively long heating varied and their diameters were as large as 50 1J m. of the milk was replaced with heating to a higher Conventional SEM of defatted samples was particularly temperature. useful in easily distinguishing between the protein and The procedure based on blending Queso Blanco and fat particles (Fig. 9). Small spherical void spaces in Ricotta cheeses with cultured high-fat cream provides the large protein particles indicated that fat globules flexibili t y in adjusting the composition of the final were present in the initial Queso Blanco curd made with product concerning t~e total solids as well as fat con whole milk as the starting material. The void spaces tents. Cultured buttermilk can be used to s t andardize observed in the micrographs were initially occupied by the fat content and provide additional flavour (17). fat which was later removed during sample preparation The manufacturer also has the option of using different (Figs. 2 and 9). The proof that the void spaces had bacterial cultures in the high-fat sour cream and also indeed been occupied by fat is provided by SEM of buttermilk to thereby tailor the flavour profile of the samples in which residues of fat globule membranes have finished product to meet various product specifica been retained (Fig. 10). Figs. ll and 12 show fat tions. retained in the protein matrix and confirm, by another preparatory technique, that the protein was indeed in a Acknowledgments corpuscular form. Sections of this product examined by TEM (Fig. 13) reveal that the core-and-lining structure Skillful technical assistance provided by Hrs. P. has persisted. Allan-Wojtas and Messrs. Ralph Cooligan, J~G. Larose, Homogenization of the protein-cream mixture at and Alan Payne is acknowledged. The authors thank Hr. approximately 70°C resulted in the disintegration of A.G. Sargant for useful suggestions. Electron Micro the protein particles and in a uniform distribution of scope Centre, Research Branch, Agriculture Canada in the fat globules from the cream (Fig. 14). The dimen Ottawa provided facilities. sions of the compact protein particles were reduced to Contribution 637 from the Food Research Institute. less than 7 1J m in diameter. A considerable amount of the protein was reduced to particles having dimensions References smaller than those of casein micelles in milk (Figs. 15 - 18). Fixation of fat and its retention in the matrix l. Allan-wojtas P, Kalab M. (1984). Milk gel struc produced micrographs showing a product in which the ture. XIV. Fixation of fat g lobules in whole-milk aqueous phase is not apparent (Fig. 16). In this re yoghurt for electron microscopy. Hilchwissen spect, formulated Cream cheese made by the new proce schaft 39(6), 323- 327. dure (14), differs from Cream cheese made according to 2. Allan-Wojtas P, KaLib H. (1984). A simple procedure the traditional method of manufacture (13). The protein for the preparation of stirred yoghurt for scan particles and the fat globules were evenly dispersed in ning electron microscopy. Food Hicrostruct. 3(2), the aqueous phase as is evident in Figs. 17 and 18. The 197-198. - small dimensions of the fat globules and their relative 3. AOAC: Official 11ethods of Analysis. (1975). Asso uniformity in the finished product are apparent from ciation of Official Analytical Chemists, 12th the micrograph. Most fat globules were 0.5-1.5 1J m in ed., Washington, DC, 1094 pp. 3(2), 197-198. diameter and a small number of the largest fat globules 4. Buchheim W, Thomasow J. (1984). Structural changes were less than 5 1.1 m in diameter. in Crerun cheese induced by thermal processing and The development of microstructure in the Cream emulsifying salts. North. Europ. Dairy J. 50, 38- cheese made from the Queso Blanco curd resembles the 44. -- Cream cheese spread made from the Ricotta cheese curd 5. Cheese Varieties and Descriptions. (1953). U.S. (12) but differs from the traditional Cream cheese as Dept. of Agriculture, Agriculture Handbook No. well as from the newly formulated and imitation Cream 54, Washington, DC, 36. cheese spreads. There is one important difference, 6. Covaevich HR, Kosikowski FV. (1977). Cream cheese however, between the Cream cheese made from the Queso by ultrafiltration. J. Food Sci. ~. 1362-1364, Blanco and the Cream cheese spread made from the Ricot 1372. ta cheese. This difference is caused by differences in 7. Harwalkar VR, Kalab M. (1980). Milk gel structure. the composition and microstructure of the respective XI. El ectron microscopy of gl ucono- cS -lactone protein bases used. Queso Blanco curd is more compact induced skim milk gels. J. Texture Stud. _!_!_, 35- than the whey protein curd; the latter curd consists of 49. small whey protein particles, approximately 100 nm in 8. Harwalkar VR, Kalab M. (1981). Effect of acidulants diameter (12). This difference is reflected in the and temperature on microstructure, firmness and spreadability of the product. The Cream cheese made susceptibility to syneresis of skim milk gels. from Queso Blanco does not spread as easily as the Scanning Electron Microsc. 1981; III: 503-513. spread made from the whey protein base (17). A higher 9. Health and Welfare Canada. The Food and Drugs Act total protein content in the Queso Blanco-based Cream and Regulations. B.08.035 Cream cheese. Dept. of cheese (10.74%) than in the Ricotta-based spread National Health and Welfare, Ottawa Canada 1980. , , (8.45%) may have been the main cause of the poorer spreadability in case of the product made from the low 10. Health and Welfare Canada. The Food and Drugs Act temperature curd (17) but not in the case of the high and Regulations. B.08.038 Cream cheese spread. temperature curd, where the protein content was compa Dept. of National Health and Welfare Ottawa rable (8.22 %) to that in the Ricotta-based spread. Canada, 1980. ' ' Rather, the more compact protein matrix of the Queso 11. Kalab M. (1981). Electron microscopy of milk 97 M. KALAB and H.W. MODLER products: A review of techniques. Scanning Elec M. V. Taranto: Does the manufacturing procedure devised tron Microsc. 1981; III: 453-472. by the authors have any advantage over the others? 12. Kalab M, Modler HW. (1985). Milk gel structure. XV. Authors: The procedure devised by the authors has sev Electron microscopy of whey-protein based Cream eral advantages. (a) It is cost-effective because the cheese spread. Milchwissenschaft 40(4), 193. total solids and butterfat can be accurately standard 13. Kal M. V. Taranto: The authors have not discussed the ef Discussion with Reviewers fect of the fat globule size distribution. I would sus pect that this structural difference would have played P. Resmini: Do the authors know the cream pH at the mo a role (at least in part) in controlling t he spread ment of preparation for electron microscopy? ability of the final product. Please cmmnent. Would the Authors: Yes , the pH of the cream was 4.4. authors also speculate on (discuss) the correlation between the observed microstructure and the mouthfeel R. T. Marshall: Postfixation with imidazole-buffered of the final Cr eam cheeses? What is the controlling osmium t etroxide does not seem to go with the second factor: the protein matrix or the fat globu le size and third treatments, i .e. freeze-fractur ing and criti distribution? This point would be important in provid cal- po int drying. Please comment. ing manufacturers sound technical means for accurately Author s : Samp l es destined for freeze- fracturing and controlling the desired final product a ttributes. critical-point drying were dehydrated in e thanol. Crit Authors: There are many factors which may affect the ical-point drying was carried out from liquid carbon spreadability of the product. The relationship between dioxide. Both sol vent s are lipophilic and would par the fat globule dimensions and spreadability was not tially extract fat unless it is fixed. This fixation studied. We agree that this relationship would be in was achieved by imidazo le-buffered osmium tetroxide teresting and should be the subject of a separat e (1) . study. 98