Micro- and macro-structure of cow, buftalo and mixed milk

Lait (1992) 72, 475-489e ElsevierilNRA

475

Original article

Micro- and macro-structure of cow,buftalo and mixed milk rasogolla:a comparative scanning electron

and light microscope study

AK Adhikari, ON Mathur, GR Patil

National Dairy Research Institute, Division of Dairy Chemistry, Karnal132 001, Haryana, India

(Received 15 October 1991; accepted 29 April 1992)

Summary - Scanning electron and Iight-microscopic studies were carried out to provide informationon the micro- and macro-structural differences between cow, buffalo and mixed milk rasogolla (a localIndian milk product obtained via acid coaqulation followed by cooking in sugar syrup). Scanningelectron microscopy of cow milk rasogolla revealed a loese, porous protein matrix with coa!escedcasein micelles forming a thread-Iike structure,.while buffalo milk rasogolla had agglomerated caseinmicelles which were large and compact, and formed a scale- or layer-type structure. Casein micelles inmixed milk rasogolla formed a combination of thread- and scale-type structures. Fat globules collapsedand the membrane shrank noticeably in ail the 3 sampi es: Larger fat globules were ~;'ost affected,while Iittle or no effect was observed in small fat globules. Ught microscopic investigation showed thatcow milk rasogolla had smooth, small oval-shaped pores, whereas buffalo milk rasogolla hadrough-edged. irregularly-shaped large pores. Mixed milk rasogolla had a heterogenous structure. TheInstron and sensory texturai examination agreed weil with the composition and' structuralcharacteristics of rasogolla.

rasogolla 1electron microscopy llight microscopy 1heat treatment 1texture 1microstructure

Résumé - Microstructure et macrostructure de rasogolla de lait de vache, de lait de bufflesseet de lait de mélange. Des études au microscope électronique à balayage et au microscope optiqueont été conduites pour déterminer les différences de micro- et macrostructure existant dans le rasogol-la (produit laitier indien local obtenu par coagulation acide suivie d'une cuisson dans un sirop de sucre)obtenu à partir de lait de vache, de lait de bufflesse ou de mélange. La microscopie électronique à ba-layage du rasogolla de lait de vache révélait une matrice protéique lâche, poreuse, avec des micellesde caséine agglomérées en cneîneties filamenteuses. Au contraire, le rasogolla de lait de bufflesseprésentait une structure compacte formée par de grosses micelles de caséine agglomérées en stratestrès denses. Le ra5Ogolla de lait de mélange avait quant à lui une microstructure intermédiaire compor-tant à la fois des oncnstnemems micellaires stratifiés et filamenteux. Les globules gras avaient uneprésentation plus ou moins dégonflée selon leur taille, la membrane des plus gros étant la plus rétrac-tée sur elle-même. L'examen au microscope optique révélait que le rasogolla au lait de vache avait

476 AK Adhikari et al

des pores petits, aux contours lisses, de forme ovale tandis que le rasogolla au lait de but/esse présen-tait des pores gros, aux contours et formes irréguliers. Le rasogolla de lait de mélange avait une struc-ture hétérogène. Les résultats de l'étude de la texture du rasogolla par l'Instron ou par analyse senso-rielle étaient en accord avec les caractéristiques de composition et de structure.

rasogolla / microscopie électronique / microscopie optique / traitement thermique / texture /microstructure

INTRODUCTION

Ultrastructural studies have been used byfood scientists for studying the topographyand internai structure of a wide variety offood products since the early fifties. In re-cent years scanning electron microscopy(SEM) has gained considerable impor-tance in the study of surface topology andto develop correlations between the struc-ture and the physical appearance of vari-ous food materials. It offers rapid and sim-ple specimen preparation, and providesgreat depth of focus, producing pictureswith a 3-dimensional impression, an excel-lent tool in the study of gel structure (Kalaband Harwalkar, 1973).

Rasogolla, an Indian sweetened milkproduct, is manufactured by cooking cha-na (an acid-coagulated soft cottagecheese analogue) in a concentrated sugarsyrup to a soft, spongy consistency.Among the Indian dairy productsIt has asignificant market value due to its extremepopularity throughout India and abroad.The preparation of a standard quality prod-uct is still carried out by sweetmeat mak-ers. Research had only been carried outon its production technology, some pro-cess modifications and its shelf-Iife (Dateet al, 1958; Jagtiani et al, 1960; Bhatta-charya and Des Raj, 1980; Soni et al,1980) but an in-depth study has not yetbeen performed.

The objective of this study was to man-ufacture good quality cow, buffalo andmixed milk rasogolla comparable to the

best market samples and to study their mi-cro- and macro-structure, texture and com-position in order to obtain an insight intothe possible interrelationships amongthese parameters.

MATERIALS AND METHODS

The manufacture of cow, buffalo and mixed milkrasogollas was carried out as depicted in theflow diagram in figure 1. Rasogolla sampieswere analysed after 12 h of soaking in 40% sug-ar syrup.

Analytical methods

The rasogolla samples were kept on a wiregauge (200-mesh size) for 2 h for completedrainage of the loosely bound sugar syrup andthoroughly homogenized using mortar and pes-Uebefore making the chemical analyses.

Moisture content was determined by the vac-uum-drying procedure (AOAC, 1975), fat by theMojonnier and Tory (1925), protein by the Kjel-dahl (AOAC, 1975) and lactose and sucrose bythe Birch and Mwangelwa method (1974). Ashcontent was determined according to the AOAC(1984) and calcium by the Davies and Whitemethod (1962).

Objective texturai analysis

Texturai properties such as hardness, cohesive-ness, springiness, gumminess and chewinesswere determined by an Instron universal testingmachine (Model 4301), equipped with a 10-Nload cell. Cylindrical samples 1.5 cm in height

Microstructure of rasogolla 477

COWmilk4%fat~9.6%SNF

Raw milk1

Filtration1

Standardization

Mixed milk 5% fat9.0%SNF1

Buffalo milk 5% fat9.0% SNF

1Boiling (3 min)

. 1Coohng at room temperature

1/ Coagulation by 1% citric acid ~

Cow milk at 65 -c / 1 ~ Mixed milk at 55 -c(pH 5.5) (pH 5.6)

Buffalo milk at 55 oC(pH 5.6)

1Chhana

1Holding for 15 min (delayed straining)

1Dipping in distilled water bath (at 50 OC)

1Washing 3 times (atter a 15-min interval)

1Pressing (1 :5, chhana to weight ratio) to 42-48% TS content

1Addition of wheat flour (2%), baking powder (0.05%),

Cardamon flavour (1.0%, solution form)

1Kneading (to the consistency coefficient of 360 ± 12 (Pa.s)

. 1Making chhana balls (1.5 cm diameter, 15-16 9 weight)

1Cooking for 15-16 min in 60 ± 2% boiling sugar syrup (101 ± 0.5 OC)

1Transfer into 40% sugar syrup (90-95 "C, 1 h)

1 •Vacuum canning

1Storage

Fig 1. Flow diagram for the manufacture of cow, buffalo and mixed milk rasogolla.Diagramme de fabrication du resoçone:

and 1.9 cm in diameter were compressed to26.70% of their original height at a cross-headspeed of 250 mrn.rnirr t. The results were meas-

ured from the force deformation curves accord-ing to Brady et al (1985) and expressed on a perunit cross-section al basis (le per mm2).


Sensory texturai analysis

Sensory texturai properties, ie firmness, crum-bliness, elasticity, stickiness, chewiness, juici-ness and overail texturai quality as definedby Szczesniak (1963) were determined on anunstructured Iinear testing scale ranging from1-10 (Lee et el, 1978).

Market samples

Best available market samples were analysedfresh for ail the parameters.

Scanning electron microscopy (SEM)

The sampies were eut into 1 x 1 x 5 mmpieces and promptly washed 3-4 times in dis-tilled water (37-40 OC). Fat and protein fixationand other procedures for SEM were carried outaccording to the method of Kalab et al (1988).Protein was fixed in 2.8% glutaraldehyde solu-tion for 2.5 h followed by thorough washing inphosphate butter (pH 7.3). This was followed bypost-fixation with 2% OS04 solution for 2 h andwashing with 0.135 mol.l-1 phosphate butter(this step was omitted for defatted samples).The samples were then dehydrated in a gradedethanol series (20, 40, 60, 80 and 100%) at 30-min intervals, defatted in chloroform for 15 min(chloroform washing was essential to removethe highly viscous sucrose-fat complexes), andthen placed in absolute ethanol for storage.Samples were freeze-fractured, critical-pointdried and mounted on SEM stubs. This was fol-lowed by gold coating (~20 mm thickness) in anion coater (GIKO -IB-3, Japan) and by examina-tion under a scanning electron microscope (Hit-achi model No -S-405, Japan) at 25 kV.

Light microscopie study

The size and number of pore distribution in therasogolla matrix were determined under a stereo-scopie compound microscope. Very thin sectionsof rasogolla samples (0.1 mm thick, 10 mmwide) were obtained and washed carefully inwarm distilled water (40' "C). This was followedby washing with an ethanol series (20-100%

conc) at 30-min intervals. Finally, the dehydratedmatrix was treated with acetone for 5 min and air-dried. The dehydrated samples were then placedon glass slides and examined under a stereo-scopie microscope at 60 x magnification. Thenumber of pores observed under an ocular mi-crometer (area 7.07 mm2) was assessed andtheir diameters were calculated with the stage mi-crometer scale (size 0.01 mm/division) in 10 dif-ferent places. A photograph of the pore structurewas taken by an Olympus automatic photomicro-graphie microscope (Model PM-10 ADS) at 50 xmagnification.

RESULTS

Figure 2 shows the scanning electron mi-crograph of a cow milk rasogolla proteinmatrix obscured with fat and sugar com-plexes. The structure shows the thickthreaded protein bodies interspersed withnumerous .volds against a dense, scale-type structure of buffalo milk rasogolla withfewer voids (fig 3). The fat globules areseen to be attached at the edge of the pro-

Fig 2. Scanning electron micrograph of cow milkrasogolla (bar 20 urn). Threaded, agglomeratedcasein micelles (C) interspersed with numerousvoids (V) and fat globules (arrow) attached atthe edge of the coalesced micelles.Micrographie MEB de rasogolla de lait de vache(barre 20 um) montrant une agglomération fila-menteuse des micelles de caséine (C), avec denombreux vides (V) et les globules gras(flèches) attachés sur le bord des micelles coa-lescentes.


Fig 3. Scanning electron micrograph of buffalomilk rasogolla (bar 20 urn) showing dense,scale-type agglomerated casein micelles (C) in-terspersed with fat globules (arrow). The obscu-red surface was mainly due to fat and proteincomplexes produced during cooking.Micrographie MEB de rasogolla de lait de buf-flesse (barre 20 pm) montrant une aggloméra-tion dense de type stratifié des micelles de ca-séine (C) entremêlées avec les globules gras(flèche). La surface obscure provenait principa-lement des complexes matière grasse/protéineproduits pendant la cuisson.

tein matrix and are fuzzy in appearance(arrow). In the case of mixed milk rasogolla(fig 4) the structure revealed a ragged pro-tein matrix with threaded as weil as scale-type agglomerated casein micelles fuseddensely together with numerous smallvoids.

As i1lustrated in figure 5, the scanningelectron micrograph of cow milk rasogollashowed a fat globule attached to the coa-lesced-threaded casein micelles. Large fatglobules (diam ~ 10 um) collapsed andlooked Iike empty balloons while the smallfat globules (::;10 urn) remained unaffected(arrow). In the case of buffalo milk rasogol-la (fig 6), the same effect was observed inthe fat globules, but the membrane hadshrunk and wrinkled to a greater extentthan the cow milk rasogolla fat globule.The fat globule was found entrapped about

Fig 4. Scanning electron micrograph of a mixedmilk rasogolla (bar 20 urn) showing threadedand scale-type coalesced casein micelles (C)admixed together.Micrographie MEB de rasogolla de lait de mé-lange (barre 20 pm) montrant une aggloméra-tion à la fois filamenteuse et stratifiée des mi-celles de caséine (C).

Fig 5. Scanning electron micrograph of a cowmilk rasogolla fat globule (bar 5 um). Collapsedfat globule (F) and shrunken membrane (largearrow), attached at the edge of the protein rna-trix (P). Small fat globules (small arrow) wereleast affected.Micrographie MEB de globule gras de rasogollade lait de vache (barre 5pm). Globule gras (F)dégonflé et à membrane rétractée (grandeflèche) attachée au bord de la matrice de pro-téine (P). Petit globule gras (petite flèche) peuaffecté.


Fig 6. Scanning electron micrograph of buffalomilk rasogolla fat globule (bar 5 um). Fat glo-bule (F) was embedded about half of its originalsize at the edge of the protein matrix (P) (obs-cured with fat and sugar syrup) and membranewrinkled severely (large arrow). Fat globule par-tially detached from the matrix (small arrow).Micrographie MEB de globule gras de rasogollade lait de bufflesse (barre 5 pm). Globule gras(F) enfoncé à environ la moitié de sa taille d'ori-gine dans les micelles de caséine agglomérées(P) sur le bord de la matrice (obscurcie à causede la matière grasse et du sirop de sucre) etprésentant une membrane plissée de façon im-portante (grande flèche). Globule gras partielle-ment détaché de la matrice (petite flèche).

50% of its original size at the edge of theagglomerated protein bodies, partially rup-tured and detached from the matrix (ar-row). In the case of mixed milk rasogolla,the fat globules (fig 7) collapsed and rup-tured to a greater extent compared to cowand buffalo milk rasogolla, while no effectwas noticed in small fat globules (arrow)which were entrapped in the coalescedprotein bodies. Study of the defatted case-in matrix of cow milk rasogolla (fig 8) re-vealed loose, agglomerated protein parti-cles with no single casein micelle present.The gritty casein micelles were folded to-gether, forming a loose, porous matrix withirregularly shaped voids which indicate theair space or the space which had initially

Fig 7. Scanning electron micrograph of mixedmilk rasoqolla fat globule (bar 5 urn). Large col-lapsed fat globule (F), partially ruptured (largearrow), was embedded in the protein matrix (P).No effect was noticed in the small fat globule(small arrow).Micrographie MEB de globule gras de rasogollade lait de mélange (barre 5 pm). Gros globulegras (F) dégonflé et partiellement rompu(grande flèche) enfoncé dans la matrice de pro-téine (P) alors qu'aucun effet n'est visible surles petits globules gras (petite flèche).

Fig 8. Scanning electron micrograph of a defat-ted casein matrix of cow milk rasogolla (bar 50urn), Agglomerated, loosely bound, gritty proteinparticles (P) with no single casein subunits.Micrographie MEB de la matrice de caséine dé-graissée de rasogolla de lait de vache (barre 50pm). Agglomération de particules de protéine(P) sableuse, liée de façon lâche, ne laissantpas apparaitre d'entité de sous-unité caséini-que.

Microstructure of rasogolla

Fig 9. Scanning electron micrograph of a defat-ted casein matrix of buffalo milk rasogolla (bar50 urn), The casein micelles (P) coalesced moredensely, forming a more compact body than thecow milk rasogolla matrix.Micrographie MEB de la matrice de caséine dé-graissée de rasogolla de lait de bufflesse (barre50 pm). Micelles de caséine (P) assemblées defaçon plus dense et formant une pâte plus com-pacte que la matrice du rasogolla du lait devache.

been filled with sugar syrup before samplepreparation.

Figure 9 shows the defatted protein ma-trix of buffalo milk rasogolla with morecompact, uneven and large clumps ofcasein micelles with fewer voids. Whereasthe defatted protein matrix of mixed milkrasogolla (fig 10) showed a more raggedsurface with threaded, loose protein parti-cles interlinked to the densely-fused largeagglomerates of casein micelles.

Figure 11 shows the scanning electronmicrograph of a defatted market rasogollasample having similar threaded and foldedprotein bodies with numerous large voids,resembling the laboratory-manufacturedcow milk rasogolla.

At higher magnification the defatted pro-tein matrix of cow, buffalo and mixed milkrasogolla (figs 12-14) confirm the aboveobservations. The cow milk rasogolla ma-trix (fig 12) revealed a loose, threaded

481

Fig 10. Scanning electron micrograph of a defat-ted casein matrix of mixed milk rasogolla (bar 50urn) showing the heterogeneous structure of thematrix having both loosely bound threaded ca-sein micelles (C) and compact large casein mi-celles (B) joined abruptly to form a more raggedand uneven protein matrix.Micrographie MEB de la matrice de caséine dé-graissée de ragosolla de lait de mélange (barre50 pm) montrant la structure hétérogène de lamatrice avec à la fois des micelles de caséine fi-lamenteuses, liées de façon lâche (C) et des mi-celles de caséine compactes et grosses (B), as-semblées irrégulièrement pour former unematrice de protéine plus désordonnée et inégale.

structure with numerous small voids. Thickthread-like micelles are arranged in a regu-lar and folded manner, whereas buffalomilk rasogolla matrix (fig 13) showed largeagglomerated clumps of casein micellesfused densely together, forming a layer- orscale-type structure. The matrix is morecompact and uneven, with fewer voidsthan cow milk rasogolla. In the case ofmixed milk rasogolla (fig 14), the structureshowed a combination of thread- andecale-type structure.

One of the best available market sam-pies studied in this context largely resem-bled the cow milk rasogolla structure (fig15) having small, gritty casein micellesjoined loosely together and with a threadedappearance. The irregular-shaped voids


Fig 11. scanninq électron micrograph of the de-fatted protein matrix of a market rasogollasample (bar 50 ..urn), The coalesced threadedprotein particles "(P) closely resemble the struc-ture of laboratory manufactured cow milk raso-golla.Micrographie MEB de la matrice de protéine dé-graissée d'un échantillon de rasogolla du com-merce (barre 50 pm). Particules de protéinesassemblées sous fôrme filamenteuse (P) res-semblant à la structure du rasogolla de lait devache fabriqué au laboratoire.

Fig 12. Scanning electron micrograph of the de-fatted protein matrix of cow milk rasogolla (bar15 urn), At higher magnification the structure re-vealed thick threaded protein particles (P)joined together by some bridging mate rial inter-spersed with numerous large voids.Micrographie MEB de la matrice de protéine dé-graissée de rasogolla de lait de vache (barre 15pm). À un grandissement supérieur, la structurerévèle des particules de protéine épaisses sousforme filamenteuse (P), reliées par quelquesponts et comportant de nombreux gros vides.

Fig 13. Scanning electron micrograph of buffalomilk rasogolla protein matrix (P) (bar 15 urn), Athigher magnification it exhibited a more com-pact and ragged surface having scale or layer-type structure and fewer voids than that of cowmilk rasogolla.Micrographie MEB de la matrice de protéine (P)de rasogolla de lait de bufflesse (barre 15 pm). Àun grandissement supérieur, elle révèle une sur-face plus compacte et désordonnée que celle durasogolla du lait de vache et présente une struc-ture de type stratifié avec moins de vides.

might have been filled with air or sugar syr-up and the round-shaped remnants mighthave been filled with fat globules beforetheir extraction during the preparation ofthe sample for SEM study.

The light microscopic study of a cross-section of each of the 3 types of rasogollamatrix was carried out to determine thestructural differences in size, shape andnumber of pores per unit area, which aresupposed to largely influence their texturaicharacteristics.

Cow milk rasogolla matrix (fig 16)showed numerous small, smooth-edgedand oval-shaped pores uniformly distribut-ed, whereas buffalo milk rasogolla matrix(fig 17) showed large, irregularly shaped,pores with corrugated edges, unevenly dis-tributed throughout the matrix. On the oth-er hand, the mixed milk product (fig 18)showed both small and large pores with a


Fig 14. Scanning electron micrograph of a defat-ted protein matrix of mixed milk rasogolla athigher magnification (bar 15 urn). The structurerevealed the combination of loose, thread-Iikeprotein partiel es (C) and compact, large scale-type protein agglomerates (6) fused loosely to-gether.Micrographie MEB de la matrice de protéine dé-graissée de rasogolla de lait de mélange à ungrandissement supérieur (barre 15 pm). Lastructure révèle la combinaison de particules deprotéine lâches sous forme filamenteuse (C) etune structure compacte, de type stratifié (B).

smooth and a rough-edged surface inter-spersed. The number of pores counted per7.07 mm2 amounted to 25-30 (av 30),22-28 (av 25) and 23-28 (av 28) for COW, buf-falo and mixed milk rasogolla respectively.The average diameter of these pores was0.90 mm, 1.60 mm and 1.50 mm for cow,buffalo and mixed milk rasogolla (table 1),respectively.

The average chemical composition ofcow milk rasogolla (table Il) showed highermoisture and sucrose contents but lowerfat, protein, lactose, ash and calcium con-tents than that in buffalo milk rasogolla.Mixed milk rasogolla had slightly higherlevels of fat, protein, lactose, ash and cal-cium but lower moisture and sucrose con-tents than that of the cow milk product.Market samples had closer compositionalmakeup to that of cow milk rasogolla man-ufactured in the laboratory.

Fig 15. Scanning electron micrograph of marketrasogolla sample at higher magnification (bar 15urn), The structure shows the gritty, looselybound threaded protein particles (P) with no evi-dence of single casein micelles, resembling thelaboratory manufactured cow milk rasogolla.Micrographie MEB d'un échantillon de rasogolladu marché à un grandissement supérieur (barre15pm). La structure manifeste des particules deprotéine peu liées, d'aspect sableaux (P) sansobservation de micelle de caséine isolée, et res-semblant beaucoup à celle du rasogolla de laitde vache fabriqué au laboratoire.

Fig 16. Light microscopie micrograph (Sax) ofcow milk rasogolla matiix (bar 350 urn), Poresare small (P), oval-shaped and srnooth-edqed(arrow) and are uniformly distributed throughoutthe matrix.Micrographie optique (Sax) de la matrice de ra-sogolla de lait de vache (barre 350 pm). Lespores sont petits (P), de forme ovale et aux re-bords lisses (flèche), et distribués uniformémentdans la matrice.


Fig 17. Ught microscopie micrograph (SOx)ofbuffalo milk rasogolla matrix (bar 3S0 urn),Pores are large {Pl, irregularly shaped, corruga-ted at the edges (arrow), and are unevenly dis-tributed throughout the matrix.Micrographie optique (50x) de la matrice de ra-sogolla de lait de bufflesse (barre 350 pm). Lespores sont gros (P), de forme irrégulière, aux re-bords irréguliers (flèche) et distribués inégale-ment dans la matrice.

Fig 18. Light microscopie micrograph (SOx)ofmixed milk rasogolla matrix (bar 3S0 urn). Thestructure shows both large, rough-edged (S)and small, smooth-edged (C) pores intersper-sed throughout.Micrographie optique (50x) de la matrice de ra-sogolla de lait de mélange (barre 350 pm). Lastructure révèle à la fois des pores gros et auxrebords irréguliers (B) et des pores petits, auxrebords lisses (C), entremêlés.

Table III shows the texturai values ofcow, buffalo and mixed milk rasogolla asobtained by the Instron. lt can be seenfrom the table that buffalo milk rasogollawas almost twice as hard as cow milk raso-golla, but mixed milk rasogolla was slightlyharder than the cow milk product. The val-ues for cohesiveness of these 3 productswere almost similar, but buffalo milk raso-golla had maximum springiness (6.20 mm),and was springier than cow and mixed milkrasogolla (4.8 and 4.2 mm respectively).Gumminess and chewiness of buffalo milkrasogolla had significantly higher values(almost double) than those of cow ormixed milk rasogolla. The market rasogollasample had closer texturai values to that ofcow milk rasogolla. No adhesive force wasrecorded for rasogolla by the Instron.

Sensory texturai scores of laboratoryand market manufactured rasogollas (tableIV) show that cow milk rasogolla had lowerfirmness, elasticity and chewiness buthigher crumbliness, stickiness, juicinessand better overall texturai quality than thatof buffalo milk rasogolla, whereas marketsamples had closer sensory texturaiscores to that of cow milk rasogolla, fol-lowed.by mixed milk rasogolla.

DISCUSSION

The structure formation of acid and heatcoagulated milk gels is influenced by sev-eral factors such as the pH, heat, salt sys-tem, culture and thickening agents (Heertjeet al, 1985). The casein micelles, a macro-molecular assembly of (X-, ~- and x-caseins, are held together by an amor-phous calcium-phosphate-citrate complexand occur as a separate entity in milk. Theultrastructure fully develops in acid-coagulated milk gels, only if the milk hadbeen heated to 90 oC, which results in theformation of x-casein and ~-Iactoglobulincomplexes (Zittle et al, 1962).


Table 1. Distribution of pores and their sizes of cow, buffalo and mixed milk rasogolla matrix as exami-ned by Iight microscope.Distribution et taille de pores de la matrice de rasogolJa de lait de vache, de bufflesse et de lait de mé-lange examinées au microscope optique.

Particulars Cow Buffalo Mixed Market

No of pores 25-35 22-28 23-28 26-36(per 7.07 m2) (av 30) (av 25) (av 28) (av 32)

Diameter of 0.80-1.10 0.90-1.90 0.80-1.80 0.70-1.25pores (mm) (av 0.90) (av 1.60) (av'1.50) (av 1.05)

Av: average.

Upon acidification of milk, micelle-boundcalcium phosphates get solubilized and thecasein-whey protein complexes becomeagglomerated by charge neutralization re-sulting in the formation of a network ofchains and clusters (Heertje et al, 1985).Micellar disintegration takes place at a low-er pH, resulting in conglomerated caseinmicelles with fat and whey proteins en-trapped in them. These· phenomena areadvocated in chhana making, which on fur-ther cooking in concentrated sugar syrup

was transformed into a typical, ragged,spongy and porous consistency, ie to thatof rasogolla.

The compact, granular casein matrix ofchhana altered markedly during cooking in60% sugar syrup to form a f1uffy, porous,thread- or scale-Iike folded structure in ras-ogolla. The strong interlinkages of the coa-lesced protein particles were severelydamaged during cooking and the compact-ness gradually disappeared to produce aragged, uneven, loose protein matrix with

Table Il. Average chemical compositon* of cow, buffalo and mixed milk rasogolla.Composition chimique moyenne du rasogolJa.

Constituents (%) Cow Buffalo Mixed Market

MoistureFatProteinLactoseAshSucroseCalcium

48.50 (1.40)6.82 (0.66)

10.30 (0.95)0.50 (0.20)0.68 (0.08)

33.20 (1.80)0.24 (0.04)

43.25 (1.65)8.60 (1.05)

14.86 (1.06)0.78 (0.35)0.95 (0.10)

31.56 (1.50)0.38 (0.08)

45.60 (1.30)7.55 (0.85)

12.64 (1.16)0.86 (0.40)0.82 (0.06)

32.53 (1.64)0.29 (0.06)

47.80 (1.70)6.35 (0.75)9.75 (1.25)0.62 (0.30)0.76 (0.12)

34.72 (2.10)0.26 (0.07)

• Average of 3 replicates. Figures in parentheses indicate standard deviations.


Table III. Texturai values' of cow, buffalo and mixed milk rasogolla as obtained by the Instron univer-sai testing machine.Caractéristiques texturales du rasogolla obtenues à l'Instron.

Texturai attributes Cow Buffalo Mixed Market

Hardness (mN) 10.01 (1.35) 18.31 (2.10) 13.21 (1.85) 11.20 (1.65)Cohesiveness 0.59 (0.08) 0.65 (0.06) 0.62 (0.05) 0.58 (0.06)Springiness (mm) 4.80 (0.35) 6.20 (0.65) 4.20 (0.50) 4.90 (0.40)Guminess (mN) 5.90 (1.35) 11.90 (1.70) 8.20 (1.40) 6.50 (1.30)Chewiness (mN.mm) 28.70 (2.45) 73.70 (3.75) 34.50 (3.60) 31.85 (3.55)

, Average of 6 replicates. Figures in parentheses indicate standard deviations.

the development of numerous large andsmall voids throughout the matrix. The fatphase became highly vulnerable and themembrane convoluted, resulting in itsshrinkage and rupture, Iiberating free fat.This presumably was because of longerand rigorous heating (~ 100 OC)in concen-trated sugar syrup which produced a highosmotic pressure difference surroundingthe fat globule membrane leading to thecollapse or rupture of the membrane. Cowmilk, which contains somewhat smaller fatglobules than those in buffalo milk (Gangu-li, 1974) was less affected, whereas thelarge fat globules (~ 10 urn) in buffalo or

mixed milk rasogolla became more proneto such heating and their globular smoothsurfaces became severely shrunken.

Creamer et al (1978) observed a folded,thread-Iike structure in casein micellesheated to 100 oC, similar to that in cowmilk rasogolla. However, Kalab et al (1988)observed that frying of cow milk paneer(another chhana-Iike Indian style cottagecheese) in oil severely altered the granu-larity of the protein matrix while it disap-peared completely in buffalo milk paneer.They also noted that the fat globule c1usteraltered during frying and acquired sharpand pointed outlines as compared to their

Table IV. Average' sensory texturai score of rasogolla on unstructured Iinear rating scale (1-10).Caractéristiques texturales du rasogolla obtenues par analyse factorielle (échelle de notaüon de 1à 10).

Sensory texturai attributes Cow Buffalo Mixed Market

Firmness 5.45 (0.55) 8.10 (0.75) 7.50 (0.60) 5.60 (0.65)Crumbliness 2.60 (0.35) 2.10 (0.40) 4.00 (0.85) 2.45 (0.45)Elasticity 7.30 (0.50) 8.80 (0.60) 8.10 (0.35) 7.50 (0.40)Stickiness 2.45 (0.25) 1.50 (0.15) 2.05 (0.20) 2.30 (0.30)Chewiness 6.85 (0.50) 8.45 (0.55) 7.40 (0.65) 7.05 (0.70)Juiciness 9.10 (0.45) 6.60 (0.50) 7.50 (0.40) 9.20 (0.50)Ove rail texturai quality 8.40 (0.50) 6.80 (0.45) 7.70 (0.60) 8.50 (0.40)

• Average of 7 judges for each of the 6 trials. Rgures in parentheses indicate standard devialions.


near-globular shape in raw paneer. Verma(1989) observed a similar thread-Iike fold-ed structure of both cow and buffalo milkrasogolla with finer and more uniformly dis-tributed casein particles in cow milk rasogol-la than that in buffalo milk rasogolla, similarto our observation. He also observed amore open texture and sponginess in cowmilk than in buffalo milk rasogolla.

Texture, which is an important funda-mental property of foods, arises from theslze, shapes and micro- and macro-structural arrangements of the food partie-ulates under differential processing condi-tions. Thus, a texturai and structural studyis essential in order to understand thephysical and mechanical behaviour offoods during processing, storage and onconsumption.

The greater hardness of buffalo milkrasogolla resulting from its denser, coarserprotein matrix might be due to the caseinmicelle makeup and typical behaviour ofbuffalo milk during processing. It was ob-served that unlike cow casein, most of thecaseins in buffalo milk are present in themicellar form (Sabarwal and Ganguli,1971). The ratio of micellar to solublecasein for buffalo milk was found to be 91as against 21 in cow milk. The authorsalso observed that buffalo milk casein mi-celles were larger in size th an those in cowmilk and also that the micellar organizationin buffalo and cow milk was not ldentlcal(Ganguli and Anavkar, 1971). lt was also::found that as the buffalo milk casein mi-celles were larger in size, the aggregationof micelles was faster and their water re-tention capacity less during rennet coagu-lation as compared to cow case in micelles(Sabarwal and Ganguli, 1971). Thus, theheterogeneity in the structure of cow, buf-falo and mixed milk rasogolla can be ex-plained by the typical micellar behaviour ofthe buffalo milk caseins and the local dif-ference in the pH values during the acidcoagulation of milk (ie during chhana prep-

aration) followed by severe physicalchanges during cooking (ie du ring rasogol-la preparation).

Mixed milk rasogolla had lower hard-ness but higher crumbliness than that ofbuffalo milk rasogolla mostly due to theheterogenous nature of the protein matrixstructure. This may be postulated as dueto the differences in micellar organization,size and typical agglomeration characteris-tics of buffalo milk caseins as compared tothose of cow milk caseins, which led tosuch an unhomogeneous micellar coales-cence, with micelles thus retaining most oftheir separate identities.

The greater springiness of the buffalomilk product might be due to the differen-tial casein micelle makeup and typical ar-rangement in the protein matrix to that ofcow or mixed milk products. Setter sugarsyrup adsorption and more juiciness incow milk rasogolla may be explained bythe differences in pore size and shapes asevidenced from the Iight microscopic stud-ies. The market sample had significantsimilarities to the structural and texturai ap-pearance of cow milk rasogolla, becausethe former is mostly manufactured fromcow milk.

lt is weil established that the firmness ofheat-induced milk gels is influenced byseve rai physical and chemical factors pre-'surnably acting at the submicroscopic lev-el. Electron microscopy was thereforeused to search for an anticipated relation-

.ship between the texture and the ultra-.:'structure of acid-coagulated followed by

heat-processed milk product (rasogolla) asaffected by varying concentrations of totalsolids, protein and other compositions. Thehigher concentration of milk solids accom-panied by higher protein, fat and calciumcontents, had a marked effect on the firm-ness'qt. rasogolla as revealed by thisstudy:'The 1.83-fold increase in hardnessof buffalo milk rasogolla to that of cow milkrasogolla was due to the increase in total


solids content of about 5% in buffalo milkrasogolla, whereas a slightly lower totalsolids content in mixed milk rasogolla alsoresulted in remarkable differences in theInstron texturai values. The microstructuralstudy showed that hardly any micelle re-tained its individuality in ail the 3 types ofrasogolla. Most of them were seen boundby fusion, forming chains or c1usters withnumerous voids in between, consequentlygiving different texturai values. Kalab andHarwalkar (1974) also observed a closerelationship between the ultrastructure andfirmness of milk gels ranging widely in firm-ness. In gels containing 40 and 50% totalsolids (14 and 17% protein respectively),caseinmicelles appeared as individual en-tities, Iinked by some bridging mate rialsbut at 60% total solids (20% protein), themicelles were fused and offered a consid-erably higher resistance to the penetrome-ter probe. The following reasons may alsobe postulated for the greater hardness ofbuffalo milk rasogolla: i), the increased ag-gregation of casein particles with strongerlinks might have reduced the capacity ofthe fat and protein phases to move in rela-tion to each other during compression bythe Instron plunger; and ii), the protein ma-trix containing a smaller proportion of waterreduced the mean free path of the caseinmicelles as compared to cow and mixedmilk rasogolla, resulting in a higher degreeof hardness in buffalo milk rasogolla.

CONCLUSION

The comparative scanning electron micro-scopic as weil as light microscopic studiesshowed that buffalo milk rasogolla with acompact, ragged and scale-Iike structurewas harder and chewier but less juicy andthus less acceptable th an that of cow milkrasogolla which exhibited a loose proteinmatrix structure with numerous voids, thusresulting in better sensory texturai ratings.

Excessive rupture of fat globules in buffalomilk rasogolla led to the release of a signif-icant amount of free fat, which may ad-versely affect the shelf-life of the productduring storage.

Thus this study revealed that by manip-ulating the casein micelle framework andaltering the chemical composition of buffa-10 milk (ie by various process treatments orchemical modifications), or by using mixedmilk, a better quality product could be pro-duced with a similar texturai and structuralappearance to that of cow milk rasogolla,which may provide some alternatives tomeeting the domestic as weil as the exportdemand for rasogolla in India.

ACKNOWLEDGMENTS

The authors thank M Kalab (Agriculture Canada,Ontario, Canada) for his valuable suggOestionsonthe SEM study and help in evaluating the photo-graphs. The skilful technical help of KN Baggaand financial support of the National Dairy Re-search Institute (India) are also acknowledged.

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Micro- and macro-structure of cow, buftalo and mixed milk

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