Viscous high-fat foods such as cream, egg yolk, or mayonnaise are co-extruded with a 3% sodium alginate solution from a syringe into a 50 mM calcium chloride solution.  The food sample passes through the inner tube of a double needle assembly while the alginate solution is simultaneously extruded through a mantle surrounding the inner needle.  As the sodium alginate solution forms a gel on contact with calcium ions, the food sample becomes encapsulated in the calcium alginate gel formed on the surface of the food sample.  The encapsulation procedure may be carried out within a temperature range between 0 C and 25 C.  Samples may be prepared for scanning electron microscopy or for transmission electron microscopy by selecting either wide or narrow bore needles, respectively

Kalab, Miloslav

Veliky, Ivan A.

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Food Structure Volume 9 Number 2 Article 11 1990 Encapsulation of Viscous High-Fat Foods in Calcium Alginate Gel Tubes at Ambient Temperature Ivan A. Veliky Miloslav Kalab Follow this and additional works at: https://digitalcommons.usu.edu/foodmicrostructure  Part of the Food Science Commons Recommended Citation Veliky, Ivan A. and Kalab, Miloslav (1990) "Encapsulation of Viscous High-Fat Foods in Calcium Alginate Gel Tubes at Ambient Temperature," Food Structure: Vol. 9 : No. 2 , Article 11. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol9/iss2/11 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 digitalcommons@usu.edu. FOOD STRUCTU RE, Vol. 9 (1990) , pp. 151 - 154 1046 - 705X/90$3 .00+ .00 Scanning Microscopy International , Chicago (A MF O'Hare), IL 60666 USA TECII NICA L NOTE ENCAPSULATION OF VISCOUS HIGH-FAT FOODS IN CALCIUM ALGINATE GEL TUBES AT AMBIENT TEMPERATURE Ivan A. Vcliky and Mil oslav Kalftb • Division of Biology, National Research Council of Canada Ottawa, Ontario, Canada K lA OR6, and • Food Research Ce ntre, Research Branch, Agriculture Canada, Ottawa, Ontario, Canada K 1 A OC6 Viscous high·fat foods such as cream, egg yo lk, or may· onnaise are co-extruded with a 3% sodium alginate solution from a !-.yringe into a 50 mM calcium chloride solution. The food sample passes through the inner tube of a double needle asse mbly while the alginate solution is simul tane-ously ex truded through a mantle surrounding the inner need le. As the sodium alginate so lution forms a gel on contact with calcium io ns, the food sample becomes e ncap-su lated in the calcium alginate ge l formed on the su rface of the food sample. The encapsu latio n procedure may he car-ri ed out within a temperature range between 0°C and 25°C. S:..~mplc~ may he prepared for scanning electron microscopy o r for transmission electron microscopy by selecting either wide or narrow bore needles, respectively. Initia l paper received April 6, 19 90 Manuscr ip t received Ju ne 15, 1990 Direct inqu iries t o M. Kal ab Telephone number: 613 995 3722 ext. 7707 Key Words: Alginate, Coat ing, Electron microscopy, En-capsulation, Gel tubes, High-fat foods. 151 lntroduction The original procedure for the encapsu lation of suspen-sions and dispersions of biological origin in agar gel tubes designed by Salyaev [7] has been modified by several au· thors fur use with foods [3-6}. In a recent mod ifica tion [6J, the sample is aspirated into a glass Pasteur pipette, the lower end of the pipette is sea led with agar ge l. and the pipette is re pea tedly dipped into a warm (40° ) agar so l to form a sleeve around the Pasteur pip ette . Th e samp le is then transferred into the agar gel tube by pulling the Pas-teur pipette out. In the Discussion with Reviewers in paper J6J Dylewski no ted that the microstructure of some heat· sensitive high-fat foods may be affected by repeated ly dip-ping the food in the pipette into the warm agar sol. In order to reduce the exposure to heat, Goff (per~onal communi-cation) has been using low~temperature gelling agarose for thi~ purpose. Viscous foods may also he a5piraretl into agar ge l tubes made around glass or metal rods [51 using the rod as a piston. and. thus, avoiding the exposure of the foods to heat , hut this procedure is more diffictllt to perform than the former procedure [6]. In particular, it is difficult to properly seal both ends of the agar gel tubes following their conta mina tion with fat. 1\ different approach is possible hy using a sodium algi-nate sol ut ion which forms a gel on contact with calcium ions. This reaction makes it possible to instantly encapsu-lat e viscous food samples at any temperature between 0°C and room temperature ( -25°C) and is the subject of this technical no te . Materials and Methods Sodium algina te (BDH Chemicals. Lt d., Poole, England) was dissolved in distilled water to form 2 to 4% solutions. Calcium chloride, 0.1 to 1.0 M solution, wa~ mixed with a 2.5% glutaraldehyde solution (1:9, vfv) and the pH was adjusted to 6.5 using 0.2N NaOH. H igh-fat food (egg yolk, cream, or mayonnaise) desti ned for examinat ion by scann ing electron microscopy (SEM) 1. A Veliky and M. Kalab was placed in a 5-mL syringe and a double need le assembly was attached to it. The double needle assembly consisted of an inner needle ( - I mm inner diameter) concentr ically positioned inside a n outer stainless steel tube (mantel) with a 0.3-mm gap between both tubes (Fig. I). Three ribs, each 3 mm long, were soldered on the inner tube, 5 mm from the tube end, as spacers in order to maintain concentric posi-tion of both needles. Another 5-ml syringe filleu with the sodium alginate solution was attached to the outer needle (Figs. I and 2). Both the food and the alginate solution were manually simultaneously extruded into the calcium chloride solution. The long column of the encapsulated food (Fig. 3) was examined under a dissecting microscope and the best parts were cut into 10-15 mm sections as soon as the calcium al· ginate tubes were firm enough to be handled, i.e., within 1-2 min. The food-conta in ing gel tube was compressed with a pair of fine tweezers and cut at that location with a scalpel. The wet cut surface was bloned with paper tissue, covered with a droplet of the sodium alginate solution, and this droplet was gelled using a droplet of the calcium chloride solution, thus sealing the food inside the cut gel tube. These shorter sections were washed with a 50 mM calcium chlo· ride solution, pH 6.5, and postfixed for 24 h at 6°C with a 2% osmium tetroxide solution made up in a combined imi · dazole and veronal-acetate buffer [1} in order to retain fat. The samples were then dehydrated in a graded ethanol se· ries and freeze-fractured [1}. The fragments were immedi· ately critical-point dried from carbon dioxide in a Samdri 3PVT apparatus; the heater of this apparatus was switched off at 35°C in the final step of drying. The dried sample fragments were mounted on aluminum stubs using a silver-based cemen t, coated with gold in a Technics Hummer II sputter coater, and examined at 20 kV in an lSI DS-130 scanning electron microscope equipped with an external oscilloscope [2]. MicrographS were taken on 35-mm 100 ASAfilm. For transmission electron microscopy (TEM), the foods were prepared in two ways. Provided that fixation and de-hydration sufficiently hardened the samples, they were cu t into smaller ( < I mm3) particles for embedding in a Spurr's low-viscosity medium (J.B. EM Service, Inc. , Pointe Claire-Dorval, Quebec, Canada). Otherwise, a similar encapsu lat-ing apparatus was used but the diameter of the inner nee· die was reduced to <0.5 mrn and the diameter of the outer needle was adjusted accordingly. Pulsed extrusion resulted in small encapsulated beads rather than tubes. The beads were better suited for TEM and were postfixed, dehy-drated, a nd embedded in the same way as other solid samples [I]. 152 Results and Discussion Co-extrusion of high-fat food and a sodium alginate so-lution th rough concentric needles into a calcium chloride solution, which contained glutaraldehyde as a fixative, re· suited in the encapsulation of a long column of the food under study in a calcium alginate gel (Fig. 3). The end of the double concentric needle assembly should be about 5 mm above the calcium chloride solution; if it is too low, the gel may form at the needle outlet and clog the needle. The diameter of the food column and the thickness of the cal-cium alginate coating depend on the diameters of the nee-dles and the rates at which each of the two components are ext ruded: rapid ext rusion of the alginate solution and slow extrusion of the viscous food produce a thick gel coating on a relative ly thin food colum n. If the extrusion rates are re -versed, an uneven gel coating may develop on the food col-umn and the column may disintegrate in the calcium chlo-ride solution. The optimum conditions founds were a 3o/o sod ium alginate and 0.05 M calcium chloride solutions. The rate of extrusion varies with the consistency of the food un-der study and should be established experimentally The need to seal the food under study following the division of the long column into shorter segments is easy to meet. Compared to an agar gel coating, which is brittle, the calcium alginate coating is elastic; the cut may be sealed with a droplet of the sodium alginate solution by gelation. Freeze·fractured cream samples which had been encap-sulated for SEM, postfixed with a 2% osmium tetroxide solution in a buffered imidazole solu tion are shown in Fig. 4. The calcium alginate gel adhered to the samples through all treatment steps. ln comparison with the dtjJping proce· <.lure using agar for encapsulation [6J, no difference could be found berween samples encapsulated according 10 this procedure and samples encapsulated in calcium alginate gels. The co-extrusion method, however, has been devel· oped to provide an alternative to the agar gel encapsulation and to prevent exposure of heat-sensitive food samples to an elevated temperatu re before fixation. Acknowledgment') Skillful technical assistance provided by Miss Gisele Larocque is acknowledged. The au thors thank Mrs. Paula Allan-Wojtas anu Dr. D. N. Holcomb for useful sugges-tions. Electron Microscope Unit, Research Branch, Agri-culture Canada in Otlawa provided facilities. Contribution NRCC No. 31643 from the Nat ional Research Couocil of Canada and Contribution 857 from the Food Research Centre. Encapsulation in Calcium Alginate Gel FiP. /. Diagram of the apparatus for the encapsulation of vl:'i-cow· food samples in calcium alginate gel rubes by co-extmd-ing the food samples with a 3% sodium alginate solution imo a 50 mM calcium chloride solwion. Fif!. 2. A photograph of the encapsulation apparatus. 153 ~ Cream (large arrows) encapsulated an calcium algi-nate gel (small affows). The long and curled encapsulated column will be cw imo shorter sections for freeze-fracturing. Fir:. 4. SEM of 2 freeze-fractured cream samples shows sm ooth fracture planes and adherence of the alginate gel r•; capsules to the samples. I. A. Veliky and M. Kalab Allan-Wojtas P, Kalab M. (1984). Milk gel structure. X IV. Fixation of fat globules in whole-milk yoghurt for electron microscopy. Milchwissenschaft 39(6), 323-327. 2. Bond EF, Kalab M. (1988). An external oscilloscope im-proves photography with the lSI DS-130 scanning elec-tron microscope. Electron Microsc. Soc. Am. Dull. 18(1), 90-91. 3. l·lenstra S, Schmidt DG. (1970). The microcapsule tech-nique. An embedding procedure for the study of suspen-sions and emulsions. LKB Application Note No. 150. 4. Jewell GG. (1981). The microstructure of orange juice. Scanning Electron Microsc. 1981;11 1:593-598. 5. KaHib M. (1987). Encapsulation of viscous suspensions in agar gel tubes for electron microscopy. Electron Microsc. Soc. Am. Bull. 17(1), 88-89. 6. Kalab M. (1988). Encapsulation of viscous foods in agar gel tubes for electron microscopy. Food Microstruc. 7(2), 213-214. 7. Salyaev RK. ( 1968). A method of fixation and embed-ding of liquid and fragile materials in agar microcapsu-lae. Proc. 4th Europ. Reg. Conf. Electron Microsc. Rome, T ipogra fi a Poliglo tt a Vat icana, Rome, Italy, n. 37-38. Discussion with Reviewe rs Reviewer I· What effect could calcium ions have on the food under study? Authors: With milk products, calcium has been recomM mended as an ingredient in buffered fixatives by several authors l9, 12, 13] in order to maintain the integrity of the casein micelles. The concentration of calcium ions pencM trating the food sample may be reduced by transferring the encapsulated food column into a glutaraldehyde solution containing 5 mM CaCJ2. Reviewer I: May any liquid food sample be examined by electron microscopy us ing the encapsulation technique de-scribed? ~There are reports in the literature that this is pos-sible [ 10, II}, but other authors mentioned problems en-countered with milk [8). Although milk may be encapsu-lated, the distribution of the casein micelles is affected by fixation with glutaraJdehyde and the removal of the soluble components of the milk serum. When the se rum is replaced with ethanol during dehydration, the fixed casein micelles sediment in the capsules irrespective whether they consist of agar or calcium alginate gels. Reviewer I: May any liquid food sample be examined by electron microscopy using the encapsulation technique de-scribed? Authors: There are reports in the literature that this is pos-sible [ 10, llJ, hut other author~ mentioned problems en-countered with milk [8]. Although milk may be encapsu-lated, the distribution of the casein micelles is affected by fixation with glutaraldehyde and the removal of the soluble components of the milk serum. When the serum is replaced with ethanol during dehydration, the fixed casein micel!es sediment in the capsules irrespective whether they consist of agar or calcium aiginate gels. Addi tional R efe re nces 8. Carroll RJ, Basch JJ , Phillips JG, Farrell HM Jr. ( 1985). Ultrastructural and biochemical investigations of ma-ture human milk. Food Microstruc. 4(2), 323-331. 9. Harwalkar VR, Allan- Wojtas 1', Kahib M. (1989). Effect of heating to 200oC on casein micelles in milk: A metal shadowing and negative staining electron microscope study. Food Microstruc. 8(2), 217-224. 154 10. Henstra S, Schmidt DG. ( 1974). Ultradtlnnschnitte aus Milch mit Hilfe der Mikrokapselmethode lUltrathin sections of milk by means of the microcapsulation method). Naturwissenschaften 57(5), 247 (in German). II. Kalab M. (1983). Electron microscopy of foods. In: Physical Properties of Foods. M Peleg and EB Bagley (eds), AVI Publishing Co., Inc .. Westport, Cf, USA, 43-104. 12. itschmann H. (1949). Elektronenmikroskopische Gr6-ssenbestimmung der Calciumcaseinatteilchen in Kuh-milch (Electron microscopic determination of calcium caseinate particles in bovine milk]. Helv. Chim. Acta 32, 1258-1264 (in German). 13. Rose D, Colvin JR. (1966). Appearance and size of mi-celles from bovine milk. J . Dairy Sci. 49, 1091-1097. 

Encapsulation of Viscous High-Fat Foods in Calcium Alginate Gel Tubes at Ambient Temperature

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Encapsulation of Viscous High-Fat Foods in Calcium Alginate Gel Tubes at Ambient Temperature

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