Food Structure Volume 5 Number 2 Article 13 1986 Relationship Between Microstructure and Susceptibility to Syneresis in Yoghurt Made from Reconstituted Nonfat Dry Milk V. R. Harwalkar Miloslav Kalab Follow this and additional works at: https://digitalcommons.usu.edu/foodmicrostructure Part of the Food Science Commons Recommended Citation Harwalkar, V. R. and Kalab, Miloslav (1986) "Relationship Between Microstructure and Susceptibility to Syneresis in Yoghurt Made from Reconstituted Nonfat Dry Milk," Food Structure: Vol. 5 : No. 2 , Article 13. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol5/iss2/13 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 MICROSTRUCTURE, Vol. 5 (1986) , pp. 287-294 0730-54 19/86$ I. 00+ . OS SEM, Inc., AMF O'Hare (Chicago). IL 60666- 0507 U.S.A. RELATIONSHIP BET WEEN MICROSTRUC TURE ANO SUSCEPTIBILITY TO SYNERESIS IN YOGHURT MAGE FRO M RECONSTITUTED NO NFAT DRY MI LK V. R. Harwalkar and Miloslav Kalab Food Research Centre. Research Branch, Agriculture Canada Ottawa. Ontario. Canada KlA OCG Introduction Increase in the density of protein matrices i n Syneres i s means separati on of the 1 iquid phase in yoghurt samples containing 10 t o 30% total milk sol ids gels. Tn yoghurt (a milk gel), syneresis is undesirable. was studied by scanning electron microscopy and was Susceptibi 1 i ty of yoghurt to syneresis depends on sever­ corr e lated with a decrease in syneresis. Casein parti­ al factors (6), particularly on the preheat treatment of cles which formed chains and clusters in the protein milk, the total solids content, and the acidity result­ matrix were largest in the 10% total solids yoghurt and ing from t h e growth of the l actic bacteria cultures. their dimensions were decreased as the total solids Culturing of milk that had been preheated to 85° to 90 °C contents were increased. This observation was confirmed results in a firm yoghurt whi ch retains the 1 !quid phase by transmission e l ectron micr oscopy of thin sections. wi thin the protein matrix. Milk wh ich had not been An attempt has been mad e to explain the discrepancy preheated forms a soft ge l from wh ich 1 iquid (whey) between results on syneresis obtained by a drainage and separates easi ly. Preheating of milk destined for yo­ a centrifugation method applied to yoghurt samples made ghurt manufacture is, therefore, pa r t of t he industrial at pi! 3.85 and 4.5. The explanation is based on a dif­ process (16). Effects of preheat treatment of milk on ference in rigidity of t he yoghurts under study and in the microstructure of yoghurt were reported earl ier (7, t he forces affecting t he protein matrix during tests for 8, 11). Casein par ticles were found to form a relatively syneresis. uniform matrix composed of branching c hains i n yoghurt made from preheated milk. The casein particles formed large cluster s ~ en unheated milk wa s cultured using a yoghurt bacterial culture (4, 9). The total so l ids content is a nother factor ~ ich affects syneresis in yoghurt. Traditionally in Ba lkan countries. yoghurt was mad e from milk that had been thickened by evaporation. A si mil ar increase in the total solids content i s presently achieved by fortifying milk by the addition of nonfat dry milk solids or milk Initial paper received June 25, 1986 protein concentrates (13, 14, 18) or by concentrating Manuscript received September 24, 1986 the milk by reverse osmosis or ultrafiltration (19). Direct inquiries to V .R. Harwalkar Microstructure of yoghurt f ortified with milk proteins Telephone number: 613 995 3722 has been studied (13, 19) but had not been related to syneres1s. In yoghurt, lactic acid produced by t he bacterial c ul ture l owers the pH below the isoelectric point and i nduces coagulation of casein. In renneted milk gels, casein micelles are destabilized by t he highly specific action of proteolytic enzymes on K- casein. Acid develop­ ment facilitates coagulation but is not essentia l for it to take place. Development of microstructure in rermeted lli lk gels, which is somewhat different from the develop­ KEY \IKJRO S: Casein particles , Centrifugation test . Drain­ ment of microstructure i n yoghurt, was studi ed by Green age test, Milk gels, Milk solids. Protein matrix, Scan­ e t al. (3) a nd Sm ith (17). Syneresis of these gels was ning electron micr oscopy, Syneresis, Yoghurt. studled in great de tail by Smith (17). Walstra et al. (20) reviewed the s yneresis in both rermeted and acid milk gels. The objective of this study was to investigate the relations hip between mi c r ostruc ture and susceptibility to syneresis in yoghurt made from heated reconstituted nonfat dry mi l k as affected by the total so lids content and final pH of the yoghurt. 287 V . R. HARWALKAR and M. KALAB Materials and Me thods Results and Discuss ion Preoaration of yoghurt Syneresis in yoghurt Yoghurt was made from reconstituted 1 ow-heat nonfat In yoghurt, syneresis is considered to be a defect dry rni lk (NOM) of commercial origin. The NOM contained rn this respect, the situation is in contrast to ren­ 94.5% total solids, 33% protein, and less than 1% fat, neted milk gels, where controlled syneresis is desirable as determined using the Off icia 1 Methods of Ana 1 ysi s and important in the manufacture of cheese (17, 20) (1). The total solids contents of the reconstituted NOM Heating of milk destined for yoghurt production is one were adjusted to 10, 12.5, 15, 20, and 30%. These milks of the processing procedures which prevents the develop­ were heated to 90°C for 10 min, cooled to 44°C, inocu­ ment of syneresis {6) in the finished product. Other lated with a coiiVIIercial yoghurt star ter culture (5%, methods of controlling syneresis may involve the use of v / v). and incubated at 44°C unti1 the mixtures gelled thickening agents such as carrageenan, gelatinized and the desired pH value was reached. This was achieved starch. or ge latin (10, 16, 18), the use of ropy bacte­ within 3 to 21 h depending on the total solids contents rial cultures (16, 18, 19). or fortification of yoghurt of the yoghurt milk. milk with milk proteins including whe y proteins {14, 16, Determination of lactose 18, 19). This latter procedure is closely related to the Lactose concentration was determined using a polar­ effect resulting from an increased level of total sol ­ imetric method (1) modified as follows: Yoghurt samples ids, which is the subject of this study. were stirred for 10 min unti 1 they formed a homogenous Electron microscopy showed (9- 12, 19) that yoghurt viscous liquid. An amount of yoghurt corresponding to consists of a protein matrix composed of chained and approximately 6 g total solids was weighed in a 100-mL clustered casein particles. Chains are convnon in yoghurt volumetric flask. Protein was coagulated by the addition made from milk which had been preheated to a minimum of of 30 mL of a mercuric iodide solution {33.2 g KI and 85°C whereas large clusters of casein particles form the g matrix of yoghurt made from unheated milk. Whey proteins 13.5 HgC1 2 was dissolved in a mixture of 200 mL of glacial acetic acid and 640 mL of water) and the volume are part of the matrix in yoghurt made from heated milk was made up with a 5% solution of phosphOtlUlgstic acid. (11). Such a matrix is characterized by interstitial The contents were shaken for 15 min and f i 1 tered. Lac ­ spaces (pores). the dimensions of which depend on the tose concentration in the filtrate was measured in a protein content in that matrix. Heertje et al. (7) Perkin- Elmer 141 Polarimeter using a 10Q-mm cell and was showed that casein micelles in milk started to disinte­ expressed in g/100 g {%). grate as the pH of the milk r eached 5.5 due to the Measurement of syneresis production of lactic acid by the bacterial culture. The Susceptibility of yoghurt to syneresis was measured disintegration was most extensive at pH 4.8 but the using a drainage and a centrifugation method described proteins re- aggregated into globular particles as the pll earlier (5, 6). In the drainage method (2). yoghurt made value was further decreased to 4.8 and lower. in 250- mL beakers was cut into 4 parts and these were Effect of total solids drained in a funnel equipped with a stainless steel It has already been mentioned that in yoghurt. ca­ screen (120 mesh). The volume of whey separated into a sein particle chains are linked at random and form a calibrated cylinder was measured at 5 min intervals for matrix with relatively uniform pores (Fig. 1) filled 60 min. In the centrifugation method, yoghurt samples with the I iquid phase (\Jley). As the total sol ids con­ were made in 15- mL calibrated tubes and were centrifuged tent in the yoghurt is increased, the casein partie] e at 6°C for 10 min at centrifugal forces ranging from 30 chains become shorter. the pol'e dimensions are dimin­ to 2000xg. The volume of the whey separated was plotted ished. and the density of the matrix is increased. This against the centrifugal force applied.