doi: 10.1111/j.1471-0307.2012.00836.x
ORIGINAL RESEARCH
Effects of poly-c-glutamic acid on the physicochemical characteristics of skim milk yoghurt SU-MIN LIM, 1 JAE-YONG SHIM, 2 S E J O N G O H , 3 MINSUK RHEE, 4 MOON-HEE SUNG 5 , 6 and JEE-YOU NG IMM 1 * 1
Department of Foods and Nutrition, Department Nutrition, Kookmin University, University, Seoul, Korea, 2 Departm Department ent of Food and Biotec Biotechnology, hnology, 3 Hankyong National National University, University, Anseong, Anseong, Korea, Depart Department ment of Animal Science, Science, Chonnam National National Univers University ity,, Gwangju Gwangju,, Korea, 4 Div Divisi ision on of Food Biosci Bioscience ence and and Technolo Technology, gy, Korea Korea Unive Universi rsity, ty, Seoul Seoul,, Korea, Korea, 5 Dep Depart artmen mentt of Advanced Advanced Ferme Fermenta ntatio tion n 6 Fusion Science and Technol Technology, ogy, Kookmin University, University, Seoul, Korea, and Bio Biolea leaders ders Corpo Corporat ration ion,, Daejeon Daejeon,, Korea
The phys physico icochem chemica icall and sens sensory ory prop properti erties es of ski skim m mil milkk yogh yoghurts urts cont contain aining ing pol polyy-c-glut -glutamic amic acid (PGA) at different levels (0.0025, 0.005 and 0.01%) were evaluated. Addition of PGA up to 0.01% to reconstituted skim milk (11%, w ⁄ v) v) did not affect the growth of lactic acid bacteria or the development of titratable acidity in yoghurt, whereas full-fat control yoghurt had reduced acid production. No changes were found in viable cell counts of PGA yoghurts during storage (4 weeks at 4 C). The addition of PGA (0.005%) significantly decreased syneresis in skim milk yoghurt and did not cause any undesirable effects in sensory acceptability. Keywords Poly-c-glutamic acid, Yoghurt, Acceptability, Syneresis.
whey protein concentrate resulted in a firm body and an d gr grai ainy ny te text xtur uree wi with th in incr crea ease sed d sy syne nere resi siss Low-fat or non-fat products are one of the fastest (Guzman-Gonzalez et al. 2000; Mistry and Hassan growing segments in the food industry, and high 1992). The addition of pre-biotics such as b-glucan consumer consum er demand continues to exist for reduced or or inu inulin lin resu resulte lted d in decr decreas eased ed sens sensory ory qual quality ity non-fat nonfat dai dairy ry pro product ducts. s. Dur During ing the las lastt deca decade, de, (Sahan et al. 2008; Guven et al. 2005). In another yoghurtt consum yoghur consumption ption has conti continuousl nuously y increas increased, ed, study, a fat replacer (Simplesse 100 ) that is prodwhich whi ch is pro probab bably ly due to the high org organo anolept leptic ic uced by mic microp ropart articu iculat lation ion of whe whey y pro protei tein n has quality, reduced lactose content and health-promot- been applied to non-fat yoghurt. When fat replacer ing effects. was used at the same concentration as anhydrous Yogh Yo ghur urtt is a ge gell ma matr trix ix of ca case sein in mi mice cellle less milk fat, yoghurt containing milk replacer did not formed at reduced pH, and the conversion of milk mee meett the tex textur tural al qual quality ity of nat natura urall yogh yoghurt urt by to yoghurt is started by the accumulation of lactic sho showi wing ng a sof softe terr cu curd rd an and d in incr crea ease sed d syn syner eres esis is acid (Lucey 2004). As fat provides yoghurt with a (Barrantes et al. 1994; Tamime et al. 1995). rich body and smooth mouthfeel, fat removal leads Poly-c-glutamic acid (PGA) is a homopolyamide to sign signific ificant ant chan changes ges in tex textur tural al char characte acteris ristic ticss composed of D- and L-glutamic acid units. PGA is and sensory quality. Non-fat yoghurts usually suf- an edible biopolymer that contains amide linkages ferr fr fe from om po poor or fla flavo vour ur an and d te text xtur uree as we well ll as between its a -amino and c -carboxylic acid groups, increased syneresis (Brennan and Tudorica 2008). and it is naturally found in the mucilage of tradiTherefore, the preparation of non-fat yoghurt that tio tional nal fermented fermented soyb soybeans eans in Japa Japan n ( Natto) an and d maintai mai ntains ns good sen sensory sory qual qualiti ities es is a pro promi minent nent Korea (Chungkookjang) (Park et al. 2005). PGA is challenge for the dairy industry (Hess et al. 1997; widel widely y used in many biological industries industries as a drug El-Sayed et al. 2002). carrier carr ier,, biol biologi ogical cal adhe adhesiv sive, e, cry cryopr oprote otecta ctant nt and To im impr prov ovee or orga gano nole lept ptic ic qu qual alit ity y of no nonn-fa fat t bitterness-relieving agent (Shih and Van 2001). yoghurt yogh urt,, incr increasi easing ng tot total al soli solid d lev level el and ⁄ or o r th thee In terms of physiological functionality, rats were addi ad diti tion on of hy hydr droco ocoll lloi oids ds was was em empl ploy oyed ed.. Th Thee fed with PGA in a form of natto mucilage (20% increa inc reased sed sol solid id cont content ent simila similarr to that of ful full-f l-fat at of diet) resulted in increased Ca solubility in the yoghurt using skim milk powder, Na-caseinate, or small intestine (Tanimoto et al. 2001), and its Ca INTRODUCTION
*Author for correspondence. E-mail:
[email protected] jyimm@k ookmin.ac.kr c.kr 2012 Society of
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absorption–promoting effect has been confirmed in post-menopausal women, especially those with lower Ca absorption than average (Tanimoto et al. 2007). Sung et al. (2005) reported that PGA administration improves antibody production and tumour regression in a B16 tumour-challenged mice. The high water-binding properties and physiological functionality of PGA led us to evaluate its performance in non-fat yoghurt as a fat replacer. This study was conducted to examine the effects of PGA on the quality characteristics of non-fat yoghurt, including fermentation speed, viable cells, microstructure, syneresis and sensory attributes. MATERIALS AND METHODS
Materials Skim milk and whole milk powder were purchased from Seoul Milk Co. (Seoul, Korea). YC-X16 was obtained from Chr. Hansen (Horsholm, Denmark) and used as a yoghurt starter. PGA (Ca salt, MW: about 5000 kDa) was kindly provided by Bioleaders (Daejeon, Korea). All other chemicals were obtained from Sigma (Sigma Chemical Co., St Louis, MO, USA) unless stated otherwise. Starter culture YC-X16 (0.02%), the mixed culture containing Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, was inoculated into a sterilised skim milk medium (11% skim milk powder) and incubated at 42 C. The starter culture was freshly made prior to making the yoghurt. Yoghurt manufacture Skim milk powder (protein, 35%; carbohydrate, 52%; lipid, 1%; ash, 7.9%; and moisture, 4.1%) and whole milk powder (protein, 25%; carbohydrate, 38%; lipid, 27%; ash, 6.2%; and moisture 3.8%) were used for the preparation of non-fat and full-fat yoghurts (11%, solid, w ⁄ v), respectively. The milk powder (11%, w ⁄ v) was reconstituted in distilled water and heated at 100 C for 20 min. Fully solubilised PGA (0.0025, 0.005 and 0.01%, w ⁄ w) was added to the reconstituted skim milk prior to heating. The starter culture (YC-X16, 2.5%, w ⁄ v) was then inoculated, and the mixtures were stored at 42 C. When the pH reached 4.6, the yoghurts were incubated at 4 C until further analysis. At least three replicates of yoghurt preparation trials were performed to obtain analytical data. Titratable acidity and viable cell count Titrable acidity (TA) was determined by the method of Dave and Shah (1998). TA is expressed as gram of lactic acid ⁄ 100 g of yoghurt. Total lactic acid bacteria (LAB) were counted in fresh and stored yoghurts (4 C, 2 and 4 weeks old) after cultivation on Elliker medium for 48 h at 37 C. Syneresis Syneresis was determined by the method of Achanta et al. (2007). Yoghurt samples (100 g) were filtered through 424
Whatman filter paper (No.4, Whatman International Ltd., Madstone, UK) on the top of a funnel. After 2 h of drainage at 4 C, the volume of separated whey was measured in a graduated cylinder as an index of syneresis.
Cryo-FE scanning electron microscope (Cryo-FESEM) A Hitachi FESEM (S-4700, Hitachi, Tokyo, Japan) was used to observe the microstructure of the yoghurts. Yoghurt samples were rapidly frozen in a jet freezing device (JFD 030, Bal-tec, Balzers, Lichtenstein, Germany), after which frozen samples were transferred (under vacuum at )140 C) to a cryochamber using the high-vacuum cryotransfer system (VCT 100, Bal-tec, Germany). The specimens were fractured and sputter-coated with Pt (15 mA, 150 s). The coated specimens were then introduced to a microscope chamber where they were examined under an accelerating voltage of 5 kV. The temperature of the cryostage was maintained at )140 C during image processing. Texture profile analysis Compression tests were carried out using a TA.XT2 texture analyzer equipped with a 50-kg load cell (Stable Micro Systems, Surry, UK). A cylindrical 25-mm-diameter probe was used to measure the hardness of the yoghurt samples prepared in 60-mL cups (5.5 cm diameter · 5 cm height). The samples (10 C) were placed under the probe, which then moved downwards at a constant speed of 2.0 mm ⁄ s (pre-test), 1.0 mm ⁄ s (test) and 1.0 mm ⁄ s (post-test). The averages of five replications were taken as the hardness values of the yoghurts. Sensory evaluation Non-fat yoghurt containing 0.005% PGA was compared with full-fat yoghurt. The samples were prepared 1 day earlier than the actual evaluations and stored at 4 C until served. Sensory evaluation was performed by 50 untrained panellists (20 men and 30 women, aged 20–30 years). Samples coded with random three-digit numbers were provided to the panellists who were asked to rinse their mouths with water after each tasting. The panellists rated appearance, taste, texture and overall acceptability using a hedonic 9-point scale, in which 9 means most liked and 1 most disliked. Statistical analysis All analytical measurements were completed at least in triplicate, and statistical analysis was performed using SPSS ver. 14.0 (SPSS Inc. Chicago, IL, USA). When the analyses of variance (ANOVA) revealed significant differences (P < 0.05), Tukey’s test was used for multiple comparison of the treatment means. RESULTS AND DISCUSSION
Titrable acidity and viable cell counts Changes in TA during incubation are shown in Figure 1. There were no significant differences in TA, except in full-fat control yoghurt. This result indicates that the addition of PGA did not 2012
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109 cfu ⁄ mL LAB after the completion of fermentation, and there were no significant differences in viable cell count up to 4 weeks. This suggests that the addition of PGA did not result in any undesirable effects on the growth and survival of the starter culture for a possible expiration date (1 month). There is no general consensus on the range of viable cell numbers in yoghurts for exerting beneficial health effects in yoghurts, although several countries including EU (EFSA 2010) have set minimum LAB levels in yoghurts and ⁄ or fermented milks.
1.5
) % ( y 1.0 t i d i c a e l b a t a r t i 0.5 T
Full fat control Non fat control PGA yoghurt (0.0025%) PGA yoghurt (0.005%) PGA yoghurt (0.01%)
0.0 0
3
6
9
12
Time (h)
Figure 1 Effects of PGA addition on titratable acidity in yoghurts. PGA,
poly-c-glutamic acid.
affect the speed of acid production. The reduced acid production of full-fat yoghurt was probably due to relatively lower lactose and protein contents, as all yoghurt samples had the same total ¨ nal et al. (2003) reported that TA of low-fat solid content. U yoghurts increases proportionally to the concentration of skim milk solids, and higher acidities are found in yoghurts with a greater amount of added protein (Modler and Kalab 1983). A variety of factors affect the viable cell numbers in yoghurts. Typically, acidity, chemical composition, inoculation practice and storage conditions significantly affect viable cell numbers (Donkor et al. 2006; Birollo et al. 2000). Figure 2 shows the changes in the viable cell counts of the yoghurts after the completion of fermentation as well as during extended refrigerated storage. All yoghurts contained about
Full fat control Non fat control PGA youghurt (0.0025%)
PGA youghurt (0.005%) PGA youghurt (0.01%)
10
9
) L m / u 8 f c 0 1
g o l ( s l l e c e l b a i V
Table 1 Effects of PGA addition on syneresis of yoghurts
7
6
5
4
Initial
Completion of fermentation
2
4
Storage (Weeks)
Figure 2 Changes in viable cell counts in yoghurts during the storage period.
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Syneresis Syneresis is the spontaneous separation of whey in set yoghurts and is considered a quality defect owing to its unattractive appearance. The effects of PGA additions on syneresis are shown in Table 1. First, the degree of whey separation was significantly influenced by fat level. Full-fat yoghurt resulted in the lowest level of syneresis, whereas the non-fat control yoghurt displayed the highest syneresis. Addition of up to 0.005% PGA to non-fat yoghurt significantly reduced syneresis, whereas no difference was found between PGA and full-fat yoghurt. However, the addition of >0.005% PGA was less effective at reducing syneresis in the non-fat yoghurt. Syneresis occurs by continuous rearrangement of the casein matrix causing gel shrinkage (Lucey et al. 1998). Harwalkar and Kalab (1986) reported that the pore size of the casein network is a critical factor governing the susceptibility of yoghurt to syneresis. In the case of full-fat yoghurt, fat can be packed into the protein network as an inert filler and thereby decrease porosity. Aziznia et al. (2008) also suggested that positive interactions occur between fat globules and the gel network in full-fat yoghurt. The decreased susceptibility of syneresis in the PGA yoghurts can be explained by the improved water-binding effect of PGA. Increased syneresis at the 0.01% PGA might have been caused by excessive bridging in the protein network. The decreased uniformity and local aggregation of a protein matrix can impair water-binding activity in a yoghurt protein matrix. A similar trend was noted in the casein micelle and a carrageenan mixture previously (Langendorff et al. 1999). Considering that the effective concentrations of yoghurt stabilizers, such as locust bean gum, guar gum high methoxyl
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Sample
Syneresis (mL ⁄ 1 00 g)
Full-fat control Non-fat control PGA yoghurt (0.0025%) PGA yoghurt (0.005%) PGA yoghurt (0.01%)
41.0c ± 44.3a ± 42.1 c ± 42.0 bc ± 43.2 ab ±
2 2 3 3 3
Each value is expressed as the mean ± SD ( n = 20). Values with different letters indicate a significant difference ( P < 0.05). PGA, poly-c-glutamic acid.
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pectin, and gelatin, were in the range from 0.1 to 0.25% (Koksoy and Kilic 2004), PGA effectively reduced syneresis at much lower concentrations.
Microstructure The microstructures of the PGA yoghurts were observed by Cryo-SEM (Figure 3). As shown in Figure 3(a) and (b), the microstructures of the whole and non-fat yoghurts were clearly differentiated by the presence of fat globules. The addition of PGA resulted in significant changes in microstructure. PGA seemed to connect and fill the pores of protein chains and consequently forming a continuous network structure. Similar to milk protein gel, the addition of PGA affected thickness and strands in tofu network, and PGA was located inside of soy protein network (Lee and Kuo 2011). A dense and less voidspaced interrelated protein network was found with 0.005% PGA (Figure 3d). The yoghurt containing 0.01% PGA showed some irregular network structure with larger void spaces (Figure 3e). These results correspond with the increased syneresis of yoghurt as mentioned previously in Table. 2.
The microstructure of yoghurt can vary depending on the type of stabilizer. Addition of gelatin (0.5%) resulted in no significant changes in yoghurt structure, whereas a fibrillar microstructure of large casein clusters was observed in the presence of carrageenan (0.4%) (Kalab et al. 1975). Lee and Kuo (2011) reported that increased electrostatic repulsion owing to the negatively charged glutamic acid moiety of PGA (1000– 1500 kDa, 0.1–0.2%) caused delay in tofu gelation. This, in turn, weakened the tofu structure by reducing the number of hydrophobic interactions between soy proteins. However, PGA cannot impose significant electrostatic repulsion in yoghurt structure formation considering the acidic pH of yoghurt.
Hardness The effects of PGA addition on the hardness of yoghurt were determined using a texture analyzer. As shown in Table 2, fat content significantly affected the hardness of yoghurt. Non-fat yoghurt demonstrated greater hardness than that of the full-fat control. This result was consistent with a report of Yazici and Akgun (2004). Sodini et al. (2004) reported that the firmness
(a)
(b)
(c)
(d)
(e)
Figure 3 Cryo-scanning electron micrographs of yoghurts. (a) Full-fat control yoghurt, (b) Non-fat control yoghurt, (c) PGA yoghurt (0.0025%), (d) PGA
yoghurt (0.005%), (e) PGA yoghurt (0.01%). PGA, poly- c-glutamic acid.
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Table 2 Effects of PGA addition on hardness values of yoghurts Sample
Hardness value (g)
Full-fat control Non-fat control PGA yoghurt (0.0025%) PGA yoghurt (0.005%) PGA yoghurt (0.01%)
47.3c ± 65.2b ± 78.9 ab ± 89.3 a ± 76.2 ab ±
5 4 9 9 3
Each value is expressed as the mean ± SD ( n = 5). Values with different letters indicate a significant difference ( P < 0.05). PGA, poly- c-glutamic acid.
of yoghurt is greatly influenced by total solid content and especially its protein content because protein would increase crosslinking in the protein network. The hardness value of non-fat yoghurt further increased as the concentration of PGA was increased up to 0.005%. The interactions between PGA and casein can be observed in Figure 3(d), and the reinforced gel internal microstructure could be a reason for increased firmness. However, the hardness value of non-fat yoghurt containing 0.01% PGA was lower than that with 0.005% PGA yoghurt. A similar trend was reported by Maroziene and Kruif (2000). In this report, pectin molecules adsorbed onto casein micelles formed a stable system at low concentrations. As the pectin concentration was increased, the casein micelles became fully covered leading to reduced flocculation.
Sensory quality The acceptability of PGA yoghurt (0.005%) was compared with that of full-fat yoghurt. As shown in Figure 4, no significant differences were found in appearance, flavour, texture and overall acceptability between the samples. Although the addition of PGA resulted in increased hardness (Table 2), it did not
10
Full fat yoghurt Non fat yoghurt (0.005% PGA) 8
e r o c s y r o s n e S
6
4
2
0
Appearance
Flavor
Texture
Overall
Acceptability
Figure 4 Acceptability of PGA yoghurt. PGA, poly- c-glutamic acid.
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have any negative effect on sensory acceptability. This result is consistent with a report by Mitsuiki et al. (1998) that indicated PGA did not exhibit its own taste. Creaminess is one of the key attributes governing the overall acceptability in low-fat ⁄ non-fat yoghurt, and the creaminess perception is inversely proportional to the fat content in yoghurt (Alting et al. 2009). On the basis of the result, the perceived creaminess could be improved in the presence of PGA in nonfat yoghurt. Consumers continue to require healthy non-fat products. PGA is a high MW natural biopolymer and has various physiological functionalities. The addition of PGA to non-fat yoghurt did not impair quality characteristics such as fermentation speed, viable cell counts and sensory quality. The syneresis of non-fat yoghurt was improved by the addition of PGA, and the high water-binding activity of PGA exerted a positive effect by providing a creamy mouthfeel. Therefore, PGA has great potential to replace conventional stabilizers such as gelatin in premium non-fat yoghurts. A C K N OW L E DG E M E N T This work was supported by a grant from the Next-Generation BioGreen 21 Program (No. PJ0083272011), Rural Development Administration, Republic of Korea. This work was also supported by a grant from Kookmin University received in 2010. The authors are thankful for the financial support.
R EF E RE N C E S Achanta K, Aryana K J and Boeneke C A (2007) Fat free plain set yoghurts fortified with various minerals. Lebensmittel Wissenschaft Technology 40 424–429. Alting A C, van de Velde F, Kanning M W, Burgering M, Mulleners L, Sein A and Buwalda P (2009) Improved creaminess of low-fat yoghurt: the impact of amylomaltase-treated starch domains. Food Hydrocolloids 23 980–987. Aziznia S, Khosrowshahi A, Madadlou A and Rahimi J (2008) Whey protein concentrate and gum tragacanth as fat replacers in nonfat yoghurt: chemical, physical, and microstructural properties. Journal of Dairy Science 91 2545–2552. Barrantes E, TamimeA Y,MuirD D and Sword A M (1994) The effectof substitution of fat by microparticulate whey protein on the quality of set-type, natural yoghurt. International Journal of Dairy Technology47 61–68. Birollo G A, Reinheimer J A and Vinderola C G (2000) Viability of lactic acid microflora in different types of yoghurt. Food Research International 33 199–805. Brennan C S and Tudorica C M (2008) Carbohydrate-based fat replacers in the modification of the rheological, textural and sensory quality of yoghurt: Comparative study of the utilization of barley beta-glucan, guar gum and inulin. International Journal of Food Science and Technology 43 824–833. Dave R I and Shah N P (1998) Ingredient supplementation effects on viability of probiotic bacteria in yoghurt. Journal of Dairy Science 81 2804– 2816.
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Donkor O N, Henriksson A, Vasiljevic T and Shah N P (2006) Effect of
El-Sayed E M, Abd El-Gawad I A, Murad H A and Salah S H (2002) Utilization of laboratory-produced xanthan gum in the manufacture of yoghurt and soy yoghurt. European Food Research and Technology 215 298–304. European Food Safety Authority (EFSA) (2010) Scientific opinion on the substantiation of health claims related to live yoghurt cultures and improved lactose digestion (ID 1143, 2976) pursuant to article 13 (1) regulation (EC) No. 1924 ⁄ 2006. European Food Safety Authority Journal 8 1763. Guven M, Yasar K, Karaca O B and Hayaloglu A A (2005) The effect of inulin as fat replacer on the quality of set-type low-fat yoghurt manufacture. International Journal of Dairy Technology 58 180–184. Guzman-Gonzalez M, Morais F and Amigo L (2000) Influence of skimmed milk concentrate replacement by dry dairy products in a low-fat set-type yoghurt model system. Use of caseinate, co-precipitate and blended dairy powders. Journal of the Science of food and Agriculture 80 433–438. Harwalkar V R and Kalab M (1986) Relationship between microstructure and susceptibility to syneresis in yoghurt made from reconstituted nonfat dry milk. Food Microstructure 5 287–294. Hess S J, Roberts R F and Wiegler G R (1997) Rheological properties of non-fat yoghurt stabilized using Lactobacillus delbrueckii ssp. bulgaricus producing exopolysaccharide or using commercial stabilizer systems. Journal of Dairy Science 80 252–263. Kalab M, Emmons D B and Sargant A G (1975) Milk-gel structure. IV. Microstructure of yoghurt in relation to the presence of thickening agents. Journal of Dairy Research 42 453–458. Koksoy A and Kilic M (2004) Use of hydrocolloids in textural stabilization of a yoghurt drink, ayarn. Food Hydrocolloids 18 593–600. Langendorff V, Cuvelier G, Launay B, Michon C, Parker A and De Kruif C G (1999) Casein micelle ⁄ iota carageenan interactions in milk: influence of temperature. Food Hydrocolloids 13 211–218. Lee C Y and Kuo M I (2011) Effect of c -polyglutamate on the rheological properties and microstructure of tofu. Food Hydrocolloids 25 1034–1040. Lucey J A (2004) Cultured dairy products: an overview of their gelation and texture properties. International Journal of Dairy Technology 57 77–84.
Mistry V V and Hassan H N (1992) Manufacture of nonfat yoghurt from a high milk protein powder. Journal of Dairy Science 75 947–957. Mitsuiki M, Mizuno A, Tanimoto H and Motoki M (1998) Relationship between the antifreeze activities and the chemical structures of oligo- and poly (glutamic acid)s. Journal of Agricultural and Food Chemistry 46 891–895. Modler H W and Kalab M (1983) Microstructure of yogurt stabilized with milk proteins. Journal of Dairy Science 66 430–437. Park C, Choi J C, Choi Y H et al. (2005) Synthesis of super-high-molecularweight poly-c-glutamic acid by Bacillus subtilis subsp. Chungkookjang. Journal of Molecular Catalysis B-Enzymatic 35 128–133. Sahan N, Yasar K and Hayaloglu A A (2008) Physical, chemical and flavor quality of non-fat yogurt as affected by a b -glucan hydrocolloidal composite during storage. Food Hydrocolloids 22 1291–1297. Shih I L and Van Y T (2001) The production of poly-( c-glutamic acid) from microorganisms and its various applications. Bioresource Technology 79 207–225. Sodini I, Remeuf F, Haddad S and Corrieu G (2004) The relative effect of milk base, starter, and process on yoghurt texture: a review. Critical Reveiws in Food Science and Nutrition 44 113–137. Sung M H, Park C, Kim C J, Poo H, Soda K and Ashiuchi M (2005) Natural and edible biopolymer poly- c-glutamic acid: synthesis, production, and applications. Chemical Record 5 352–366. Tamime A Y, Kalab M, Muir D D and Barrantes E (1995) The microstructure of set-style, natural yoghurt made by substituting microparticulate whey protein for milk fat. International Journal of Dairy Technology 48 107–111. Tanimoto H, Mori M, Motoki M, Torii K, Kadowaki M and Noguchi T (2001) Natto mucilage containing poly- c-glutamic acid increases soluble calcium in the rat small intestine. Bioscence Biotechnology and Biochemistry 65 516–521. Tanimoto H, Fox T, Eagles J, Satoh H, Nozawa H, Okiyama A, Morinaga Y and Fairweather-Tait J (2007) Acute effect of poly- c-glutamic acid on calcium absorption in post-menopausal women. Journal of American College of Nutrition 26 645–649. ¨ Unal B, Metin S and Is ¸ ikli N D (2003) Use of response surface methodology to describe the combined effect of storage time, locust bean gum and dry
Lucey J A, Teo C T, Munro P A and Singh H (1998) Microstructure, permeability and appearance of acid gels made from heated skim milk. Food Hydrocolloids 12 159–165. Maroziene A and Kruif C G (2000) Interaction of pectin and casein micelles. Food Hydrocolloids 14 391–394.
matter of milk on the physical properties of low-fat set yoghurt. International Dairy Journal 13 909–916. Yazici F and Akgun A (2004) Effect of some protein based fat replacers on physical, chemical, textural, and sensory properties of strained yoghurt. Journal of Food Engineering 62 245–254.
acidification on the activity of probiotics in yoghurt during cold storage. International Dairy Journal 16 1181–1189.
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