FoodInfo Online Features -->6 August 2004
In the second part of his review,
Ernest Mann concentrates on the scientific aspects of yoghurt production.
The second part of this review concentrates predominantly on more scientific aspects of yoghurt production. A recent study in Australia (30) looked at seasonal variations in physical characteristics of both set and stirred yoghurts. The effects of variations in solids-not-fat contents (10–14 per cent) on gel strengths, whey drainage and other properties were examined. It was found that standardisation of total solids content with dried skim milk was not sufficient to produce yoghurts with consistent physical characteristics throughout a season. Another group of Australian scientists (31) studied the effects of fortification of milk with two per cent of either whey powder (WP), skim milk powder (SMP) or whey protein concentrate (WPC) on composition, pH, firmness, viscosity, syneresis and microstructure of yoghurt. The results indicated that WP and SMP supplementation reduced viscosity and firmness, while WPC supplementation increased these values.
Another report from the same laboratory (32) looks at the effects of adding proteolytic strains of Lactobacillus delbrueckii subspecies bulgaricus to commercial ABT starter cultures or a mixed starter on the texture of yoghurt and the effects on the survival of starter bacteria. Amongst other things, the results indicated that the viability of probiotic bacteria improved in yoghurt made with the mixed starter with the added Lactobacillus delbrueckii strain.
In the USA, researchers have reported (33) on the effects of stirred yoghurt thickness, flavour and colour on the sensory perceptions of 120 school children. A German study (34a) has reported on the enzymatic cross-linking of milk proteins on the functional properties of set-type yoghurt, involving transglutaminase treatment of milk prior to fermentation with a starter culture. The results indicated that, while the aroma and consistency of yoghurts from enzyme-treated milk were less 'yoghurt specific', they were more creamy than those produced from untreated milk. This suggests that a transglutaminase treatment may simulate fat in fermented products.
The effect of fermentation temperature between 37 and 46°C on the formation and rheology of yoghurt has been studied in the UK (34b) using reconstituted skim milk preheated and fermented with either a ropy or non-ropy starter culture. A study in Spain (35) investigated the effects of manothermosonication treatment (MTS) of milk on the rheological properties of yoghurt made from MTS milk. The results indicated that MTS treatment, which is the simultaneous application of heat and ultrasound under moderate pressure, could result in yoghurts with rheological properties superior to those found in control yoghurts made from untreated milk.
Studies in the USA (36) have compared yield stress, microstructure, shear values and water holding capacity (WHC) in full fat set yoghurt prepared from fortified milk. This had been subjected to either thermal processing (85°C for 35 minutes) or high hydrostatic pressure processing (193 or 676 MPa for 5 or 30 minutes). Amongst other things, the results indicated that yoghurts made from milk treated at 193 MPa and untreated milk (control) exhibited low yield stress, low WHC and large clusters of coalesced micelles. In a more general article (37) on the effects of different ingredients and manufacturing processes on the texture of set, stirred and probiotic yoghurts, the costs of various commercially available yoghurt texturisers are also considered.
An Austrian study (38) on the influence of starter culture on the relationship between dry matter content and the physical properties of stirred yoghurt formed part of an EU project that aimed to develop a standard procedure for the laboratory-scale production of set-type and stirred yoghurt. The first of three reports from the USA (39) was concerned with the evaluation of the textural properties of two types of yoghurt (light or blended), using a trained sensory analysis panel and an instrumental compression/penetration test in combination with a novel data analysis method.
The second report (40) presented a comparison of the effects of aspartame and sucrose on the microstructure of yoghurt. The third study from the USA (41) reports the development of a method for manufacturing a milk protein powder rich in both casein and whey proteins, as well as being free of lactose. The powder was used successfully in the manufacture of non-fat yoghurts without the addition of gelling and stabilising agents. Russian workers (42) have developed a stabiliser for heat-treated yoghurt which is based on a modified maize starch and a texturising agent containing guar gum, xanthan gum and pectin.
The synthesis and utilisation of folate by 32 strains of yoghurt starter cultures and probiotic cultures was examined by Australian scientists (43). Brazilian workers (44) conducted a study of composition and sensory quality of whole stirred yoghurt made from milks with somatic cell counts ranging from below 400 000 to above 800 000 cells per ml. Cell counts higher than 400 000 per ml had a detrimental effect on the sensory quality of yoghurt. A report from Egypt (45) looked at the utilisation of laboratory produced xanthan gum in the manufacture of natural and soya yoghurt. It concluded that the highest sensory scores were obtained when xanthan gum additions of 0.01 and 0.005 per cent were employed with the two types of yoghurt respectively. Acetaldehyde is regarded as a major indicator of flavour in yoghurt.
In a recent Turkish study (46), the effects on acetaldehyde levels in yoghurts, of the use of viscous and non-viscous cultures plus amino acids, treatment with ß-galactosidase and the use of heat-shocked cultures were investigated. The results indicated that the highest acetaldehyde levels in yoghurt were obtained when non-viscous starter cultures were used alone.
The effects of processing and refrigerated storage on the distribution and stability of aflatoxin M1 in yoghurt, artificially contaminated with different levels of the toxin, has been investigated in Greece (47). Spanish scientists (48) have developed a new multiresidue method for determining trace amounts of organochlorine pesticides and polychlorinated biphenyls in yoghurt, the method proving quick, accurate and repeatable. In another Spanish study (49) the survival of E. coli 0157:H7 in yoghurt at different storage temperatures (4–22°C) was compared with that of a non-pathogenic strain of E. coli. The results showed that counts of the pathogenic strain during storage at 4 and 8°C were higher than those of the non-pathogenic strain. Turkish scientists (50) have studied the microbiological properties of labneh concentrated yoghurt stored under oil at room temperature and refrigerator temperature.
An Italian study looked into the occurrence of yeast contamination in yoghurt (51). Examination of 40 samples of fruit yoghurt with obvious signs of blowing, produced by two different firms, revealed the presence of three different species of yeast - Pichia fermentans, Torulospora delbrueckii and Claviospora lusitaniae. This is believed to be the first recorded time that these yeast species have been associated with yoghurt blowing. Several reports are associated with health-nutritional aspects of yoghurt. The first of these, from Argentina (52), gives the results of trials where mice were fed with different doses of yoghurt as part of a milk re-nutrition diet on recovery of the intestinal barrier and mucosal immune function. The results suggest that, although they were obtained with an animal model, the consumption of yoghurt by malnourished children may accelerate the restoration of gut function following intestinal malfunction.
In trials in the USA (53), the addition of three servings of yoghurt daily to the diet of 29 postmenopausal women with habitually low calcium intakes resulted in a significant reduction in the urinary excretion of N-telepeptide, a marker for bone resorption. The nutrients added with the yoghurt greatly improved overall diet quality as compared with a nutrient-poor snack diet. Somewhat inconclusive results were obtained in Italy (54) during a study of the effects of yoghurt feeding as a dietary supplement for 12 healthy, elderly people. However, significantly lower faecal clostridia counts were obtained in comparison with the controls, which suggests a beneficial effect of yoghurt feeding.
Polish scientists (55) have reported on the effects of certain factors on the properties of set-type ewes' milk yoghurt. The factors studied included different starter cultures and fat contents and their effects on overall quality, rheological and sensory properties when fresh and following storage for up to 14 days at 5°C. The same team (56) has also studied the sensory quality and physico-chemical properties of ewes' milk yoghurt made from milk with fat contents ranging from zero to six per cent. Quality parameters varied with fat content and storage time, and optimum results for sensory quality were obtained for non-fat yoghurts.
A Greek study (57) looked at the effects of frozen storage of ewes' milk on the microbiological and physicochemical properties, as well as its yoghurt-making ability. The overall quality of yoghurt made from frozen stored ewes' milk was as high as that made from fresh milk. Another Greek report (58) described a method for the detection of bovine milk in ovine yoghurt by electrophoresis of para-k-casein.
Finally, it was concluded from a study carried out in Croatia (59), that goats' milk yoghurt fermented with probiotic bacteria and fortified with 1.5 per cent inulin shows good potential as a functional food.
References
(30) Cheng, L J et al (2002) Austral J Dairy Technol 57(3)187
(31) Bhullar, Y S et al (2002) Milchwissenschaft 57(6)328
(32) Shihata, A et al (2002) Internat Dairy J 12(9)765
(33) Brennan, E M et al (2002) J Food Sci 67(7)2785
(34a) Lorenzen, P C et al (2002) Internat J Dairy Technol 55(3)152
(34b) Haque, A et al (2001) Food Hydrocolloids 15(4/6)593
(35) Vercet, A et al (2002) J Agric & Food Chem 50(21)6165
(36) Harte, F et al (2002) J Food Sci 67(6)2245
(37) Meester, R 2002) Food Ingred & Anal Internat 24(1)12
(38) Jaros, D et al (2002) Milschwissenschaft 57(8)447
(39) Carson, K et al (2002) J Food Sci 67(3)1224
(40) Haque, Z Z et al (2002) Food Sci & Technol Res 8(1)21
(41) Mistry, V V (2002) Lait 82(4)515
(42) Dunchenko, N I et al (2002) Moloch Promyshl No 10 p27
(43) Crittenden, R G et al (2002) Internat J Food Microbiol 80(3)217
(44) Oliveira, C A F et al (2002) Austral J Dairy Technol 57(3)192
(45) El-Sayed, E M et al (2002) Europ Food Res & Technol 215(4)298
(46) Ozer, B et al (2002) Internat J Dairy Technol 55(4)166
(47) Govaris, A et al (2002) Food Additives and Contam 19(11)1043
(48) Yague, C et al (2002) J AOAC Internat 85(5)1181
(49) Bachrouri, M et al (2002) J Food Sci 67(5)1899
(50) Say, D et al (2002) Milchwissenschaft 57(9/10)528
(51) Vallone, L et al (2001) Industrie Alimentari 40(407)1001
(52) Gauffin-Cano, P et al (2002) J Dairy Res 69(2)303
(53) Heaney, R P et al (2002) J Amer Dietet Ass 102(11)1672
(54) Canzi, E et al (2002) Lait 82(6)713
(55) Bonczar, G et al (2002) Food Chem 79(1)85
(56) Bonczar, G et al (2002) Zywnosc 9(1)109
(57) Katsiari, M C et al (2002) Food Chem 77(4)413
(58) Kaminarides, S E et al (2002) Food Chem 78(1)53
(59) Bozanic, R et al (2002) Mljekarstvo 52(2)93
This review was originally published in the August 2003 edition of Dairy Industries International
