ORIGINAL CONTRIBUTION

Use of ultrafiltration and reverse osmosis to improve goats’ milk yogurt VALERIE MARSHALL and EKRAM EL-BAGOURY* Food Research Institute, Shinfield, Reading RG2 9AT, UK

Acid and acetaldehyde production from yogurt starter was compared in goats’ milk fortified by ultrafiltration (UF). reverse osmosis (RO) and addition of goats’ milk powder. Starter activity was impaired when milk was concentrated by RO and aroma was poor in milk fortified by addition of dried milk, while yogurts made from milk concentrated by UF had good acidity and aroma. Threonine (0. I %) addition to unfortvied milk and milk fortified by dried milk or UF produced yogurts with high levels of the aroma compound acetaldehyde.

Modern manufacture of yogurt requires a fortification process because the increase in total solids strengthens the yogurt coagulum, increases viscosity and prevents syneresis (Galesloot, 1958; Ashton, 1963). An economical and easy method of fortification is to add skimmed milk powder, but as dried goats’ milk powder is not widely available this is not always possible for making goats’ milk yogurt. As goats’ milk may be used as a replacement for cows’ milk in the diet of those suffering from allergy to cows’ milk, addition of dried cows’ milk is precluded for the making of goats’ milk yogurt. However, other methods of fortification, for example, ultrafiltration (UF) and reverse os- mosis (RO), have been suggested and assessed for making cows’ milk fermented products (Bundgaard et al, 1972; Chapman et al, 1974; Jepsen, 1979). For example, Abrahamsen & Holmen (1980) found that for yogurt there was better viscosity and curd firmness using UF membranes and that the milk could therefore be concentrated to a lesser degree and still achieve good texture using this method. More recent work (Modler & Kalab, 1983; Tamime et al, 1984) suggests that the ratio of casein:whey protein also influences structure as fortification with whey protein produced weak coagula.

This study compares the effect on yogurt starter activity of increasing total solids in goats’ milk by UF, RO and addition of goats’ milk powder.

MATERIALS AND METHODS Goats’ milk was obtained from the goat herd at the National Institute for Research in Dairying (now Food Research Institute, Reading). Spray-dried goats’ milk powder was obtained from Tregaron Foods Ltd, Station Road, Dyfed, Wales. DL-threonine was purchased from BDH Ltd, Poole, Dorset, England and was prepared as a 10% (w/v) solution and sterilized by filtering through a sterile Millex-HA filter unit SLHA 025 BS (Millipore Ltd, Harrow, Middlesex, England) Cultures Streptococcus thermophilus NCDO 2393 and Lactobacillus bulgaricus NCDO 2394 originally isolated from yogurts sold in the UK were used. Each culture was grown overnight in sterilized cows’ skimmed milk at 37”C, and 1.5% of each was used as starter to ferment the milk.

Methods of concentration (a) Ultrafiltration Raw goats’ milk was concentrated 2- and 3-fold using a type B module, Paterson Candy International, with polysulphone BX5 membranes with a nominal 100%

*Present address: Food Science & Technology Department, Faculty of Agriculture, Suez Canal University, Esmailia, Egypt

rejection of molecules larger than 20,000 mw. (b) Reverse osmosis Raw goats’ milk was concentrated 2-and 3-fold using De Daiisk Sukkerfabrikker equipment at an operating pressure of 4.0 bar (560 lb/in2). The membranes had a nominal 100% rejection of molecules larger than 200 mw. (c) Addition of goats’ milkpowder Milk powder was added to goats’ milk to increase total solids to ca 13%, 15% and 17% by addition of 3g, 5g and 7g powder to lOOml milk. Preparation of yogurt Milk was steamed for 5 min and cooled to 42OC, and 3% of mixed starter was addcd. Incubation at 42°C continued until the pH fell to 4.2-4.5 and the time taken to attain this pH was noted. After overnight storage at 4°C yogurts were assessed for acidity (pH) and aroma (acetaldehyde).

pH was measured usilng a Digital p H meter model 7065 (Electronic Instruments Ltd, Kent, England).

Acetaldehyde was ineasured in yogurts made in Universal bottles fitted with siliclsne rubber septa; lml head-space vapour could then be withdrawn for injection on to a gas chromatograph (Perkin Elmer model 900, Beaconsfield, England) fitted with a flame ionization detector.

RESULTS The dominant flavour components of yogurt, that is, lactic acid and acetaldehyde, were measured (the lactic acid by pH) and used as indicators of starter activity. The results in Table 1 also show the time taken for the different milks to reach pH 4.3-4.6 when coagulation can easily be seen. This period of time was considerably lengthened for milk concentrated by RO. Milk with 15.25% TS took cver 5% h to coagulate, and over 1 I h when concentrated to 24% ‘TS, whereas milk concentrated to 17.4% TS by U F took only 4% h to reach pH 4.5. Not only did the pH fall slowly in milk concentrated by RO but acetaldehyde levels were also low, while in milk concentrated by UF there was a marked positive effect on acetaldehyde content. This increased from 16.8 ppm for unfortified milk to 27.6 pprn for 2-fold concentrated milk anti to 30.8 ppm for a 3-fold concentration. This increase in acetaldehyde was not reflected when milk was fortified by addition of goats’ milk powder. Although acid development was gocd for this method of fortification, there was no increase in aci:taldehyde content.

Duitschaever (1978) also used goats’ milk powder as a method of fortification and, finding little difference in the rate of acid production, suggested that addition of goats’ milk powder had little effect on starter activity. However, Abrahamsen et al, (1978) noted poor acetaldehyde levels in goats’ milk yogurt compared with cows’ milk yogurt when acidity was good, indicating a change in starter metabolism. The amino acid

Journal of the Society of Dairy Technology, Vol. 39, No. 2, April 1986 65

TABLE 1 Acid and acetaldehyde production by yogurt starter in milk

fortified by UF, RO and goats’ milk powder (GMP)

TS Incubn time pH CH,CHO (%) (min) (PPm)

Goats milk’ 9.5 249 4.4 18.4

Conc X 2 (UF)’ 13.0 25 1 4.45 27.6 (7.7-10) (246-250) (4.1-4.5) (10-23)

. , (1 1-16) (250-300) (4.3-4.6) (19-41)

Conc X 3 (UF)z 17.4 270 4.53 30.8

Conc X 4 (UF)* 21.5 275 4.6 40

Conc X 2 (RO)’ 15.25 33 1 4.45 23

Conc X 3 (RO)> 23.5 680 4.65 14.25

(4-205) (265-275) (4.4-4.6) (23-38)

(18.9-24.0) (270-280) (4.5-4.7) (25-45)

(15.0, 15.5) (320, 342) (4.5, 4.5) 16.5, 29.5)

(23, 24) (670, 690) (4.6, 4.7) 13.5, 15.0) GMP’ ca. 13 305 4.4 20

GMP’ ca. 15 330 4.4 22

GMP’ ca. 17 360 4.5 22

(280, 339) (4.3, 4.5) (16.5, 23)

(300, 360) (4.3, 4.5) (16, 28)

(330-290) (4.4, 4.6) (16, 28)

I Means of 8 samples ’Means of 4 samples range shown in parentheses

’Means of duplicate samples

TABLE 2 Effect of addition of threonine (0.1%) to fortified goats’ milk

TS PH CH, CHO (%) fuum)

U F 13 4.5 78.5

UF

GMP

(4.46, 4.52) (67, 90)

(4.45-4.47) (90, 90) 17.4 4.5 90

13 4.4 57.5 (4.25, 4.54) (50, 65)

(4.32-4.55) (35-55)

(4.4-4.52) (37-60)

(4.13-4.29) (36.5-54.0)

pH and acetaldehyde values after 4-6 h incubation and overnight at 4°C. Duplicate values are shown.

GMP 15 4.43 45

GMP 17 4.46 48.5

Unfortified 10 4.2 1 45.3

threonine has been shown to stimulate acetaldehyde production in Lactobacillus acidophilus (Marshall & Cole, 1983), and the effect of addition of this amino acid to fortified goats’ milk is shown in Table 2. Threonine stimulates acetaldehyde production by starter bacteria in U F milk to levels higher than those required for a good quality yogurt, while in unfortified milk and milk fortified by addition of powder the acetaldehyde levels were between 35 and 65 pprn and produced yogurts with a good aroma.

DISCUSSION To achieve an acceptable yogurt using goats’ milk, attention must be paid to both texture and flavour of the final product.

Fermentation of unfortified goats’ milk results in poor texture and a weak coagulum. Fortification with goats’ milk powder is reported to produce a better texture, but when the product was compared with cows’ milk similarly fortified, its gel was weaker and the stirred product less viscous (Duitschaever, 1978). Yogurt has been successfully made from cows’ milk concentrated by U F and RO, and these processes have now been applied to the making of goats’ milk yogurt. The present study indicates impairment of starter activity in milk concentrated by RO which is not paralleled in milk concentrated by UF. This may be due to an increase in sugar content of RO milk or to an increase in antibiotic residues. Both these types of compound would be expected to increase with RO but not with UF where the membranes are permeable to compounds of molecular weights less than 20,000. Concentration by RO and by addition of milk powder results in an increase in protein, lactose, vitamins and salts, whereas U F concentrates only the protein, and as goats’ milk protein contains more threonine residues than cows’ milk protein (Duitschaever, 1978; Rasic & Kurmann, 1978) this might also explain the increase in acetaldehyde levels in yogurt produced from this milk. That threonine contributes to acetaldehyde production by starter bacteria has been suggested by Lees & Jag0 (1978)and is clearly demonstrated in Table 2 where acetaldehyde was increased from 27.6 and 30.8 ppm to 78.5 and 90 pprn in UF milk and from 20 and 22 ppm to 57.5 and 45 ppm in milk fortified with goats’ milk powder. Threonine also increased the acetaldehyde content of yogurts made with unfortified milk.

CONCLUSION This paper examines the quality of yogurts made from milk fortified by UF, RO and addition of goats’ milk powder. Coagula were improved by addition of powder and by concentration by UF. RO did not provide a useful method of fortification; yogurts were slow to coagulate and acidity and aroma were poor. Aroma was markedly improved with the UF concentrated milk which may have been a consequence of an increase in threonine residues as protein levels increased.

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66 Journal of the Society of Dairy Technology, Vol. 39, No. 2, April 1986