CA1170493A - Yogurt and method therefor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Novel yogurt is prepared by inoculating pasteurized milk with conventional Lactobacillus and Streptococcus F, and incubating the milk. The resulting yogurt is smooth and creamy, without graininess, and shows little tendency to undergo syneresis on storage. Preferably the mil is thermally treated in the presence of added serum protein so as to produce a casein-serum protein complex and reduce the coagulation of casein-serum protein complex and reduce the coagulation of casein on fermentation.

Description

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YOGURT ~ND METHOD THEREFOR
FIELD OF_INVENTION
This invention relates to fermented milk products; it particularly relates to improvements in the production of yogurt and yogurt produced thereby.
BACKGROUND OF INV~NTICN
Yogurt is prepared in accordance with conventional processes by pasteurizing milk, inoculating the pasteurized milk with a culture ccmprising Lactobacillus bulgæ icus and optionally one or more other bacterial species the most notabl-e -of which is Stre~tococcus thermophilus, and incubating theinoculated milk. The cultures are generally lactose assimilating and produce lactic acid as a metabolite. In the course of the fenmentation a change in the physical structure of the milk is observed, first as an increased viscosity and ul-timately, if the fermentation is co~tinued, as a phase separation.
The increased viscosity is recognized as resulting frcm changes in the nature of the casein micelle. As the pH of the fermentation product decreases concomitantly with the production of lactic acid, a growth in the size of the micelle and a change ::

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in the chemical } up thereof is observed. Control of the change is difficult on an industrial scale due in part to variations in the make up of the natural starting products.
The t~ickened product so produced is generally considered to have an unpleasant, grainy texture. It is unsuitable for distribution through normal ccmmercial channels, where a shelf life of up to abGut 45 days is desirable, due to the instability of the naturally gelled liquid system, with syneresis often occurring in the course of storage.
In a typical commercial process wherein the fermentation is carried out in bulk, the natural fermented yog~rt is subjected to m~chanical working to produce a sm~oth product. Such mechanical working reduces the curd tension of the fermented product, and it is generally necessary to add thickening agents, for example, starches, gelatine and cellulose based derivatives, to compensate for such reduction.
In a second commercial process wherein the fenmentation is carried out in the vessel in which it is distributed at retail level, the so-called fermentation in a cup method, it is not generally feasible to mechanically work the product. This methcd is normally used in the production of ready to consume fruit flavoured yogurt wherein fruit and sugar are added prior to fermentation. m e increased sugar level in the fermented mixture decreasesthe stability of the naturally gelled liquid system, generally exacerbating the a~ove problem of producing a smooth textured product with gocd shelf life.

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AIMS AND OBJECTS CF INVENTICN
It is an object of the present invention to provide a new yogurt product.
It is a further object of the present invention to provide a new yogurt product with improved texture and flavour.
It is anothex object of the invention to provide a new yogurt product with improved shelf life.
It is yet another object of the invention to provide a new yogurt product with improved resistance to syneresis.
It is a still further object of the invention to provide a fermented in the cup yogurt with improved propPxties as afore-said.
It is anothex object of the invention to provide a new process for the production of yogurt whexeby a smcoth, creamy product may be formed withDut necessitating mechanical working or the addition of thickening agents.
SUMM~RY QF INVENTION
In accordance with a broad aspect of my invention, a yogurt is produced by pasteurizing milk and inoculating the pasteurized milk with a conventional yogurt culture and a culture of a bacteria isolated from dairy milk which will be further descxibed herein and to which I refer as Streptococcus F. The inoculated milk is incubated at a suitable temperature, generally about 42 C, for a suitable time to produce a product having a desired viscosity characteristic.
The nature of the yogurt so produced can vary between - " 11 704g3 wide ex-tre~es, depending upon the balance of bacterial species used for the inoculation, for example. When milk is inoculated exclusively with Streptococcus F, the resulting fermented product is extremely viscous and bland, having a pH of about 5 to 5.2.
There is relatively little agglomeration of the casein micelle-at this pH, and the prcduct has almost no perceptible graininess.
Whilst I do not wish to be bcund by theory, it is believed that the hi~h viscosity results from the in-situ generation of polysaccharides as metabolic products of fermentation.
In general, I prefer to inoculate the pasteurized milk with about equal amounts of conventional yogurt producing bacteria and Streptccoccus F, so as to produce a fermented product wherein the texture is smooth and creamy and wherein there is a suitable development of flavour. Where there is too great an imbalance between the initial concentration of the conventional yogurt producing bacteria and the Streptococcus F the one type tends to overgrow and suppress the other type. This is particularly so where Streptococcus F is initially dominant. For this reason I
prefer bo cultivate separate bulk starters for the yogurt producing bacteria and the 5treptocoocus F which are used for the inocula-tion of the pasteurized milk on plant scale, so as to exercize proper control over the bacterial ratios. It may be appreciated that should it be desired, milk may be fermented on a plant scale in a first stream wherein the milk is inoculated solely using the conventional ycgurt producing bacteria bulk starter, and in a second stream wherein the Streptococcus F bulk starter is used :` ` ' ' : : .

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alone, and the resulting fermented products blended. However such method is nok preferre~, for it is generally desirable not to subject the fermented products to the degree of mechanical handling khat would be necessary to blend the streams, as such handling rupkures khe microstructure of the coagulum.
It will be apparent from the aforesaid that the prime purFose of the conventional yogurt producing backeria in the instant processr apar-t from any beneficial health-giving properties that may accrue therefrom, is in the production of a suitable flav~ur in the finished product. It is neither necessary nor desirable that the casein protein be agglomerate~ and coagulated in the course of the fermentation to form a curd with a high curd tension such as is normally formed in the production of yogurt, as the novel yogurt of my invention is preferably thickened primarily by the metakolic products of Streptococcus F.
m e pH of the medium is norm~lly permitted to decrease in the course of the fermentation to a value of about 4.3 to about 4.7 so as to develop a desired flavour, when the fermentation is normally arrested. It will be appreciated that such pH values are close to the iso-electric point of casein, and a heavy curd -~ould normally result. In order to reduce the degree to which the casein agglomerates and coagulates I find it desirable to comple~
the casein prior to the inoculation step. Such camplexing is effected by increasing the serum protein content of the milk so as to provide a milk base having a serum protein nitrogen to casein protein nitrogen ratio desirably in the range of about 1:3 "

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to about 1:4.5, a preferred value of the ratio being 1:4, and subjecting the milk base to a thermal treatment generally in the range of a~out 85C to g3 & , and preferably in the range of about 88C to about 91& for a suitable time, generally in the range of about 5 to 10 minutes, to produce a serum-casein cQmplex. The optimum time of the thermal treatment may be gauged -from the effect thereof on the texture and properties of the finished fermented product, for it is surprisingly found that if the optimum time is unduly exceeded the firmness and viscosity of the finished product is decreased, and there is an increased probability of syneresis occuring where the thermal treatment takes place at temEeratures outside the range generally specified above.
The process is particularly suited to the formation of fruit flavoured yog~rts, eslpecially fenmented in the cup yogurts.
Thus the fermented product has a relatively low acid content which does not need to be offset by the addition of læ ge a~unts of a sweetening agent~ Also, the mild nature of the fermented product penmits the flavour of relatively delicately flavoured fruits to 2Q ke experienced without necessitating the addition of artificial flavour enhancement. In part it is surmised that flavour improvement may be due to the viscous nature of the yogurt whereby the yogurt clings longer to the taste buds of the tongue than do prcducts of a more conventional nature.

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CULTURE CHAR~CIERISTICS OF STREæTCCOC~US F.
-The bacterial strain used herein was isolated from dairymilk. It has a minimum growth temperature in the range of akout 8& to lO&, a maximum growth temperature of akout 52& and an optimum growth temperature in the range of about 37 & to about 45C, The practical incubation temperature is akout 42C.
It is a gram positive short chain diplococci, that ferments milk to a normal pH after incubation of about 5.2, producing acid slowly without formation of C02. Flavcur ccmponents such as acetoin, diacetyl and acetaldehyde are not produced in fermentation, and lactic acid is produced very slowly.
Milk protein is not fermented or degraded. Citric acid is not fermented.
Streptococcus F exhibits an excellent growth rate in the milk base used hRrein for the production of yogurt, and no transference of the culture.
Freeze dried samples of Streptococcus F are on deposit in the ~ulture collection of the University of Western Ontario, London, (Canada) under the Serial Number UW~ 521.

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- 8 - 1326 P/4 Ch The follcwnng exanples are illustrative of the invention as it nHy be practis~d co~rcially.
EXAMPLE 1: Prep~ration of Streptococcus F Bulk Culture Milk adjusted to 11% MSNF is fortified by thé addition of 1.5% nutrients and sterilized by autoclaving for 30 minutes.
It is then cooled and about a 200 ml aliquot is inoculated with freeze dried culture and incubate~ at 43 & for 8 h~urs to provide a mother culture. An intermediate culture is similarly prepared, but inoculated with 5% of the mother culture and incubated at 42 C for 7 hcurs Milk for the bulk culture is adjusted to 11% MSNF, and heated to 89 & for 45 minutes, $hen cooled to 42 C and inoculated with 5% of the intermediate cult~re. The inoculated milk is incubated at 42 & until the pH is 5.2, when the bacterial count is akout 3 x 108 ~1 .
EX~MPLE 2: Preparation of ~ilk base for inoculation.
100 kg of wh~le da~ry n~lk was partially ski~med to provide a desired fat`content of 2%, and the non-fat milk solids content thereof is adjusted to within the range of about 13 to 14 percent W.V. by the addition of 4.5 kg of low heat skim milk Fc~der having a minimum protein nitrogen value of 6 mg/g of po~-der. The temperature of the milk was increased to abcut 50 C, and 0.9 kg of serum milk protein fraction ~as added to provide a milk base having a serum protein nitrogen to casein protein nitrGgen ratio of 1:4. The serum milk ~rotein possessed the follcwing characteristics:

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CQMPCNEN~ WEIGHT %
Protein (N x 6.38) 53,4 Lactose 26.4 Milk fat 2.5 Eactic acid 2.1 Citxic acid 2.4 Ash 10.2 Water 3.0 The corrected protein efficiency ratio is 3.2, and the pH of a 5% solution is 6.78.
The milk base was hcmogenized at 65C in a two stage homogenizex at 1900 psig in the first stage and 500 psig in the second stage. The temperature of the homogenized milk was increased to 91C in a plate heat exchanger and the hot milk kase was passed to a serpentine to provide a residence time of 8 minutes. The outlet temperature of the serpentine was 88C.
The pasteurized, thermally complexed milk base so formed was xapidly cooled in a plate cooler to 42C and held in a bulk tank equipped with a stirxex and heating/cooling coils.
EXAMPLE 3: Bulk fermentation.
100 kg of milk kase prepared as in Example 2 is inoculated by metexing 1.5 kg of each of a commercial yogurt culture having a bacterial ccunt of about 5.8 x 108 ml ccmprising-approximately e~ual quantities of Lactobacillus bulgaxicus and Streptococcus thermophilus, and a seFarately grown bulk staxter of Streptococcus F pxepar0d as in Example 1 `` 1 ~ 7~9.~

into the cooled nLIk base flow line. m e mixture is incubated at 42C in the fermentation tank without stirring until the pH
is reduced to 4.5 m e te~perature of the coagulum is then reduced by the internal cooling coils to about 38C, and the coagulum is Fumped using a positive pres Æ e ~mp having a large diameter piston at a rate not exceeding akout 80 strokes/minute so as to not rupture the microstructure of the coag~m unduly.
A milk plate is located in the discharge line of the pump to cool the coagulum to a temperature of akout 16C to about 24 & .
The discharge is received in Folystyrene distribution cups and chilled by refrigeration to a temperature within the range of about 5C to about 10C, and is permitted to set for some 24 hours before distribution. The set yogurt has a smocth, creamy consistency similar to that of sc~r cream, and is devoid of grainy texture characterizing y~gurt produced in accordance with more traditional metho s. The product was stored at temperatures in the range of abcut 5C to ahout lo& for 42 days. The texture of the yogurt remained essentially unchanged on storage, and no syneresis was noticeable. Changes in the pH and viscosity were observed as follcws:
Ti~e (days) 0 1 3 7 14 21 28 35 42 pH 4.30 4.20 4.20 4 lS 4.15 4.10 3.90 3.90 3.85 Viscosity C.P. 3940 3690 4025 4425 6830 7250 6840 5150 5010 The above viscogity measurements were made at 22C usin~
a Bkookfield Viscometer, spindle 4. The average of two readings made at 20 r.p.m. and 10 r.p.m. was taken for each measurement.

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EXAMPLE 4~
me procedure of Example 3 is generally followed, except that 4.0 kg of sucrose is added to the milk base with the serun milk protein. The cooled coagulum is p~mped into blending tanks and 12 kg of crushed strawberries are added and mixed at low shear rates using a slowly rotating paddle stirrer. The prepared yoglrt was packaged in re~ail cups and chillad as before. The fruit particles remained in suspension after storage at 5C 10C for 45 days. No syneresis was observed, and the texture of the product was generally unchanged over this period, EX~MPLE 5 The procedure of Example 3 was followed, except that 0.7 kg of a commerc;ally available bulk starter of Lactobacillus acidopholus are additionally added at the inoculation stage.
Similæ results were achie~ed~
EXAMPLE 6:
Sweetened, thermally treated and inoculated milk base prepared as described in Example 4 is inoculated with about 2.5%
by volume of each of mixed Lactobacillus culture and Streptococcus F culture, and is metered into 180 ml polystyrene cups containing 14 g of crushed apricots in the bottom thereof. The cups were ~aintained at a temperature of 42C until a pH of 4.7 was reached when the titratable acidity of the coagulum was 0.89% b~ weight, expressed as lactic acid. The cups were then cooled in a cold chamber equipped with air circulation fans to a temperature of 15 C to 18& , prior to storage in the range of 5 C - 10 C. No `` I ~ 7~
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syneresis or change in texture were apparent after storage within this temperature range for 45 days. The mild flavcur and relatively low acidity of the yogurt base enhanced the fruit flavour of the yoyurt.
. The above examples are for illustrative purposes only without way of limitation. It is to be understood that the Claims appended hereto are intended to cover changes and mcdifications which do not depart fron the spirit and scope of the invention.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Method for the production of yogurt comprising inoculating pasteurized milk with bacterial species Lactobacillus bulgaricus and Streptococcus F, and incubating said inoculated milk.

2. A method as defined in Claim 1 wherein said pasteurized milk is inoculated with about equal quantities of Lactobacillus bulgaricus and Streptococcus F.

3. A method as defined in Claim 1 wherein said incubating is carried out at about 42°C.

4. A method as defined in Claim 1, 2 or 3 wherein said milk is thermally treated in the presence of added serum protein prior to said inoculating.

5. A method as defined in Claim 1, 2 or 3 wherein prior to said inoculating serum protein is added to said milk to provide a serum protein nitrogen to casein protein nitrogen ratio in the range of about 1:3 to about 1:4.5, and the milk is then pasteurized at a temperature in the range of about 85°C to about 93°C for a time sufficient to produce a serum-casein complex.

6. A method as defined in Claim 1, 2 or 3 wherein said milk is thermally treated at a temperature in the range of about 88°C to about 91°C for a time in the range of about 5 minutes to about 10 minutes in the presence of added serum protein.

7. A method as defined in Claim l, 2 or 3 wherein said incubating is carried out until the pH of the product is in the range of about 4.3 to about 4.7.

8. A method for the production of yogurt comprising preparing a first bulk culture consisting essentially of Streptococcus F in milk and inoculating pasteurized milk with said bulk culture and a second bulk culture containing Lactobacillus bulgaricus, and incubating the so inoculated milk.

9. The method of Claim 8 wherein said milk is enriched with serum protein and thermally treated for a time and at a temperature to complex said serum protein with the casein protein of said milk so as to produce in said yogurt a soft curd.

10. Yogurt when produced by the process of Claim 1, 2 or 3 wherein prior to said inoculating serum protein is added to said milk to provide a serum protein nitrogen to casein protein nitrogen ratio in the range of about 1:3 to abut 1:4.5 and the milk is heated to a temperature in the range of about 85°C to about 93°C for a time to produce a serum-casein complex,

11. Yogurt when produced by the process of Claim 1, 2 or 3 wherein said milk is thermally treated at a temperature in the range of about 88°C to about 91°C for a time in the range of about 5 minutes to about 10 minutes in the presence of added serum protein so as to produce in said yogurt a soft curd.

12. Yogurt as defined in Claim 1, 2 or 3 wherein said milk contains added serum protein so as to provide a serum protein nitrogen to casein protein nitrogen ratio in the range of about 1:3 to about 1:4.5, and the serum protein is complexed with the casein protein so as to provide a soft curd in said yogurt.

13. Yogurt when produced by the process of Claim 8 or 9.

14. A thickened yogurt produced by the fermentation of milk with a yogurt producing lactobacillus and Streptococcus F.