Note: Descriptions are shown in the official language in which they were submitted.
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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
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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.
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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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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
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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
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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.
