| Abstract | Brački sir je punomasni, tvrdi ovčji sir te se kao i većina hrvatskih tradicijskih sireva
proizvodi od sirovog ovčjeg mlijeka, o čijem sastavu i sposobnosti zgrušavanja u najvećoj
mjeri ovisi i njegova kakvoća.
Stoga je cilj ovog istraživanja bio utvrditi: kemijski sastav, fizikalna svojstva i
higijensku kvalitetu sirovog ovčjeg mlijeka, proteolitičke i teksturne promjene bračkog sira
tijekom zrenja te povezanost navedenih promjena sa sastavom mlijeka.
Istraživanje je provedeno na 60 sireva proizvedenih u 15 proizvodnih šarži. Analiza
kemijskog sastava, fizikalnih svojstava i higijenske kvalitete mlijeka te kemijskog sastava
sira je provedena standardnim metodama. Sposobnost sinereze sirnog gruša je utvrđena
centrifugiranjem i izražena je kao udio otpuštene sirutke u odnosu na ukupnu masu
uzorka. Tvrdoća sirnog gruša je utvrđena pomoću cilindrične sonde i teksturnog
analizatora prema uputama proizvođača. Proteolitičke promjene tijekom zrenja bračkog
sira su utvrđene standardnim metodama dok su njegove teksturne značajke utvrđene
metodom analize teksturnog profila.
Provedenim istraživanjem je utvrđeno da je s povećanjem omjera kazeina i
mliječne masti u ovčjem mlijeku značajno (P<0,01) povećana lomljivost i čvrstoća bračkih
sireva dok se vrijednosti WSN (% TN), TCA-SN (% TN), ukupnih slobodnih aminokiselina,
Iγ/β i Iα nisu značajnije mijenjale. Statistički značajno (P<0,01) najveća vrijednost lomljivosti
i čvrstoće je utvrđena u bračkim sirevima proizvedenim od mlijeka s koncentracijom uree
u rasponu od 25,01 do 35,00 mg/100 g dok su vrijednosti teksturnih pokazatelja ispod
odnosno iznad navedenog raspona bile gotovo podjednake. Sirevi proizvedeni od mlijeka
s koncentracijom uree manjom od 25,00 mg/100 ml su imali značajno (P<0,05) najveću
vrijednost Iγ/β te utvrđenu sekundarnu proteolitičku aktivnost. Nastavak povećanja
koncentracije uree iznad 25,01 mg/100 ml u ovčjem mlijeku je značajno (P<0,05) smanjio
vrijednost navedenih pokazatelja u bračkom siru. Povećanje broja somatskih stanica u
ovčjem mlijeku je značajno (P<0,05) smanjilo vrijednosti Iα ali nije utjecalo na vrijednosti
Iγ/β u bračkom siru. Značajno (P<0,01; P<0,05) najveću lomljivost i čvrstoću ali i intenzitet
sekundarne proteolitičke aktivnosti je utvrđen u sirevima proizvedenim od mlijeka s brojem
somatskih stanica iznad 501.000/ml.
Rezultati ovog istraživanja pokazuju da je kakvoća sirovog ovčjeg mlijeka jedan od
ključnih čimbenika koji utječu na tijek razgradnje parakazeinske strukture bračkog sira kao
temeljnog biokemijskog procesa koji mijenja njegove teksturne značajke. |
| Abstract (english) | Introduction
Brač cheese is produced from raw sheep milk and its quality is mainly affected by
the composition of milk as in most of Croatian traditional cheeses. If good hygiene is
practised during milking of healthy sheep, wholeness and intact levels of milk constituents
(especially casein) are retained. In the coagulation process of milk, casein forms the
continuous and hard para-casein matrix which in relation to the other milk constituents
mostly occludes the milk fat. Hard para-casein matrix also keeps its functional properties
during syneresis process and represents basic substrate for proteolytic enzyme activity
during process of cheese ripening. Proteolytic changes during cheese ripening are
fundamental process which plays a vital role in development of characteristics of cheese
texture. Therefore, the motivation for this study is to evaluate effect of raw sheep milk
composition on textural curd characteristics and intensity of proteolytic and textural
changes during the ripening of Brač cheese.
Hypothesis and aims of the research
The hypothesis of the research is that raw sheep milk composition has effect on
curd syneresis and texture. The present research also aimed to assess the hypothesis
that proteolytic and textural changes during ripening of brač cheese depends on raw
sheep milk composition. Because casein network in cheese influence its mechanical
behaviour, it can be hypothesized that intensity of proteolysis in brač cheese changes its
textural characteristics during ripening.
The objective of this research is to determine:
• physicochemical composition and hygienic quality of raw sheep milk,
• texture and syneresis characteristics of curd as indicators of sheep milk
coagulation ability,
• effect of sheep milk composition on texture, syneresis ability and whey expulsion
of curd at four different times of coagulation,
• chemical, textural and proteolytic changes in Brač cheese during ripening,
• relationship between sheep milk composition and textural, proteolytic and
composition changes of Brač cheese during ripening,
• relationship between proteolytic pattern and texture of Brač cheese.
Materials and methods
The research was conducted on a family farms in the area of Pražnice and
Supetar on the Brač island. Milk samples were collected twice a month throughout the
different lactation periods (Supetar – 125 and Pražnice – 75 days in milking). Raw milk
samples were collected into a laboratory bottles, transported in a portable refrigerator to a
laboratory and stored at 4⁰ C until its coagulation within the next 24 hours. The milk
samples intended for physicochemical analysis were filled in 100 ml capacity polyethylene
laboratory bottles, preserved with azidiol and sent to the Reference Laboratory for Milk
and Dairy Products at the Faculty of Agriculture in Zagreb. Raw sheep milk was analysed
for fat, protein, casein, lactose, dry matter, solids-non-fat, urea, total and ionic calcium
content as well as somatic cell counts, total number of bacteria and acidity.
Milk was poured in a laboratory beaker and warmed in water bath at 32⁰ C. After
rennet addition milk was immediately transferred into eight glass beakers and conical
bottom centrifuge tubes. A hundred milliliters of milk for curd preparation was added in
each of eight glass beakers. Furthermore a thirty grams of milk for syneresis test was
weighed in each of sixteen centrifuge tubes. Each analysis was done in duplicate. Milk
samples were placed in water bath at 25°C and allowed to coagulate for 30, 45, 60 and 75
minutes in laboratory under climatized conditions at same temperature. Strength of
obtained curd in glass beakers were measured by texture analyser (Lloyd instruments,
model TA Plus) equipped with a 500 N load cell (model XLC -0500-A1) and cylindrical
probe (model FG/CY3, 12,5 mm in diameter). Syneresis was measured by centrifuging
the curd at 5000 RPM for 15 min. After centrifugation the free water was separated,
weighed and expressed as percentage of the total weight of milk (30 g).
Fifteen batches of Brač cheese were produced with four cheeses in each batch.
The cheeses were periodically sampled after 0, 30, 60, 90 and 120 days. Using a drill,
cheese samples (about 20 g) were taken from the lateral side of each cheese in order to
determine chemical composition and proteolytic changes of cheese during ripening period
of four months. Cheese samples for texture measurement were taken by cork borer
parallel to the direction of pressing of cheese between the edge of the top and bottom
surface. Cheese cores were cut into cylindrical samples (diameter=17 mm; height=25
mm). One part of the cheese, with core, was cut at a depth of 1 cm which was nearest the
surface, and was then replaced in the hole in the cheese. The hole was coated with
cheese wax, covered with plastic mash and finally sealed with cheese wax before the
cheese was replaced in the ripening room. Cheese were analysed for content of fat,
protein, dry matter and salt as well as pH value. Proteolysis in cheese was monitored by
polyacrylamide gel electrophoresis and determining content of water soluble nitrogen
(WSN) as a percentage of total nitrogen (% TN), trichloroacetic acid-soluble nitrogen
(TCA-SN) as a percentage of total nitrogen as well as free amino acid (as g of leucine/100
g dry matter). The texture profile analysis (TPA) was carried out using the texture
analyser (Lloyd instruments, model TA Plus). The textural characteristics of Brač cheese
were analysed for hardness 1, strain at hardnes 1, fracturability, strain at fracture,
hardness 2, adhesiveness, cohesiveness and springiness.
Descriptive statistics, analysis of variance (ANOVA) using a GLM procedure,
pairwise comparisons of mean values with Fisher’s LSD test, Pearson's correlation
coefficients and linear regression analysis were performed with SPSS (version 21).
Results and conclusions
The results showed that composition of sheep milk had a greater effect on curd
syneresis ability in comparision to its strength. Casein and fat content, casein to fat ratio
as well as urea concentration in sheep milk had no significant effect on curd strength.
Increasing of casein and fat content in sheep milk significantly (P<0.01; P<0.05) reduced
syneresis ability of curd while increasing casein to fat ratio significantly (P<0.01) enhanced
its syneresis ability. Enhanced curd strength and its syneresis ability were significantly
affected by increase of total (P<0.01; P<0.05) and ionic calcium (P<0.01) in sheep milk.
The increase of urea concentration in sheep milk had no significant effect on curd strength
but obtained curd showed significant reduction (P<0.01) in syneresis ability. Somatic cell
count in sheep milk significantly (P<0.05) affected curd strength but did not affect its
syneresis ability.
The increase of casein content in sheep milk significantly (P<0.01; P<0.05)
reduced the extent of secondary proteolysis in Brač cheese. The level of β-casein and its
ripening index were significantly (P<0.01; P<0.05) affected by increase of casein content
in sheep milk. The levels of the rest of caseins in Brač cheese did not show statistically
significant differences due to the increase of casein content in sheep milk. The research
showed that increase of casein content in sheep milk had significant effect (P<0.01;
P<0.05) on hardness 1 and hardness 2, fracturability and springiness of Brač cheese
while changes in adhesiveness, cohesiveness as well as strain at fracture and hardness 1
did not show statistically significant differences.
The content of TCA-SN (%TN) in Brač cheese was significantly (P<0.01) reduced
as a result of an increase of milk fat in sheep milk. The increase of the aforementioned
parameter in sheep milk also reduced the content of free amino acid and WSN (% TN) in
Brač cheese but without statistical significances. The increase of milk fat in sheep milk
significantly (P<0.05) reduced level of γ-caseins in Brač cheese while the levels of the rest
of caseins in Brač cheese did not show statistically significant differences. The
fracturability, hardness 1 and hardness 2 of Brač cheese significantly (P<0.01) decreased
with increase of milk fat in sheep milk. On the other hand, adhesiveness, cohesiveness as
well as strain at fracture and hardness 1 of Brač cheese were not affected by fat content
in sheep milk.
The extend of secondary proteolysis in Brač cheese was not changed regardless
of casein to fat ratio in sheep milk. The increase of casein to fat ratio in sheep milk caused
a significant increase in levels of β and γ-caseins in Brač cheese. Although level of αs1casein
in Brač cheese did not undergo statistically significant changes, level of αs1-I-
casein was significantly modified with increase of casein to fat ratio in sheep milk. The
fracturability, hardness 1 and hardness 2 of Brač cheese significantly (P<0.01) increased
as a result of an increase of casein to fat ratio in sheep milk. Increase of casein to fat ratio
significantly (P<0.05) affected strain at fracture of Brač cheese but did not affect its
adhesiveness, cohesiveness, springiness and strain at hardness 1.
The increase of urea concentration in sheep milk significantly (P<0.01) modified
levels of β-caseins but did not affect content of the rest of caseins in Brač cheese. The
extent of secondary proteolysis in Brač cheese was significantly (P<0.05) affected by the
increase of urea concentration in sheep milk. The fracturability, hardness 1 and hardness
2 of Brač cheese were significantly (P<0.01) modified with increase of urea concentration
in sheep milk. However, no significant differences were found between urea concentration
in sheep milk and adhesiveness, cohesiveness, springiness as well as strain at fracture
and hardness 1.
Extend of secondary proteolysis in Brač cheese were significantly (P<0.01;
P<0.05) affected by increase of somatic cell count in sheep milk. The levels of α and βcasein
significantly (P<0.05) increased with increasing somatic cell count in sheep milk
while changes in γ and αs1-I caseins did not show statistically significant differences. The
increase of somatic cell count content in sheep milk significantly increased fracturability
(P<0.05) and hardness 2 (P<0.01) as well as significantly reduced adhesiveness (P<0.01)
in Brač cheese. Changes in strain at fracture and hardness 1 as well as springiness were
also significantly (P<0.01; P<0.05) affected by increase of somatic cell count in sheep
milk. There were no significant differences in hardness 1 and cohesiveness in Brač
cheese due to increase of somatic cell count in sheep milk.
The content of TCA-SN (%TN) in Brač cheese was significantly (P<0.01) reduced
as a result of advancement of lactation. The stage of lactation had no significant effect
both on the content of free amino acid and WSN (% TN) in Brač cheese. The level of βcasein
in Brač cheese was significantly (P<0.05) reduced in Brač cheese with
advancement of lactation. The levels of the rest of caseins in Brač cheese did not show
significant differences due to stage of lactation. With advancement of lactation, the
cheeses became significantly (P<0.01) softer. The major consequences were a significant
(P<0.01) increase of fracturability, hardness 1 and hardness 2 with stage of lactation. The
other texture parameters were not affected by stage of lactation.
The levels of αs1 and β-casein were significantly (P<0.01; P<0.05) reduced and the
levels of its degradation products were significantly (P<0.01) increased during the 120-day
maturation period of Brač cheese. The extend of secondary proteolysis in Brač cheese
significantly (P<0.01) increased by increasing of its ripening time. On the other hand, Brač
cheese became significantly (P<0.01) harder, less springy and less cohesive. |
