Memilih istri yang baik adalah perkara yang sangat penting. Tidak ada seorang manusiapun yang berselisih mengenai hal ini. Akan tetapi, yang diperselisihkan adalah bagaimana pilihan itu dikategorikan baik, apa berdasar pada harta, keturunan, kecantikan atau agama ? Maka, penasehat yang terpercaya adalah Rasulullah SAW sebagai berikut :
"Seorang wanita dinikahi karena 4 hal: (1) hartanya, (2) Nasab, keturunannya, (3) Kecantikannya, dan (4) Agamanya. Maka pilihlah yang taat pada agama, niscaya kamu akan beruntung (HR Buchari).
Ketahuilah bahwa, sebaik-baik wanita adalah wanita yang solehah, sebagaimana sabda Nabi,
"Dunia adalah perhiasan dan sebaik-baik perhiasan adalah wanita solehah." (HR Muslim).
Wanita solehah adalah:
1. Wanita yang menyenangkan hati suaminya
2. Senantiasa mengajak untuk berbuat kebaikan
3. Melarang dari keburukan
4. Memotivasi suaminya untuk beribadah
5. Bersabar dalam segala keadaan
(Dikutip dari Syaamil Al-Quran, The Miracle).
Istri Salehah
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Sifat Fisik dan Mikrostruktur dari Kamaboko yang ditambahkan dengan Eksopolisakarida Lactobacillus bulgaricus
SIFAT FISIK DAN
MIKROSTRUKTUR DARI KAMABOKO YANG DITAMBAH DENGAN EKSOPOLISAKARIDA Lactobacillus
bulgaricus
(Physical Properties
and Microstucture of Kamaboko Added with Lactobacillus bulgaricus
Exopolysaccharide)
Ratmawati Malaka and
Metusalach
ABSTRACT
Exopolysaccharides (EPS) are bioactive compounds produced
by specific microbial that are exreted out the cell wall. Their applications in foods, especially in
milk products, have been studied. In
this experiments, the EPS extracted from L.
bulgaricus was used in kamaboko, which is a pasta fish product. Physical properties and microstructure of
kamaboko were used to determine the effects of EPS in this product. Physical properties evaluated were cooking
lost and tenderness, and the structure formed between the EPS and kamaboko
proteins was analyzed histologically.
Results showed that EPS improved the quality of the kamaboko by
decreasing the cooking lost and increasing the texture of the kamaboko as
confirmed by the microstructure properties.
Key words : exopolysaccharide, kamaboko, microstructure, physical properties, Lactobacillus bulgaricus
ABSTRAK
Eksopolisakarida adalah produk bioaktif yang dihasilkan
oleh mikroba tertentu yang dieksresikan ke luar sel. Aplikasi pada berbagai produk pangan telah
pernah dicoba khususnya pada produk susu.
Pada penelitian ini aplikasi EPS dari L. bulgaricus dilakukan pada produk ikan yaitu kamaboko yang berupa
produk pasta ikan. Untuk melihat
bagaimana pengaruh EPS pada produk ikan ini, dilakukan pengujian fisik dan
mikrostruktur. Pengujian fisik yang
diukur adalah susut masak dan keempukan, dan kemudian reaksi yang terjadi
antara EPS dan protein kamaboko dilihat dengan pembentukan struktur dengan
membuat preparat histology. Hasilnya
menunjukkan bahwa terjadi peningkatan kualitas dengan terjadinya penurunan
susut masak dan peningkatan tekstur yang dibuktikan dengan penampakan
mikrostruktur.
Kata kunci : eksopolisakarida,
kamaboko, mikrostruktur, sifat fisik, Lactobacillus
bulgaricus
PENDAHULUAN
Eksopolisakarida (EPS) yang
dihasilkan oleh L. bulgaricus merupakan hidrokoloid heteropolisakarida
yang mampu membentuk gel ketika dipanaskan.
Aplikasi EPS bakteri seperti curdlan, dekstran, pullulans, gum xantan
pada produk pangan telah dilaporkan oleh sejumlah peneliti (Harada et al.,
1991; Nakao et al. 1991; Malaka, 1997; Broadbent et al., 2003).
Aplikasi EPS pada produk ikan belum banyak diteliti.
Secara biokimia, struktur dan fungsi
otot ikan serupa dengan hewan darat,
tetapi dalam proses pengolahan, myosin daging ikan kurang stabil
dibandingkan dengan myosin pada daging hewan darat khususnya mamalia (Bechtel,
1986). Hal ini disebabkan sifat
miofibril ikan khususnya daya ikat air yang rendah sehingga mudah kehilangan
daya emulsi dan kemampuan membentuk gel.
Kamaboko merupakan salah satu produk
daging ikan yang dihaluskan dan berbentuk pasta. Produk ini populer di Jepang yang semakin
berkembang dari tahun ke tahun antara lain, Chikuwa (kamaboko berbentuk
silinder), satsumage (kamaboko berbentuk lembaran) dan sosis kamaboko
(Pigott dan Turker, 1990).
Sifat fungsional kamaboko dapat
ditingkatkan dengan menambahkan bahan pengikat (binders) dari bahan hidrokoloid
yang umumnya berupa polisakarida. Oleh
sebab iru pada penelitian ini dicoba digunakan EPS dari L. bulgaricus
yang telah pernah digunakan pada produk susu, untuk melihat pengaruhnya
terhadap sifat fisik dan pembentukan
struktur pada kamaboko.
METODE
PENELITIAN
Pembuatan Kamaboko
Surimi dibuat dari ikan teri putih
segar yang diambil dari Tempat Pelelangan Ikan Paotere, Makassar. Surimi dibuat berdasarkan formula Pigott dan
Tucker (1990) yaitu ikan dicuci dengan air kran. Kepala beserta isi perutnya dibuang dan
dicuci kembali sampai bersih. Selama
pencucian ditambahkan garam 0,3% untuk meningkatkan kelarutan protein
miofibril. Ikan kemudian digrinder dan
ditambahkan garam 0,3% dan es batu 10% kemudian dicuci dengan air sebanyak tiga
kali. Surimi kemudian dapat disimpan
beku dan dibuat produk ikan berbasis surimi, seperti kamaboko dan sosis.
Kamaboko kemudian dibuat dari surimi
dengan ditambahkan tepung jagung sebanyak 30% dan 0,3% merica dan bawang
putih. EPS ditambahkan dengan
konsentrasi 0%, 0,5% dan 1,0%; kemudian dibuat pasta dan dicetak berbentuk
selinder, lalu dikukus.
Rancangan dan Desain Penelitian
Rancangan penelitian yang diterapkan
adalah Rancangan Acak Lengkap. Variabel bebas yang diuji adalah konsentrasi EPS
yang ditambahkan pada produk ikan, yaitu :
K1 = 0,0%
K2 = 0,5 %
K3 = 1,0%
Variabel
tak bebas yang diuji adalah sifat fisik yaitu daya putus (keempukan), dan daya
ikat air dengan mengukur susut masak.
Disamping itu untuk melihat pembentukan struktur reaksi antara protein
dan EPS maka dibuat preparat histologi dengan pewarnaan Haematoxilin-Eosin dan
diamati perubahan strukturnya menggunakan mikroskop dengan pembesaran 400 kali.
Pengukuran susut masak
Susut masak daging adalah perbedaan
antara bobot daging sebelum dan sesudah dimasak yang dinyatakan dalam
persentase (%). Metode pemasakan dalam
air dilakukan dengan menggunakan metode Bouton et al. (1976) dalam
Soeparno (1994). Untuk perhitungan berat
yang hilang selama pemasakan (cooking lost) digunakan rumus sebagai
berikut :
% Susut Masak
= 
x 100%
x 100%
Pengukuran
daya putus /keempukan (Creuzot dan Dumont 1983 dalam Abustam, 1993).
Keempukan kamaboko diukur dengan
menggunakan CD-Shear Force yang dinyatakan dalam kg/cm2. Sampel dipotong dengan luas penampang 1 cm2. Sampel kemudian dimasukkan ke dalam lubang
CD-shear Force. Nilai skala CD-Shear
Force dimasukkan dalam rumus sebagai berikut :
A = 
Keterangan: A =
Nilai daya putus (kg/cm2)
A’
= Tenaga yang digunakan (kg)
r =
Jari-jari pada lubang CD-Shear Force (0,635 cm)
p = 3,14
Analisa Data
Data
dianalisa dengan analisis ragam dan BNT dengan menggunakan program SPSS. Model analisis matematik yang digunakan
adalah :
Yij
= m
+bi
+ eij
Dimana :
Yij = Nilai
pengamatan pada satuan percobaan ke-j dari perlakuan ke-i (taraf ke i dari
faktor konsentrasi EPS)
m = Nilai tengah perlakuan
bi
= Pengaruh perlakuan ke-i dari (faktor konsentrasi EPS) terhadap susut masak
dan keempukan.
eij
= Pengaruh galat penelitian pada
pengamatan ke-j yang mendapatkan perlakuan ke-i
Mikrostruktur
dijelaskan secara deskriftif yang disertai dengan tampilan gambar-gambar.
HASIL DAN PEMBAHASAN
Susut masak
Gambar 1 menunjukkan persentase
susut masak dari kamaboko, yang semakin menurun dengan meningkatnya konsentrasi
EPS. Pada penambahan 1% EPS susut masak
mendekati nol dalam artian hanya sedikit sekali terjadi susut masak. Hal ini menunjukkan bahwa terjadi kenaikan
kemampuan dari matriks kamaboko dalam menahan air sehingga kehilangan air dari
matriks menjadi rendah. Pengaruh
pencampuran komponen saat pembuatan kamaboko menyebabkan terjadinya interaksi
yang saling menunjang dalam pembentukan matriks protein-polisakarida melalui
proses pemanasan pada suhu 80oC (Sobstad, 1988). EPS dapat berfungsi sebagai bahan pengental (viscosifying agents), bahan
penstabil (stabilizers), pengemulsi (emulsifiers), bahan
pembentuk gel (gelling agents) dan bahan pengikat air (water-binding
agents). EPS BAL telah
dipertimbangkan sebagai bahan pengikat (thickening agents) generasi baru pada
produk pangan, EPS mampu meningkatkan
sifat fisik dan rheologi produk dengan hanya menggunakan konsentrasi yang
sangat rendah (Yang, 1999).
Gambar 1. Daya putus (kg/cm2) dan susut
masak (%) kamaboko dengan penambahan EPS konsentrasi 0,0, 0.5 dan 1%. SmKAM = Susut Masak Kamaboko, DPKAM = Daya
Putus Kamaboko.
Analisis
ragam menunjukkan bahwa konsentrasi EPS berpengaruh nyata terhadap susut masak. Analisa BNT menunjukkan perbedaan yang nyata
dari masing-masing konsentrasi terhadap susut masak. Hal ini memberikan indikasi bahwa pengunaan
EPS dalam pembuatan kamaboko akan meningkatkan kualitas kamaboko dilihat dari
susut masak. Sifat fungsional daging
ikan seperti kemampuan membentuk emulsi dan kemampuan mengikat air secara
ektensif dikembangkan pada pembuatan
produk ikan seperti kamaboko dan jenis sosis ikan yang mulai popular di Jepang
dan Amerika Serikat (Bechtel,
1986). Kesuksesan dalam pembuatan
kamaboko sangat berhubungan dengan sifat miofibril ikan, khususnya daya ikat
air, dimana pada umumnya daging ikan memiliki kemampuan daya ikat air sangat
rendah, mudah kehilangan daya emulsi dan kemampuan membentuk gel.
Kamaboko
dari ikan teri tidak dipisahkan dari tulang sehingga diharapkan mengandung
cukup kalsium untuk komsumsi mulai anak-anak hingga orang tua. Kamaboko merupakan salah satu produk daging
ikan cincang yang perlu dipelajari untuk meningkatkan kualitasnya.
Di
Jepang, umumnya dibuat dari ikan-ikan kualitas yang tidak cukup baik untuk
dijual dalam keadaan segar, salah satunya adalah ikan teri. Produk ini berbasis surimi yaitu ikan yang
dihaluskan dan dicuci air untuk mengeluarkan bau ikan dan protein larut
air. Produk-produk lainnya yang berbasis
surimi semakin berkembang dari tahun ke tahun antara lain chikuwa (kamaboko
berbentuk silinder), satsumage (kamaboko berbentuk lembaran) atau sosis
kamaboko (Pigott dan Turker, 1990).
Daya Putus (Keempukan)
Daya
putus yang menunjukkan keempukan kamaboko dapat dilihat pada Gambar 1. Semakin tinggi konsentrasi EPS maka kamaboko
semakin kurang empuk. Dalam pengukuran
kualitas suatu produk, ukuran keempukan adalah bersifat relatif. Produk yang terlalu lembek kadang tidak
begitu disukai konsumen, tetapi produk yang lebih kompak lebih disukai. Hal ini
memberikan indikasi bahwa EPS dapat meningkatkan kekompakan atau kekenyalan
produk kamaboko.
Hasil
analisis ragam menunjukkan bahwa konsentrasi EPS berpengaruh nyata terhadap
daya putus kamaboko, dan uji BNT
menunjukkan bahwa terdapat perbedaan yang nyata antara masing-masing
konsentrasi EPS. Penambahan EPS konsentrasi
0,5% telah meningkatkan nilai daya putus yang menggambarkan peningkatan
kekenyalan dari produk ikan. Hasil
tersebut memberikan indikasi kemungkinan terjadinya interaksi antara miofibril
ikan-polisakarida maizena dan EPS yang menyebabkan terjadinya peningkatan
struktur matriks kamaboko.
Mikrostruktur
Gambar 3 menunjukkan mikrostruktur kamaboko yang
memberikan illustrasi bahwa penambahan EPS menyebabkan peningkatan tekstur
dengan terbentuknya struktur yang lebih kompak.
Mikrostruktur ini memperjelas hasil yang diperoleh dari pengukuran daya
putus dan susut masak. Daging halus yang
baik adalah jika lubang-lubang yang terbentuk mempunyai diameter kurang dari 5
– 10 mm, dan pada pasta yang dihomogenkan sebaiknya lubang-lubang kurang dari 1
mm. Untuk meningkatkan tekstrur maka
dapat digunakan bahan pengikat seperti tepung pati, albumin, gula, atau protein
tanaman (Pigott dan Tucker, 1990).
Penambahan bahan-bahan ini selain dapat meningkatkan sifat-sifat daya
ikat air, juga dapat meningkatkan cita rasa.
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Gambar
2. Mikrostruktur kamaboko dengan
penambahan EPS (pembesaran 400x)
Amano
(1992) menyatakan bahwa untuk membuat struktur jenis sosis atau kamaboko
menjadi elastis dan kompak maka dapat ditambahkan bahan yang dapat membuat
rantai molekul myosin menjadi panjang
sehingga dapat menahan air. Jika tekstur
tidak kompak dan terlalu lembek maka eksudat air dari produk selama penyimpanan
akan membentuk lekukan-lekukan air kecil yang cocok untuk pertumbuhan
bakteri. Kamaboko yang berkualitas baik
akan membentuk jaringan myosin yang kompak.
Ada tiga teori
pembentukan ikatan dalam matriks gel pada produk pangan. Yang pertama adalah struktur terorganisasi
dalam situasi seimbang yaitu energi potensial dalam sistim adalah minimum. Pada sistim hidrokoloid dengan molekul
padatan yang tinggi, maka kekuatan tolakan elektrostatik diantara molekul yang
berdekatan dapat diminimalisasi jika molekul mempunyai konfigurasi
tertentu. Yang kedua adalah pembentukan
ikatan kovalen yaitu kation divalensi secara efektif membentuk jembatan antara dua kelompok anion
yang berdekatan. Yang ketiga adalah
pembentukan ikatan hydrogen yaitu ion hydrogen bebas dalam air mengikat kelompok
molekul yang mempunyai rantai terbuka misalnya kelompok karboksil pada molekul
yang berdekatan (Printice, 1990).
Pembentukan struktur kamaboko dengan penambahan EPS kelihatannya berada
pada teori yang ketiga. Hal ini disebabkan karena pada umumnya
polisakarida mampu mengikat sejumlah besar air (Aberle et al,
2001).
KESIMPULAN
Eksopolisakarida L. bulgaricus dapat
meningkatkan kualitas kamaboko yang ditandai dengan peningkatan kualitas
fisiknya yaitu peningkatan daya ikat air dan tekstur. Penambahan EPS menyebabkan terjadinya
peningkatan jaring-jaring matriks kamaboko yang lebih kompak.
UCAPAN TERIMA KASIH
Terima kasih atas dukungan dana
dari DIKTI melalui Proyek Hibah Bersaing.
Terima kasih kami sampaikan pula kepada
Budi dan Sapariyadi atas bantuannya di laboratorium, dan kepada Bapak
Prof. Herry Sonjaya atas bantuannya membaca manuscript ini.
DAFTAR PUSTAKA
Aberle, E.D., J.C. Forrest, D.E. Gerrard, E.W. Mills, H.B. Hedrick,
M.D. Judge, R.A. Merkel. 2001. Principles of Meat Science. Fourth Edition. Kendall/ Hunt Publishing Company. Iowa.
Abustam, E. 1993. Peranan Maturasi (Aging) terhadap Mutu Daging
Sapi Bali yang Dipelihara Intensif dan dengan Penggemukan. Laporan Hasil Penelitian. Fakultas Peternakan, Universitas Hasanuddin, Makassar.
Amano, K. 1992. Fish Sausage Processing. In Seafood Science
and Technology. Fishing News Books. Cambridge.
Bechtel, P.J. 1986. Muscle as Food. Academic Press, Inc. New
York.
Broadbent, J.R., D.J. McMahon, D.L. Welker, C.J. Oberg, and S.
Monineau. 2003. Biochemistry, genetics, and applications of
exopolysaccharide production in Streptococcus thermophilus: a review. J. Dairy Science 86: 407 – 423. Http://jds.fass.org/cgi/content/full/82/2/407. Diakses 9 Oktober 2004.
Harada, T., Y. Kanzawa, K. Kanenaga, A. Koreeda, and A. Harada. 1991.
Electron microscopic studies on the ultrastructure of curdlan and other
polysaccharides in gels used in foods.
Food Structure 10: 1-18.
Malaka, R. 1997. Effects of Curdlan, a Bacteria Polysaccharide
on the Physical Properties and Microstructure of Acid Milk Curd by Lactic Acid
Fermentation. Master thesis. Faculty of Agriculture, Miyazaki
University. Japan.
Nakao, Y., A. Konno, T. Taguchi, T. Tawada, H. Kasai, J. Toda, and M.
Terasaki. 1991. Curdlan: Properties and application to
foods. J. of Food Science 56 (3): 771 –
776.
Pigott, G.M. and B.W. Tucker.
1990. Seafood: Effects of Technology on Nutrition. Marcell Dekker, Inc. New
York.
Printice, J.H. 1990. Measurements in the Rheology of
Foodstuffs. Elsevier Applied Science
Publishers. London.
Sobstad, G.E. 1988. Thermal Processing, Evaporation and Drying of
Fish-Meat Product. In Physical,
Chemical and Biological Changes by Thermal Processing. Applied Science Publishers Limited. London.
Soeparno. 1994. Ilmu dan Teknologi Daging. Cetakan kedua. Gadjah
Mada University
Press. Yokyakarta.
Diterbitkan di Jurnal terakreditasi Torani 2006
Rheological Properties and Microstructure of Acid Milk Curd by Curdlan Addition, a Polysaccharide from Bacteria
Rheological
Properties and microstructure of Acid Milk
Curd by Curdlan Addition, a
Polysaccharide from Bacteria
(Sifat-Sifat Reologi dan Mikrostruktur Curd Susu Asam
dengan Penambahan Curdlan, Polisakarida Bakteri)
Abstrak
Dalam industri susu, penggunaan
polisakarida yang diproduksi bakteri adalah penting dalam peningkatan bentuk
dan tekstur produk akhir. Curdlan
ditambahkan dalam makanan untuk meningkatkan kualitasnya dan juga digunakan
untuk membuat makanan baru. Tujuan penelitian ini adalah untuk mengetahui
sifat-sifat reologi dan mikrostruktur curd susu asam dengan penambahan
curdlan yang difermentasi dengan bakteri asam laktat. Curd susu asam dibuat dari susu skim
rekonstitusi 10%, ditambahkan dengan 0 – 1% curdlan, kemudian dipanaskan pada
suhu 85oC selama 30 detik, diinokulasi dengan 1% Lb. delbrueckii
subsp. bulgaricus B-5b dan diinkubasi pada 37oC selama 16
jam. Viskositas, pH, hardness dan
breaking energy curd susu asam yang mengandung curdlan meningkat dengan
meningkatnya konsentrasi curdlan. Pada
kontrol curd susu asam tanpa curdlan hanya kasein misel dan Lb. delbrueckii
subsp. bulgaricus B-5b yang terdapat dalam mikrostruktur gel. Secara umum, sedikit meningkat derajat ikatan
kasein misel menjadi rantai dan kluster dengan adanya curdlan dibandingkan
dengan curd susu asam kontrol. Curdlan
saat berinteraksi dengan kasein misel, kelihatannya seperti massa benang halus
yang bergabung seperti tali antara satu kasein dengan kasein lainnya, dan/atau
sebagai lapisan massa benang halus pada permukaan kluster kasein misel.
Kata Kunci:
Polisakarida Bakteri, Curdlan, Curd Susu Asam, Sifat-Sifat Reologi,
Mikrostruktur.
Abstract
In dairy industry, the use of
polysaccharide producing bacteria is of interest with respect to improvement of
body and texture of milk product. Curdlan is added to foods to improve their
properties and is also used to make new foods.
The objective of this study was to investigate the rheological
properties and microstructure of acid milk curd with curdlan addition,
fermented by lactic acid bacteria. The acid milk curd was made from 10%
reconstituted skim milk (RSM), added with 0 - 1% of curdlan, heated at 85ºC for
30 sec, inoculated with 1% of Lb.
delbrueckii subsp. bulgaricus
B-5b, and incubated at 37ºC for 16 h.
The viscosity, hardness and breaking energy of acid milk curd containing
curdlan increased with increasing curdlan concentration. In a control acid milk curd containing no
curdlan (0% of curdlan), only casein micelles and Lb. Delbrueckii subsp. bulgaricus
B-5b composed the gel microstructure.
In general, there was somewhat higher degree of linking casein micelles
into chains and clusters in the presence of curdlan than in control acid milk
curd. When curdlan interacted with casein micelles they appeared as fluffy mass
joined by string between one-casein and the other, and/or a fluffy mass film on
the surface of clusters of casein micelles.
Key
word : Microbial polysaccharide curdlan, acid milk curd, rheological properties, Microstructure.
Introduction
There have been many investigations
involving optimization of milk curd texture. These studies have demonstrated
that the total solid and fat levels in the milk, heat treatment of the milk
prior to inoculation, homogenization, incubation conditions, and handling of
the ripened coagulum will all affect the body of the final milk product.
Another major way to affect the body yogurt is through the addition of
stabilizers such as gelatin, pectin or another polysaccharides. Stabilizers are
added to the product to increase viscosity as well as to decrease
susceptibility to syneresis (Schellhaass and Morris, 1985).
In dairy industry, the use of
polysaccharide producing bacteria is of interest with respect to the
improvement of body and texture of yogurt, in particularly in France and
Netherlands where addition of plant or animal stabilizers is prohibited.
Curdlan is extracellular slime
polysaccharide of Alcaligenes faecalis var.
myxogenes strain 10C3 where found by
Harada for the first time at 1965. Harada was isolated, purified this
polysaccharide and he concluded that curdlan is shown to contain about 10%
succinic acid, 70 - 80% glucose, and small amounts of galactose and mannose and
it seems to have beta-glycosidic linkages (Takahashi et al., 1986). Curdlan is added to foods to improve their
properties and is also used to make newfoods (Harada, 1992).
Although polysaccharide has been widely investigated, little information
exists on how effect polysaccharide in reconstituted skim milk fermented by
lactic acid bacteria. This study determined the influence of curdlan on the
rheological properties and microstructure of acid milk curd by lactic acid
bacteria fermentation.
Materials and Methods
Lactic
acid bacteria
Lb. delbrueckii subsp.
bulgaricus B-5b were obtained
from Japan Milk Product Technology Association, Tokyo, Japan. During the course
of the investigation the culture were routinely propagated in 10% RSM. The RSM
was autoclaved at 121ºC for 15 min. and tempered to 37ºC prior to
inoculation. A 0.1% inoculum was added
to the RSM and the culture was allowed to incubate at 37ºC over night.
Preparation of acid milk curd
Acid milk curd was made from 10%
RSM. These milks
were added with curdlan (Takeda Chemical Industries Ltd., Osaka, Japan) with
different concentration (0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%), heated at 85ºC for
30 sec., cooled to 37ºC, inoculated with 1%(v/v) Lb. delbrueckii subsp. bulgaricus
B-5b, and incubated at 37ºC for 16 h.
Rheometric properties
Curdlan added acid milk curds were studied
toward the viscosity by using a viscometer (Tokimec Inc., Visconic
ED-model). Steady shear rate of 1-100/s
along with a MK 50 rotor assembly and NV sensor system operating at 25ºC,
viscosity was expressed as millipascals per sec.
The hardness of acid milk curd was measured with a Sanwariken JK-T type
rheometer (Sanwariken Ltd., Tokyo) as a curd knife cut the surface of milk
curd. These measurement conditions were 1.44mm/sec in penetration speed of the
curd knife, 18 cm/min in chart speed and 0.1V in sensitivity of a recorder.
Breaking energy of acid milk curd
(stress x strain) was estimated on the basis of curd harness, cross section of
the curd knife (0.1963 cm), length of milk curd (4.9363 cm), and penetration
depth of the knife (0.4807 cm). Elastic modulus, stress/strain was a measure of
elasticity in the instance of solid matter. The detailed calculation of
breaking energy and elastic modulus for milk curd was described previously
(Ohashi et al., 1983).
From the chart, value for X and Y
were determined on the methods described by Ohashi et al., 1978), using the following equations and conditions.
Hardness = dyn
Breaking energy (dyn/cm2) =
(G/A) x (a/L)
Elastic Modulus (dyn/cm2) =
(G/A) / (a/L)
Where:
G = Hardness (dyn), A = Transversal area of knife,
a = Knife penetration, L = Height of sample.
Surface
structure of Lb. delbrueckii subsp. bulgaricus B-5b
Appropriate dilutions of overnight Lb. bulgaricus subsp. Bulgaricus B-5b were spread on 5% skim
milk agar plates with composition as follows:
Skim milk 50.0 g Yeast extracts 2.5 g
Peptone 5.0 g Glucose 1.0 g
Agar 15.0 g
These plates were incubated for
24-70 h at 37ºC. A 2-4 mm cube of the
agar with a colony on it was cut from each plate. The sample were fixed in 2.5% glutaraldehyde
solution for 24 h and post-fixed in 1% osmium tetraoxide solution for 5 h. The samples were then soaked in a series of
ethanol distilled water solutions (50, 60, 70, 80, 90, 95, 99.5% (v/v) ethanol)
as intermediate fluid. Samples were allowed to stay for 10 min in each
concentration, dried in a Hitachi HCP-2 type critical point drier (Hitachi
Ltd., Tokyo), coated with gold in a Hitachi E-1030 type sputtercoater ((Hitachi
Ltd., Tokyo), and examined with a Hitachi S-4100 type scanning electron
microscope (SEM, Hitachi Ltd., Tokyo) at an accelerating voltage of 1.0 kV.
microstructure of milk curd
A template was made by gluing 4x10
mm glass rods to the inside surface of a petridish cover. A 3% agar solution (60ºC) was pored 13 mm
deep into the petridish. The template
was then placed into the agar solution.
The template was removed after the agar had solidified, which resulted
in the formation of cylindrical pores in the agar. The coagulated milk curd was
then pipeted into the pores. The surface
was overlaid with 3% agar, which had been tempered to 45ºC. After the agar overlay had solidified, 6 mm
cubes containing a single cylindrical pore of coagulated milk, were cut out of
the agar. The agar cubes were fixed in
2.5% glutaraldehyde solution buffered at pH 7.0 with 0.1 M phosphate buffer,
and then post-fixed in 1% osmium tetraoxide solution. Samples were dehydrated in a graded alcohol
series as described above, and then dried in a Hitachi HCP-2 type critical
point drying apparatus (Hitachi Ltd., Tokyo), coated with gold in a Hitachi JFC-1
type, ion type sputter coater (Hitachi Ltd., Tokyo), and viewed in a Hitachi
S-4100 type scanning electron microscope (Hitachi Ltd., Tokyo) at an
accelerating voltage 1.0 kV.
Results and Discussion
Rheological properties
Four kinds of rheological characterization
have been examined in the experiment is viscosity, hardness, breaking energy
and elastic modulus. However, there are many reason concerning with food
quality control.
The effect of curdlan concentration
on the apparent viscosity of milk curd was determined as shown in the fig. 1. Curdlan have two types of gels,
'low set gel' if its heated at 60ºC and
'high set gel' if its heated at 95ºC (Takahashi et al., 1986) or
above 80ºC (Harada et al., 1991).
We used heating at 85ºC for 15 sec.
The viscosity of acid milk curd containing curdlan increased with
increasing curdlan concentration. Viscosity of acid milk curd on addition of
curdlan (0-1%) and heated at 85ºC for 15 sec followed by inoculation with 1% Lb. delbrueckii subsp. Bulgaricus B-5b and incubation at 37ºC
for 16 h, increased from 64.7 to 342.2 mpa/sec. The viscosity increased slowly
between 0% and 0.4% concentration of curdlan, and increased linearly with an
elevation of concentration above 0.4% curdlan concentration. The increasing viscosity may be due to the
fact that some casein, particularlyβ-casein,
start to dissociate from the micelle, and dissolved casein molecules have a
higher hydrodynamic volume. The addition of curdlan probably affected the
hydrodynamic volume of casein micelle and acid milk curd formation. The use of
curdlan in acid milk curd/yogurt increases the apparent viscosity and pH,
however increasing pH is undesirable in yogurt making. In yogurt, lactic acid produced by the
bacterial culture lowers the pH below the iso-electric point and induced
coagulum of casein (Harwalkar and Kalab, 1986).
In the acid milk curd heated at
85ºC, hardness and breaking energy increased with increasing curdlan
concentration (%). The breaking energy and hardness were not significant
recognized between not-supplemented acid milk curd samples (0% of curdlan) and
those with 0.2% curdlan concentration, and increased linearly with an elevation
of concentration above 0.4% curdlan concentration. This result can be explained in the microstructure
of acid milk curd. The size of clusters of casein micelle was increased with
increasing curdlan concentration, and curdlan added-acid milk curd, the pore
dimensions are diminished, and the density of the matrix was increased. The
individual casein may self associate or form associations with other fractions
through hydrophobic or electrostatic interactions.
Elastic modulus was demonstrated polynomial chart with increasing
curdlan concentration. Compared with
acid milk curd with 0% curdlan (no addition curdlan), in the acid milk with 0.2%
curdlan, elastic modulus decreased until 0.87x105 dyn/cm2,
from 2.58x105 dyn/cm2.
Elastic modulus in acid milk curd with 0.4% of curdlan increased slowly
from 0.76 to 2.50 x 105 dyn/cm in the acid milk curd with 1% of
curdlan.
Microstructure of acid milk curd
Scanning electron micrographs of the Lb.
Delbrueckii subsp. Bulgaricus
B-5b demonstrated that the surface appendages (slimy) is not present on the
cell surface, indicated that the bacteria is non-ropy lactic acid bacteria.
Thickening agents are used to improve the texture, increase the
firmness, and prevent syneresis in yogurt.
This is important to help maintain good textural and visual properties
during transportation and storage.
The application of SEM to explain rheological behavior when studying
exopolysaccharide-producing bacteria was used to help understand the mechanism
and influence the physical properties.
One of the reasons may be the
relative difficulty of subjecting acid milk curd/yogurt to electron microscopy,
particularly to scanning, because the fine yogurt network is very susceptible
to electron beam damage (Kalab and Emmons, 1975). Fixation, washing, and subsequent drying
preserved the spatial configuration of the protein in the initial network. Electron microscopy shows that the casein
micelles were fused into chains and clusters, yet essentially retaining their
globular shape in spite of the presence of bacteria possessing some proteolytic
activity. The arrangement of the micelles and the formation of a protein skeleton
created large free spaces inside the network, which are best seen under a
scanning electron microscope. The casein micelles are easily distinguishable
from lactic bacteria at magnifications over 2000x (Kalab and Emmons, 1975).
It has already been mentioned that
in yogurt, casein particle chains are linked at random and firm a matrix with
relatively uniform pores (Harwalkar and Kalab, 1986) filled with the liquid
phase (whey). The milk curd containing
curdlan, the pore dimensions are diminished, and the density of the matrix is
increased. In heated induced milk gels
with high concentrations of milk proteins (14-20%), the protein network
consisted of casein micelles either connected by short bridges or fused into
long chains a clusters (Kalab and Emmons, 1975).
Electron microscopy showed that
yogurt consists of a protein matrix composed of chained and clustered casein
particles. Chains are common in yogurt made from milk, which had been preheated
to a minimum of 85ºC whereas large clusters of casein particles from the matrix
of yogurt made from heated milk. Such a matrix is characterized by interstitial
spaces (pores), the dimensions of which depend on the protein contention that
matrix. The casein micelles in milk
started to disintegrate as the pH of the milk reached 5.5 due to the production
of lactic acid by the bacterial culture.
The disintegration was most extensive at pH 4.8 but the proteins
reaggregated into globular particles as the pH value was further decreased to
4.8 and lower (Harwalkar and Kalab, 1986). Yogurt made from heated skim milk,
changes were not particularly conspicuous: individual micelles lost their sharp
and smooth outlines and became ragged, grew somewhat in size and fused together
into clusters and chains (Kalab and Emmons, 1975).
In a control acid milk curd
containing no curdlan, only casein micelles and lactic acid bacteria (Lb. delbrueckii subsp. bulgaricus B-5b) composed the gel
microstructure (Fig. 2A). In general, there was a slightly higher degree among
linking casein micelles into chains and clusters in the presence of curdlan
than in control acid milk curd (Fig. 2B-2F).
The lack of differences was attributed to the low curdlan concentration
(0, 0.2, 0.4, 0.6, 0.8 and 1.0%) in the acid milk curd. Kalab and Emmons (1975) found that in yogurt
contain gelatin may be visible to electron microscopy in yogurt containing 2%
and 10% gelatin. Teggazt and Morris (1990) suggested that in ropy cultures, the
EPS in attached to the bacterial cell surface and also interacts with the
casein. When curdlan interacted with casein micelle they appeared as fluffy
mass join by string between one-casein micelles and other casein micelles,
and/or fluffy mass film in the surface of cluster of casein micelles. In acid milk curd with 0.6% curdlan
concentration, string fluffy mass film were lower than acid milk curd with 1%
curdlan concentration.
Conclusion
The
result from this investigation can be concluded that curdlan increased
rheological properties of acid milk curd.
Scanning electron microscope of acid milk curd demonstrated that curdlan
improve degree among linking casein micelles into chains and clusters in
microstructure of acid milk curd.
Acknowledgments
We
would like to thank Professor Tomio OHASHI and Professor Kiyoshi YAMAUCHI for
supervision. We are grateful for the financial support Takeda Chemical Ind.,
Ltd., OSAKA for providing the samples of curdlan used in the experiments and
Laboratory of Biochemistry and Technology of Miyazaki University, Japan.
References
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Takahashi F., Harada T., Koreeda A. and A. Harada.
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Figure 1. Relationships of curdlan concentration to
the viscosity, harness, breaking energy and elastic modulus of acid milk curd,
fermented by Lb. Delbruckii subsp. Bulgaricus B-5B at 37 ºC for
16 hours
A B
C D
E F
Figure 2. Microstructure of acid milk curd (10% skim
milk). A) 0% of curdlan (control), B) added 2% of curdlan, C) added 0.4% of curdlan, D) added 0.6% of curdlan, E) added 0.8% of curdlan, F) added 1.0% of curdlan, a) Lb.
Delbrueckii subsp. Bulgaricus
B-5b, b) casein micelles
Figure 3.
Microstructure of acid milk curd (10% reconstituted skim milk) with
addition of 0.6% curdlan (18,000x).
Figure 4.
Microstructure of acid milk curd (10% reconstituted skim milk) with
addition of 1.0% curdlan (18,000x).
1 Staf
Jurusan Produksi Ternak, Fakultas Peternakan Universitas Hasanuddin
Di publikasi pada BIPP (terakreditasi) VIII (2): 142 - 147 (2001)
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