Analisis Kadar Air . . . 22 B. METODE DESTILASI

Analisis Kadar Air . . .

B. METODE DESTILASI• Didasarkan pada penentuan langsung jumlah air yang

diuapkan dari sampel. • Mencampurkan sampel dengan pelarut yang mempunyai

titik didih tinggi (sedikit lebih tinggi dari air), berat jenis berbeda dengan air, dan tidak larut dalam air.

• Air dan pelarut akan menguap, terkondensasi, mencair dan terpisah. Pelarut kembali bercampur dengan sampel.

• Pelarut yang digunakan: Toluene, Xylene, dan Benzene. • Digunakan untuk analisis kadar air:

a. Rempah-rempah (AOAC No. 986.21) b. Pakan ternak (AOAC No. 925.94) c. Cheese (AOAC No. 969.19)

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FST 393.1.0 FOOD SCIENCE & TECHNOLOGY PRACTICALS – V 2014

AS2011743

x and be safe to use.

The flask containing the sample and the organic solvent is attached to a condenser by a side arm and the mixture is heated. The water in the sample evaporates and moves up into the condenser where it is cooled and converted back into liquid water, which then trickles into the graduated tube. When no more water is collected in the graduated tube, distillation is stopped and the volume of water is read from the tube.

Practical Considerations

There are a number of practical factors that can lead to erroneous results:

(i) emulsions can sometimes form between the water and the solvent which are difficult to separate

(ii) water droplets can adhere to the inside of the glassware, (iii) decomposition of thermally labile samples can occur at the elevated temperatures used.

Advantages and Disadvantages

x Advantages: I. Suitable for application to foods with low moisture contents;

II. Suitable for application to foods containing volatile oils, such as herbs or spices, since the oils remain dissolved in the organic solvent, and therefore do not interfere with the measurement of the water;

Dean-Stark apparatus set up;

1: Stirrer bar/anti-bumping granules

2: Still pot

3: Fractionating column

4: Thermometer/Boiling point

temperature

5: Condenser

6: Cooling water in

7: Cooling water out

8: Burette

9: Tap

10: Collection vessel

Sterling-Bidwell Moisture Trap


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Potensi Kesalahan (Error) pada Metode Distilasi:

1. Terbentuk emulsi (air dan pelarut) yang tetap bertahan (tidak rusak). • Ditunggu hingga dingin, sebelum dilakukan pembacaan

volume air yang dihasilkan. 2. Tetesan air menempel pada permukaan alat (gelas)

• Mencuci alat-alat gelas hingga bersih. 3. Dekomposisi sampel yang menghasilkan air.

• Menggunakan metode lain dengan suhu lebih rendah.



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KELEBIHAN DAN KEKURANGAN METODE DESTILASI Kelebihan: 1. Cocok untuk aplikasi untuk makanan dengan kadar air

rendah; 2. Cocok untuk aplikasi untuk makanan yang mengandung

minyak atsiri, seperti jamu atau rempah-rempah, karena minyak tetap terlarut dalam pelarut organik, dan karena itu tidak mengganggu pengukuran air;

3. Peralatan relatif murah, mudah untuk setup dan beroperasi; Metode Distilasi telah resmi disetujui untuk sejumlah aplikasi makanan;

Kekurangan: 1. Merusak; 2. Relatif memakan waktu; 3. Melibatkan penggunaan pelarut yang mudah terbakar; 4. Tidak berlaku untuk beberapa jenis makanan.Jurusan Teknologi Hasil Pertanian

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C. METODE KIMIA (KARL FISCHER)

• Didasarkan pada penentuan langsung jumlah air yang dikandung sampel.

• Dapat menentukan kadar air hingga 100 ppm. • Untuk bahan yang KA rendah pada bahan tinggi kadar

gula, tinggi gula reduksi dan protein, atau tinggi kadar lemaknya.

• Digunakan untuk analisis kadar air: a. Sayuran dan buah kering (AOAC No. 967.19 E-G) b. Permen dan cokelat (AOAC No. 977.10) c. Kopi sangrai, serta lemak dan minyak (AOAC No.

984.20)Jurusan Teknologi Hasil Pertanian

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• Penetuan KA berdasarkan reaksi: Reduksi Iodine (I2) oleh SO2 dengan adanya molekul air (H2O):

2H2O+SO2 +I2 → C5H2SO4 +2HI . . . . . . . . . . (1) • Selanjutnya dimodifikasi dengan penambahan Pyridine

(C5H5N) dan Methanol (CH3OH): C5H5N·I2 + C5H5N·SO2 + C5H5N+H2O→ 2C5H5N·HI + C5H5N·SO3 . . . . . . . . . . . . . . (2)

C5H5N·SO3 + CH3OH → C5H5N(H)SO4·CH3 . . . . (3) • 1 mol H2O memerlukan 1 mol I2, 1 mol SO2, 3 mol C5H5N,

dan 1 mol CH3OH (Karl Fischer Reagent). • 3.5 mg H2O = 1 ml Reagent.


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• Karl Fischer Reagent tidak stabil, perlu di tentukan distandardisasi dengan menghitung Karl Fischer Reagant Equivalent (KFReq).

• KFReq ditentukan dengan mentitrasi larutan air standar: a. Standar campuran air-metanol (=1 mg Air/ml larutan). b. Sodium Tartrate Dihydrate (Na2C4H4O6.2H2O) (=15.66%

air).• KFReq ditentukan berdasarkan Sodium Tartrate Dihydrate:

• Ket: St = berat Sodium Tartrate. A = ml KFR untuk titrasi.

96 Part II • Compositional Analysis of Foods

together) and two-component reagents (solvent andtitrant components separate) have been prepared.The one-component reagent may be more convenientto use, but the two-component reagent has greaterstorage stability.

Before the amount of water found in a foodsample can be determined, a KFR water (moisture)equivalence (KFReq) must be determined. The KFReqvalue represents the equivalent amount of moisturethat reacts with 1 ml of KFR. Standardization mustbe checked before each use because the KFReq willchange with time.

The KFReq can be established with pure water,a water-in-methanol standard, or sodium tartratedihydrate. Pure water is a difficult standard to usebecause of inaccuracy in measuring the small amountsrequired. The water-in-methanol standard is pre-mixed by the manufacturer and generally contains1 mg of water/ml of solution. This standard canchange over prolonged storage periods by absorb-ing atmospheric moisture. Sodium tartrate dihydrate(Na2C4H4O6·2H2O) is a primary standard for deter-mining KFReq. This compound is very stable, contains15.66% water under all conditions expected in thelaboratory, and is the material of choice to use.

The KFReq is calculated as follows using sodiumtartrate dihydrate:

KFReq (mg H2O/ml)

=36 g H2O/mol Na2C4H4O6 · 2H2O × S × 1000

230.08 g/mol × A

[8]

where:

KFReq = Karl Fischer reagent moisture equivalenceS = weight of sodium tartrate dihydrate (g)A = ml of KFR required for titration of sodium

tartrate dehydrate

Once the KFReq is known, the moisture content ofthe sample is determined as follows:

%H2O =KFReq × Ks

S× 100 [9]

where:

KFReq = Karl Fischer reagent water (moisture)equivalence

Ks = ml of KFR used to titrate sampleS = weight of sample (mg)

The major difficulties and sources of error in theKarl Fischer titration methods are as follows:

1. Incomplete moisture extraction. For this rea-son, fineness of grind (i.e., particle size) is

important in preparation of cereal grains andsome foods.

2. Atmospheric moisture. External air must notbe allowed to infiltrate the reaction chamber.

3. Moisture adhering to walls of unit. All glass-ware and utensils must be carefully dried.

4. Interferences from certain food constituents.Ascorbic acid is oxidized by KFR to dehy-droascorbic acid to overestimate moisture con-tent; carbonyl compounds react with methanolto form acetals and release water to overesti-mate moisture content (this reaction also mayresult in fading endpoints); unsaturated fattyacids will react with iodine, so moisture contentwill be overestimated.

6.5 PHYSICAL METHODS

6.5.1 Dielectric Method

The electrical properties of water are used in thedielectric method to determine the moisture contentof certain foods, by measuring the change in capac-itance or resistance to an electric current passedthrough a sample. These instruments require calibra-tion against samples of known moisture content asdetermined by standard methods. Sample density orweight/volume relationships and sample temperatureare important factors to control in making reliable andrepeatable measurements by dielectric methods. Thesetechniques can be very useful for process control mea-surement applications, where continuous measure-ment is required. These methods are limited to foodsystems that contain no more than 30–35% moisture.

The moisture determination in dielectric-typemeters is based on the fact that the dielectric constantof water (80.37 at 20◦C) is higher than that of most sol-vents. The dielectric constant is measured as an indexof capacitance. As an example, the dielectric methodis used widely for cereal grains. Its use is based onthe fact that water has a dielectric constant of 80.37,whereas starches and proteins found in cereals havedielectric constants of 10. By determining this properlyon samples in standard metal condensers, dial read-ings may be obtained and the percentage of moisturedetermined from a previously constructed standardcurve for a particular cereal grain.

6.5.2 Hydrometry

Hydrometry is the science of measuring specific grav-ity or density, which can be done using several dif-ferent principles and instruments. While hydrometryis considered archaic in some analytical circles, it isstill widely used and, with proper technique, is highly

t


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• Setelah KFReq ditentukan, Kadar Air (KA) sampel dapat dihitung denga rumus:

96 Part II • Compositional Analysis of Foods

together) and two-component reagents (solvent andtitrant components separate) have been prepared.The one-component reagent may be more convenientto use, but the two-component reagent has greaterstorage stability.

Before the amount of water found in a foodsample can be determined, a KFR water (moisture)equivalence (KFReq) must be determined. The KFReqvalue represents the equivalent amount of moisturethat reacts with 1 ml of KFR. Standardization mustbe checked before each use because the KFReq willchange with time.

The KFReq can be established with pure water,a water-in-methanol standard, or sodium tartratedihydrate. Pure water is a difficult standard to usebecause of inaccuracy in measuring the small amountsrequired. The water-in-methanol standard is pre-mixed by the manufacturer and generally contains1 mg of water/ml of solution. This standard canchange over prolonged storage periods by absorb-ing atmospheric moisture. Sodium tartrate dihydrate(Na2C4H4O6·2H2O) is a primary standard for deter-mining KFReq. This compound is very stable, contains15.66% water under all conditions expected in thelaboratory, and is the material of choice to use.

The KFReq is calculated as follows using sodiumtartrate dihydrate:

KFReq (mg H2O/ml)

=36 g H2O/mol Na2C4H4O6 · 2H2O × S × 1000

230.08 g/mol × A

[8]

where:

KFReq = Karl Fischer reagent moisture equivalenceS = weight of sodium tartrate dihydrate (g)A = ml of KFR required for titration of sodium

tartrate dehydrate

Once the KFReq is known, the moisture content ofthe sample is determined as follows:

%H2O =KFReq × Ks

S× 100 [9]

where:

KFReq = Karl Fischer reagent water (moisture)equivalence

Ks = ml of KFR used to titrate sampleS = weight of sample (mg)

The major difficulties and sources of error in theKarl Fischer titration methods are as follows:

1. Incomplete moisture extraction. For this rea-son, fineness of grind (i.e., particle size) is

important in preparation of cereal grains andsome foods.

2. Atmospheric moisture. External air must notbe allowed to infiltrate the reaction chamber.

3. Moisture adhering to walls of unit. All glass-ware and utensils must be carefully dried.

4. Interferences from certain food constituents.Ascorbic acid is oxidized by KFR to dehy-droascorbic acid to overestimate moisture con-tent; carbonyl compounds react with methanolto form acetals and release water to overesti-mate moisture content (this reaction also mayresult in fading endpoints); unsaturated fattyacids will react with iodine, so moisture contentwill be overestimated.

6.5 PHYSICAL METHODS

6.5.1 Dielectric Method

The electrical properties of water are used in thedielectric method to determine the moisture contentof certain foods, by measuring the change in capac-itance or resistance to an electric current passedthrough a sample. These instruments require calibra-tion against samples of known moisture content asdetermined by standard methods. Sample density orweight/volume relationships and sample temperatureare important factors to control in making reliable andrepeatable measurements by dielectric methods. Thesetechniques can be very useful for process control mea-surement applications, where continuous measure-ment is required. These methods are limited to foodsystems that contain no more than 30–35% moisture.

The moisture determination in dielectric-typemeters is based on the fact that the dielectric constantof water (80.37 at 20◦C) is higher than that of most sol-vents. The dielectric constant is measured as an indexof capacitance. As an example, the dielectric methodis used widely for cereal grains. Its use is based onthe fact that water has a dielectric constant of 80.37,whereas starches and proteins found in cereals havedielectric constants of 10. By determining this properlyon samples in standard metal condensers, dial read-ings may be obtained and the percentage of moisturedetermined from a previously constructed standardcurve for a particular cereal grain.

6.5.2 Hydrometry

Hydrometry is the science of measuring specific grav-ity or density, which can be done using several dif-ferent principles and instruments. While hydrometryis considered archaic in some analytical circles, it isstill widely used and, with proper technique, is highly

s

• Ket: Ss = berat sampel. Ks = ml KFR untuk titrasi sampel.

Analisis Kadar Air - Karl Fischer. . .


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TERDIRI DARI 2 METODE ANALISIS: 1. Volumetric Titration.

• Digunakan untuk sample dengan KA ≥ 0.03%. • Akhir titrasi di tandai dengan perubahan warna (visual). • I2 dan SO2 ditambahkan (titrasi) ke sample, analisis

selasai dengan ditandai perubahan warna (kuning - coklat).

2. Coulometric Titration. • Digunakan untuk sample dengan KA < 0.03%. • Meenggunakan 2 eletroda platinum, dimasukan dalam

ruang titrasi. • Arus (listrik) dialirkan melalui eletroda, menyebabkan

Iodine menghasilkan elektron (2I ———> I2 + 2e). • Perubahan arus (elektron) pada akhir titrasi diukur

dengan Galvanometer). Jurusan Teknologi Hasil Pertanian Fakultas Pertanian Universitas Lampung

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Analisis Kadar Air - Karl Fischer. . .

Penyebab Error (variasi) analisis Metode Karl Fischer: 1. Ekstraksi (air) tidak sempurna.

• Ukuran partikel penting. 2. Terkontaminasi oleh air bukan dari sampel.

• Air dari udara dan alat. 3. Air hasil reaksi reagent dengan komponen dalam sampel.

• Asam askorbat dioksidasi oleh reagent. • Komponen karbonil bereaksi dengan metanol. • As. lemak tak jenuh bereaksi dengan iodine.


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D . M E T O D E F I S I K A Analisis Kadar Air . . .

1. Dielectric Method. • Metode análisis KA dengan memanfaatkan kemampuan

serapan energi listrik molekul air dalam bahan. • Analisis KA didasarkan pada perubahan arus listrik yang

dilewatkan pada suatu sampel. • Perubahan arus listrik tersebut dibandingkan dengan

standar yang telah diprogram dalam instrumen, senaljutnya KA dapat diketahui.

• Digunakan untuk bahan (bĳi-bĳian dan serealia) dengan kadar air maks. 35%.

• Movie.


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Analisis Kadar Air - Dielectric Method . . .


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Analisis Kadar Air - Metode Fisika . . .

2. Hydrometer. • Menentukan KA berdasarkan berat jenis (specific

gravity) sampel (bahan). • Digunakan untuk sampel cair (minuman, larutan gula,

atau larutan garam). • Pengukuran berat jenis (KA) didasarkan pada Hukum

Archimedes, yaitu benda yang dimasukan ke dalam cairan akan mendapat gaya ke atas sebesar zat cair yang dipindahkan.

• Berat/volume ccairan ditentukan dengan mentukan volume cairan yang dipindahkan oleh benda yang diketahui beratnya (berat standar)

• Standar tersebut adalah Hydrometer.


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Analisis Kadar Air - Hydrometer . . .

Jenis Hydrometer:

1. Lactometer2. Baume Hydrometer

3. Brix Hydrometer4. Alcoholometer


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Analisis Kadar Air - Metode Fisika . . .

3. Refractometer. • Untuk menentukan Index Refrative yang dikalibrasi

(dikonversi) menjadi kadar (persern) padatan atau kadar (persen) gula dalam cairan.

• Mekanisme: Ketika sinar dilewatkan melalui dua medium dengan kerapatan berbeda, sinar tersebut akan dibelokan atau dibiaskan.

• Besarnya pembiasan merupakan fungsi dari media dan sinus sudut pembiasan, dan bergantung pada suhu dan tekanan.

• Menggunakan prisma amici untuk menghasilkan cahaya pada panjang gelombang 589 nm.

• Dikalibrasi pada Brix (g sukrosa/100 g larutan).


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Analisis Kadar Air - Refractometer . . .


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Analisis Kadar Air - Refractometer . . .


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Analisis Kadar Air . . . 22 B. METODE DESTILASI

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