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ANALISIS HASIL PERTANIAN

A N A L I S I S K A D A R A I R

Bahan Bacaan: Nielsen, S. 2010. Food Analysis 4th Ed. Hal.: 85-104. Nielsen, S. 2010. Food Analysis: Laboratory Manual 2nd Ed. Hal.: 17-28. Pomeranz, Y dan C. E. Meloan. 2000. Food Analysis: Theory and Practice. Hal. : 575-601.

Jurusan Teknologi Hasil Pertanian Fakultas Pertanian Universitas Lampung

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FUNGSI ANALISIS KADAR AIRAnalisis Kadar Air Sangat Penting: 1. Mematuhi aturan dan labeling.

• Ada batasan kadar air maksimum dan minimum • Merupakan standar komposisi. • Menetukan nilai gizi.

2. Ekonomi. • Air adalah komponen paling murah, ditambahkan

sebanyak-banyaknya. • Biaya pengangkutan, penyimpanan.

3. Stabilitas terhadap mikroba. • Pertumbuhan mikroba dipengaruhi oleh kadar air. • Kadar air dibawah kadar air kritis

Analisis Kadar Air . . .

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Analisis Kadar Air Sangat Penting: 4. Kualitas pangan (hasil Pertanian).

• Mempengaruhi laju kerusakan. • Mempengaruhi stabilitas bahan hasil pertanian. • Menentukan karakteristik bahan hasil pertanian

(tekstur, rasa, penampakan). 5. Proses pengolahan.

• Diperlukan untuk mengetahui sifat bahan hasil pertanian untuk prngolahan (pencampuran, pengeringan, daya alir, dan pengemasan).

• Analisis material balance dan penyusutan selama proses.

Fungsi Analisis Kadar Air . . .

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JENIS AIR DALAM BAHANJenis Air dalam Bahan Hasil Pertanian: 1. Air Bebas (Free/Bulk Water).

• Air yang bebas dari bahan lainnya. • Tiap molekul air dikelilingi oleh molekul air lainnya. • Mempunyai sifat sama dengan sifat air murni (titik leleh,

titik didih, densitas, panas penguapan, spektrum penyerapan gelombang).

2. Air Kapiler (Capillary/Trapped Water). • Air dalam saluran kapiler, ditahan oleh daya kapiler. • Terperangkap secara fisik, sehingga sulit keluar. • Meliputi air bebas, air terikat (fisik dan kimia).

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Jenis Air dalam Bahan …


Jenis Air dalam Bahan Hasil Pertanian: 3. Air Terikat secara Fisik (Physically Bonded Water).

• Molekul air yang tidak diselimuti oleh molekul air lannya, melainkan kontak dengan komponen lainnya (protein, karbohidrat, dan mineral).

• Memiliki sifat yang berbeda dengan air bebas, karena mempunyai ikatan yang berbeda.

• Ikatan kovalen 4. Air Terikat secara Kimia (Chemically Bonded Water).

• Molekul air yang terikat secara kimia dengan molekul lainnya.

• Disebut juga sebagai air kristalisasi (hidrat), NaSO4.10H2O). • Ikatan kimia (lebih kuat dari ikatan antar molekul air).

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METODE ANALISIS KADAR AIR


Kadar Air : • Jumlah atau berat molekul air dalam sample yang telah

diketahui beratnya. Pemilihan metode: • Tergantung pada sifat bahan yang dianalisis dan kegunaan

informasi yang diperlukan. • Pengembangan teknik analisis kadar air makanan tergantung

pada kemampuan untuk membedakan air ( analit) dari komponen lain dalam makanan (matriks).

• Sifat umum air untuk analisis: titik didih rendah; polaritas tinggi; mampu mengalami reaksi kimia yang unik dengan reagen tertentu; spektrum penyerapan elektromagnetik yang unik; dan sifat fisik yang khas (density, kompresibilitas, konduktivitas listrik, dan indeks bias).

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Metode Analisi Kadar Air …


Variable yang perlu dipertimbangkan: a. Bahan hasil pertanian adalah bahan heterogen yang

mengandung air dengan proporsi jenis air yang berbeda; (air bebas, kapiler, terikat secara kimia dan fisik).

b. Bahan hasil pertanian mungkin berisi air dalam beberapa fase: gas, cair atau padat. - Menyebabkan sifat fisik-kimia berbeda.

Untuk alasan di atas, sejumlah metode analisis lainnya telah dikembangkan untuk mengukur kadar air, didasarkan: a. Pengukuran langsung dari massa air dalam makanan. b. Air dalam makanan dapat dibedakan dari komponen lain

dalam beberapa cara terukur.

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Metode Analisi Kadar Air …


PENYIAPAN SAMPEL

Sumber Kesalahan dalam Analisis: 1. Pengambilan sampel yang tidak representatif. 2. Terjadi perubahan sampel, sebelum dianalisis. 3. Terjadi penambahan (penyerapan) dan pengurangan

(penguapan) air.

Pencegahan Kesalahan: 1. Tidak terpapar ke udara terbuka (atmosfer). 2. Suhu penyimpanan tidak berfluktuasi terlalu besar. 3. Head space penyimpanan minimal.

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METODE ANALISIS KADAR AIR


A. METODE PENGERINGAN Metode Oven, Oven Vacum, microwave, dan Infra Merah, dan Rapid Moisture Analyzer.

B. METODE DESTILASI Sterling-Bidwell Method (Reflux Destillation)

C. METODE KIMIA Karl Fischer Titration,

D. METODE FISIK Dielectric Method, Hydrometry (hydrometer dan Pycnometer), Refractrometry, Infrared Analysis, Freezing Point.

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Metode Analisis Kadar Air . . .

A. METODE PENGERINGAN (GRAVIMETRI)• Didasarkan pada pengukuran pengurangan berat bahan yang

dikeringkan. • Banyak air yang akan dianalisis menentukan jenis oven yang

digunakan, kondisi dalam oven, waktu dan suhu pengeringan. • KA 0.1-99.9 %. Banyak metode ini diakui oleh AOAC

International.

•Kelebihan: ➡ Akurat, relatif mudah, murah, banyak metode yang

digunakan, banyak sample dianalisis bersamaan.

•Kekurangan: ➡ Merusak, memerlukan waktu lama, dan tidak dapat

dilakukan untuk beberapa jenis bahan pangan.

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Metode Pengeringan …

ALAT METODE PENGERINGAN: 1. Convection Oven:

• Sampel dipanaskan dalam oven pada suhu dan waktu tertentu hingga beratnya konstan.

• Variasi suhu dalam oven hingga 10oC.

2. Force Draft Oven: • Ditambahkan kipas dalam oven. • Variasi suhu dalam oven 1oC. • Tidak sesuai untuk sampel berkabohidrat dan bahan

volatil tinggi.

3. Vacuum Oven: • Tekanan dalam oven 25-100 mmHg, suhu <100oC (70oC). • Suhu rendah, mencegah kerusakan.

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Chapter 6 • Moisture and Total Solids Analysis 91

with the matrix using a glass rod. Heat dish, matrix,cover, and glass rod for at least 4 h at 100◦C, reweigh.The difference between weighing must be less than0.5 mg for any suitable matrix (12).

6.2.1.7 Calculations

Moisture and total solids contents of foods can becalculated as follows using oven drying procedures:

%Moisture (wt/wt) =wt H2O in samplewt of wet sample

× 100 [2]

%Moisture (wt/wt)

=wt of wet sample − wt of dry sample

wt of wet sample× 100 [3]

%Total solids (wt/wt) =wt of dry samplewt of wet sample

× 100 [4]

6.2.2 Forced Draft Oven

When using a forced draft oven, the sample is rapidlyweighed into a predried moisture pan covered andplaced in the oven for an arbitrarily selected time if nostandardized method exists. Drying time periods forthis method are 0.75–24 h (Table 6-2), depending on thefood sample and its pretreatment; some liquid samplesare dried initially on a steam bath at 100˚C to minimizespattering. In these cases, drying times are shortenedto 0.75–3 h. A forced draft oven is used with or with-out a steam table predrying treatment to determine thesolids content of fluid milks (AOAC Method 990.19,990.20).

An alternative to selecting a time period for dry-ing is to weigh and reweigh the dried sample andpan until two successive weighings taken 30 min apartagree within a specified limit, for example, 0.1–0.2 mgfor a 5-g sample. The user of this second method mustbe aware of sample transformation, such as brown-ing which suggests moisture loss of the wrong form.Lipid oxidation and a resulting sample weight gaincan occur at high temperatures in a forced draft oven.Samples high in carbohydrates should not be dried ina forced draft oven but rather in a vacuum oven at atemperature no higher than 70◦C.

6.2.3 Vacuum Oven

By drying under reduced pressure (25–100 mm Hg),one is able to obtain a more complete removal of waterand volatiles without decomposition within a 3–6-hdrying time. Vacuum ovens need a dry air purge inaddition to temperature and vacuum controls to oper-ate within method definition. In older methods, avacuum flask is used, partially filled with concentratedsulfuric acid as the desiccant. One or two air bub-bles per second are passed through the acid. Recentchanges now stipulate an air trap that is filled with cal-cium sulfate containing an indicator to show moisturesaturation. Between the trap and the vacuum oven isan appropriately sized rotameter to measure air flow(100–120 ml/min) into the oven.

The following are important points in the use of avacuum drying oven:

6-2table Forced Draft Oven Temperature and Times for Selected Foods

ProductDry on

Steam Bath

OvenTemperature

(◦C ± 2)Time inOven (h)

Buttermilk, liquid Xa 100 3Cheese, natural type only 100 16.5 ± 0.5Chocolate and cocoa 100 3Cottage cheese 100 3Cream, liquid and frozen X 100 3Egg albumin, liquid X 130 0.75Egg albumin, dried X 100 0.75Ice cream and frozen desserts X 100 3.5Milk X 100 3

Whole, low fat, and skim 100 3Condensed skim 100 3

Nuts: almonds, peanuts, walnuts 130 3

From (6) p. 492, with permission. Copyright 2004 by the American Public Health Association,Washington, DC.aX = samples must be partially dried on steam bath before being placed in oven.

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Force Draft Oven:

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Vacuum Oven

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ALAT METODE PENGERINGAN: 4. Microwave Oven:

• Mengeringkan, menimbang, dan menghitung KA secara automatik. Waktu analisis cepat (4 menit).

• Terdiri: Mode 1 (diatur power dan waktu) dan Mode 2 (diatur power, waktunya ditentukan oleh alat).

5. Infrared Drying: • Menggunakan lampu halogen yang mengeluarkan sinar

infrared. Mengeringkan dan menimbang secara automatis.

• Waktu 10-25 menit.

6. Rapid Moisture Analyzer: • Menggunakan metode mikrowave atau infra merah. • Cepat.

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Table A l . l . l Recommended Moisture Protocols

Matrix Recommended oven type Conditions

Animal feeds Ice cream/frozen desserts Cheese Dried milk Seafood Meat Meat and poultry products Dried eggs Frozen french-fried potatoes Tomato/tomato products Canned vegetables Dried fruits Fruivfruit products Nutshut products Flour Sugars Sugars

Vacuum Convection Vacuum Vacuum Convection Convection Microwave Convection Convection Microwave Vacuum Vacuum Vacuum Vacuum Vacuum Convection Vacuum

2 hr at 95"- 100°C 3.5 hr at 100°C 4 hr at 100°C 5 hr at 100°C 4 hr at 100°C 16-18 hr at 100°C Mode 2 5 hr at 98"- 100°C I6 hr at 103°C Mode I , 4 min Constant weight at 70°C 6 hr at 70°C Constant weight at 70°C 5 hr at 95"- 100°C 5 hr at 98"- 100°C 3 hr at 70°C 3 hr at 100°C

through the sulfuric acid, and into the oven at a rate of -2 bubbledsec. Dry sample 4 hr.

7. Isolate vacuum pump by closing oven outlet valve and carefully admitting dried air into oven (by slowly opening air inlet valve fully), increasing the pressure inside oven until the door can be easily opened. Remove drying dish with sample and cool 30 min in a desiccator.

8. Weigh cooled dish with sample.

9. Repeat steps 5 to 8, except dry for 1 hr.

10. If weight has not changed, test is done. If weight is lower, continue drying for 1-hr periods and reweighing until constant weight is achieved.

Most samples take -4 hr to dry.

1 1. Calculate moisture as the percent loss in weight after drying.

MEASURING MOISTURE USING A MICROWAVE MOISTURE ANALYZER Water is measured in a sample using a microwave moisture analyzer that uses microwave energy to remove water. The instrument weighs the sample both before and after drying and calculates the percent moisture automatically. The analyzer can measure an effective range of 0.1 % to 99.9% water, but may have problems at the lower range due to burning or scorching, especially if there is a high sugar content.

The microwave moisture analyzer allows one to dry a sample by two modes. Mode 1 dries a sample at a specific power setting for a given time period. Mode 2 dries a sample at a specific power setting using a time period that is determined by the analyzer itself, ending the run when no weight change is achieved within a preset time interval criterion. This protocol looks only at Mode 1 operation.

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ALTERNATE PROTOCOL 2

Gravimetric Measurements of Water

A1.1.3



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Microwave Oven Infrared Drying

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Rapid Moisture Analyzer

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P E R T I M B A N G A N T E K N I S :

a. Penurunan Kadar Air. • Didasarkan pada suhu penguapan air (100oC, pada P atm),

tetapi karena adanya bahan terlarut, suhu harus lebih tinggi. • 1 mol bahan terlarut (solute) meningkatkan suhu uap air

sebesar 0.512oC. • Bahan tertentu, diperlukan dua-tahap pengeringan:

i. Produk cair (juice, susu): Steam bath (predried) dan Oven (drying).

ii. Tepung dan bĳi-bĳian: Air dried (penjemuran), giling, dan Oven dried.

• Ukuran partikel, distribusi ukuran partikel, ukuran sampel, dan luas permukaan sampel mempengaruhi laju dan efisiensi penguapan air.

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Pertimbangan Metode Pengeringan …

b. Kerusakan bahan. • Kerusakan bahan terjadi karena suhu terlalu tinggi

dan/atau waktu terlalu lama. • Contoh:

a. Karbohidrat terdecomposisi pada pemanasan 100C, mengikuti rumus:

C6H12O6 ————> 6C + 6H2O b. Hidrolisis sukrosa, menyerap air. c. Kehilangan bahan volatil, seperti: asam asetat,

propionat, butirat; alkohol; ester; dan aldehid.

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c. Variasi Suhu • Suhu dalam oven dapat bervariasi, bertgantung pada jenis

oven yang digunakan. • Variasi suhu antara 1-10C.

d. Pemilihan dan Penanganan Cawan • Pemilihan cawan bergantung pada jenis bahan yang diuji. • Disarankan yang disposable. • Gunakan penutup cawan. • Gunakan spatula untuk memindahkan cawan.

e. Penggumpalan (Clumping) Pembentukan Kulit (Crust). • Beberapa bahan dapat membentuk kulit (crust) dan gumpal. • Hasil análisis salah dan tidak seragam. • Dicegah dengan menambahkan pasir kering dari tanah

diatom.

Pertimbangan Metode Pengeringan …

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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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Analisis Kadar Air . . . KEUNTUNGAN DAN KERUGIAN

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.

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C. METODE KIMIA (KARL FISCHER)

• Didasarkan pada penentuan langsung jumlah air yang diuapkan dari 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)

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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.

METODE KARL FISCHER . . . .

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Analisis Kadar Air . . . METODE KARL FISCHER . . . .

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 tirtasi. • Arus (listrik) dialirkan melalui eletroda, menyebabkan Iodine

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

Galvanometer).

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Analisis Kadar Air . . . METODE KARL FISCHER . . . .

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

• KRFeq 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).

• KRFeq 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

29


• 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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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


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 - 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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