🔎 Lecture 9: Proximate Analysis

Proximate Analysis: Lecture Objectives

Understanding Procedures

- Understand and describe procedures used in proximate analysis

Calculations

• Calculate different aspects of proximate analysis

• Determine nutrient content on a dry matter (DM) basis and wet/as-fed basis

Proximate Analysis of Feed

Main Components and Procedures

• Dry Matter (DM) → determines moisture content by measuring what remains after water is removed

• Crude Protein (CP) → determined using the Kjeldahl procedure

• Ether Extract (EE) → determined using ether extraction with an organic solvent, such as diethyl ether

• Crude Fiber (CF) → determined by boiling the sample in acid, then alkali

• Ash → determined by burning the sample to measure inorganic residue

Kjeldahl Procedure: Measuring Crude Protein

1. Digestion → sulfuric acid and copper sulfate

2. Distillation → strong base (NaOH) releases ammonia

3. Titration → distilled ammonia is collected in boric acid solution and titrated with HCl

Memory Cue

• CP → Kjeldahl

• EE → Ether extraction

• CF → Acid + alkali

• Ash → Burning

Proximate Analysis of Feed

Definition

- Series of methods used to determine the nutritive value of food

Main Components

• Dry Matter (DM) → moisture content

• Ether Extract (EE) → crude fat

• Ash → inorganic matter/minerals

• Crude Protein (CP) → nitrogen content multiplied by a correction factor

• Crude Fiber (CF) → plant cell wall components

• Nitrogen-Free Extract (NFE) → soluble/digestible carbohydrates

Alternative Fiber Analysis: Van Soest Method

• Neutral Detergent Fiber (NDF)

• Acid Detergent Fiber (ADF)

Proximate Analysis: Sample Processing

Sample Preparation

• Sample is ground to pass a #20 sieve

• Oven-dried at 98–100°C (Analytical Chemists)

• Produces an air-dry sample for analysis

Main Analysis Procedures

- Ash

• Measures inorganic mineral matter

- Ether Extract (EE)

• Extracts lipids using a solvent

• Measures crude fat

- Kjeldahl Nitrogen

• Nitrogen content × 6.25 = crude protein

- Crude Fiber (CF)

1. Acid digestion → residue remains

2. Alkaline digestion → remaining residue is crude fiber

Dry Matter (DM) and Moisture Content

Definition

- Dry Matter (DM) → food material remaining after water is removed

Determining Moisture Content

1. Weigh the sample before drying

2. Dry sample in an oven at 100°C

3. Reweigh the sample

4. Repeat drying and weighing until weight no longer changes

Calculation

- Moisture content = Pre-drying weight − Post-drying weight

Dry Matter (DM): Maillard Reaction

Potential Problems During Drying

• Maillard reaction → reaction between amino acids (AA) and sugars

• Produces melanoidins, which give browned food its flavour

• Causes amino acid loss, primarily lysine

• Can lead to overestimation of available protein

Ash: Measuring Mineral Content

Definition

- Ash → inorganic (mineral) content of a food item

Method

1. Weigh a portion of the dried sample

2. Heat sample in an oven at 500–600°C for 8 hours

3. Reweigh the remaining sample

Result

- Remaining ash = mineral (inorganic) content of the food item

STEPS ****IMPORTANT

Ash: Potential Problems

• Some trace minerals may be lost through volatilization during heating; error is small

• Measures total inorganic content but does not identify or quantify specific minerals

Crude Protein (CP): Measuring Nitrogen Content

Definition

- Crude Protein (CP) → estimated from the nitrogen content of a food item

Kjeldahl Method

1. Analyze the dried sample for nitrogen using the Kjeldahl method

2. Determine the nitrogen content

3. Multiply nitrogen by a conversion factor to estimate crude protein

Calculation

• %CP = %N × 6.25

• Total Kjeldahl nitrogen × conversion factor = crude protein

Crude Protein (CP): Potential Problems

• Not all nitrogen comes from protein → nitrogen can also come from urea, ammonia, uric acid, and DNA

• Assumes all proteins contain 16% nitrogen → uses a conversion factor of 6.25 because 100 / 16 = 6.25

Ether Extract (EE): Measuring Crude Fat

Definition

- Ether Extract (EE) → crude fat content of a food item

Method

1. Weigh the dried sample

2. Extract sample with diethyl ether, which dissolves lipids

3. Separate the ether from the sample and evaporate it

4. Weigh the remaining residue

Result

- Remaining residue = Ether Extract (crude fat)

Ether Extract (EE): Potential Problems

• Assumes all dissolved lipids are fats, but may include non-nutritive lipids, such as waxes

• Error is generally small, but can be greater in plant products

Crude Fiber (CF): Measuring Insoluble Carbohydrates

Definition

- Crude Fiber (CF) → insoluble carbohydrate fraction, including cellulose, hemicellulose, and lignin

Method

1. Weigh the fat-free portion remaining after ether extraction

2. Boil sample in dilute acid, then filter and wash

3. Boil sample in dilute alkali, then filter and wash

4. Weigh the remaining residue

Result

- Remaining residue = Crude Fiber (CF)

Crude Fiber (CF): Potential Problems

• Underestimates total fiber → some fiber is solubilized and lost during analysis, especially hemicellulose

• Does not distinguish between fiber sources

• Considered the most unsatisfactory analytical procedure in proximate analysis

Nitrogen-Free Extract (NFE): Soluble Carbohydrates

Definition

- NFE → soluble carbohydrate fraction, including starch and sugars

Method

- Calculated rather than measured directly through laboratory analysis

Calculation

- %NFE = 100 − (%EE + %ash + %CP + %CF + %moisture)

Nitrogen-Free Extract (NFE): Potential Problems

• Accumulates errors from all other proximate analyses because NFE is calculated rather than measured directly

• Crude fiber (CF) contributes the largest error

Van Soest Fiber Analysis

Forage Components

• Cell Contents → protein, starch, sugars, organic acids, pectins

• Neutral Detergent Fiber (NDF) → hemicellulose, cellulose, lignin

• Acid Detergent Fiber (ADF) → cellulose and lignin

• Lignin Residue → cellulose and lignin are further separated using 72% sulfuric acid, leaving lignin

Analysis Procedure

1. Neutral detergent digestion → separates cell contents from NDF

2. Acid detergent digestion → separates hemicellulose from cellulose and lignin

3. 72% sulfuric acid digestion → removes cellulose, leaving lignin

Van Soest Fiber Analysis: Neutral Detergent Step

Definition

- Measures the insoluble carbohydrate fraction using Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF)

Step 1: Neutral Detergent Digestion

1. Weigh the dried sample

2. Boil in neutral detergent (pH 7)

3. Filter, wash, and dry the sample

4. Separates the sample into NDF and NDS

Fractions

• NDF (Neutral Detergent Fiber) → cellulose, hemicellulose, and lignin

• NDS (Neutral Detergent Solubles) → soluble carbohydrates, protein, and fats

Van Soest Fiber Analysis: Separating Fiber Fractions

Step 1: Neutral Detergent Digestion

- Separates forage into:

• NDS (Neutral Detergent Solubles) → protein, starch, sugars, organic acids, pectins

• NDF (Neutral Detergent Fiber) → hemicellulose, cellulose, lignin

Step 2: Acid Detergent Digestion

- Separates NDF into:

• Hemicellulose → removed into solution

• ADF (Acid Detergent Fiber) → cellulose and lignin

Step 3: 72% Sulfuric Acid Digestion

- Separates ADF into:

• Cellulose → removed into solution

• Lignin → remains as residue

Memory Cue

• NDF = Hemicellulose + Cellulose + Lignin

• ADF = Cellulose + Lignin

Van Soest Fiber Analysis: Acid Detergent Step

Step 2: Acid Detergent Digestion

1. Weigh the dried NDF sample

2. Boil in acid detergent (pH below 7)

3. Filter, wash, and dry the sample

4. Separates hemicellulose from the remaining ADF

Fiber Fractions

• NDF → cellulose, hemicellulose, and lignin

• ADF → cellulose and lignin

Key Relationship

- NDF − ADF = Hemicellulose

Van Soest Fiber Analysis: Lignin Separation

Step 3: Concentrated Sulfuric Acid Digestion

1. Weigh the dried ADF sample

2. Boil in concentrated sulfuric acid (72%)

3. Filter, wash, and dry the sample

4. Separates cellulose from acid detergent lignin

Fiber Fractions

• ADF (Acid Detergent Fiber) → cellulose and lignin

• ADL (Acid Detergent Lignin) → lignin

Key Relationships

• ADF − ADL = Cellulose

• ADL = Lignin

Calculating Nitrogen-Free Extract (NFE)

Given Values

• Moisture = 5%

• Crude Protein (CP) = 8%

• Ether Extract (EE) = 12%

• Ash = 4%

• Crude Fiber (CF) = 5%

NFE = 100 - (% moisture + % CP + % EE + % ash + % CF)

CF = NDF

Remember: Use crude fiber (CF) in the NFE formula, not NDF or ADF.

As-Fed Basis vs. Dry Matter Basis

As-Fed Basis (AFB)

- Nutrient content of food including moisture

Dry Matter Basis (DMB)

• Nutrient content of food excluding moisture

• Represents nutrients in the dry portion only

Moisture Content in Feed

Definition

- Moisture → amount of water contained in an ingredient or feed

Ways to Express Moisture

• Wet weight → amount of water by weight (kg)

• Moisture percent → percentage of the feed's total weight that is water (%)

As-Fed (As-Is) Feed Composition

Total Feed Weight

• Includes both Dry Matter (DM) and moisture

• As-fed/as-is = total feed weight = 100%

Key Relationship

- DM (%) + Moisture (%) = 100%

Dry Matter (DM): Definition and Measurement

Definition

• Dry Matter (DM) → dry portion of an ingredient or feed after removing its moisture

• Contains the nutrients in the feed, including carbohydrates, proteins, minerals, and fats

Ways to Express Dry Matter

• Dry weight → weight of the feed remaining after water removal (kg)

• DM (%) → percentage of the feed's total as-fed weight that is dry matter

• DM (%) + Moisture (%) = 100%

As-Fed Basis vs. Dry Matter Basis

Definitions

• As-Fed Basis (AFB) → nutritional composition of food as it is fed, including moisture

• Dry Matter Basis (DMB) → nutrient composition of the food after moisture is removed

Reporting Nutrient Values

• Nutrient profiles (e.g., proximate analysis) and metabolizable energy (ME) can be expressed on either an AFB or DMB

• Always check which basis is used when comparing values

Why Use AFB for This Course and Project?

• Represents the food in the form the animal actually consumes, including its natural moisture

• Allows nutrient content and energy to be calculated based on the total amount of feed offered or eaten

As-Fed Basis vs. Dry Matter Basis

Definitions

• As-Fed Basis (AFB) → nutritional composition of food as it is fed, including moisture

• Dry Matter Basis (DMB) → nutrient composition of the food after moisture is removed

Reporting Nutrient Values

• Nutrient profiles (e.g., proximate analysis) and metabolizable energy (ME) can be expressed on either an AFB or DMB

• Always check which basis is used when comparing values

Why Use AFB for This Course and Project?

• Represents the food in the form the animal actually consumes, including its natural moisture

• Allows nutrient content and energy to be calculated based on the total amount of feed offered or eaten

Converting Dry Matter Basis (DMB) to As-Fed Basis (AFB)

Conversion Formula

• % Nutrient (AFB) = % Nutrient (DMB) × (% DM ÷ 100)

• % DM = 100 − % moisture

Example: Crude Protein (CP)

• CP (DMB) = 7%; moisture = 22%

• DM = 100 − 22 = 78%

• CP (AFB) = 7 × (78 ÷ 100) = 5.46%

Key Point

- Converting DMB to AFB accounts for moisture, so the nutrient percentage decreases.

Converting Dry Matter Basis (DMB) to As-Fed Basis (AFB)

Conversion Formula

- % Nutrient (AFB) = % DM of food × (% Nutrient (DMB) ÷ 100)

Example: Crude Protein (CP)

• CP (DMB) = 7%; moisture = 22%

• Step 1: Determine DM = 100 − 22 = 78%

• Step 2: Calculate CP on an AFB

  • % CP (AFB) = 78 × (7 ÷ 100)

  • % CP (AFB) = 5.46%

Nutrient Composition: Water and Dry Matter

On an AFB

• Dry matter = 75%

• Moisture = 25%

• Protein = 25%

• Carbohydrates = 25%

• Fat = 25%

On a DMB

• Dry matter = 100%

• Protein = 33.3%

• Carbohydrates = 33.3%

• Fat = 33.3%

• Protein + carbohydrates + fat = 100% of DM