NUTR 3210 Lecture Notes - Lecture 2: Short-Chain Fatty Acid, Kjeldahl Method, Freeze-Drying
NUTR3210 – Food Composition Analysis & Digestive Systems
Methods used in the chemical analysis of foods and feeds:
• Proximate analysis: basic determination of moisture, crude protein (Kjeldahl analysis of
nitrogen), crude fat (ether extract), crude fibre, ash (minerals) and available CHO (N free
extract; NFE) by subtraction
• Newer Methods: replace or extend traditional components of proximate analysis (ex. Gas
chromatography for specific fatty acids)
• Southgate/Van Soest Methods: methods to replace nitrogen free extract (NFE) and crude
fibre for modern CHO labelling
Why look at food composition?
• Food analysis – development, application and study of analytical methods for
characterizing foods and their constituents
• Info about foods allow the consumer to make informed decisions
• Government regulation – stipulate nutrition facts, ingredient lists and health claims
• Quality control in industries – ensures consistent food composition
Nutrition Facts:
• Serving size; servings per container
• Amount per serving (calories per serving and number of calories from fat)
• Calories – measure of fuel you get from food
• & vitamins and
Proximate Analysis – early methodology for quantification of nutrients in foods
• developed in Germany in 1800s
• some methodologies are still in use, some have been adapted or replaced
• General comments:
o No info on digestibility
o No info on specific amino acids, minerals, lipids or carbs **
o Still used in food labelling and animal feed analysis
o Has provided the basis for developing more advanced analyses
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1. Moisture content (water; an essential nutrient) – loss in weight after drying is
moisture content
-errors: loss of other volatile FA and alcohols can cause overestimation of
moisture
-modern improvements: freeze drying is more accurate
-importance: further analyses require dried sample, agricultural systems work
with food data on a dry matter and human is on wet weight, water content has
effects on palatability and shelf life **
-moisture = water ; % dry matter = dry weight/wet weight X 100% ;
% moisture = 100- %dry matter
2. Crude fat – ether extract
-dried sample is extracted with ether (non-polar solvent) ; ether soluble lipids/fats
remain and are weighed
-errors: ether is poor at extracting phospholipids (lipoproteins) and the method
does not identify specific types of FA
-modern improvements: chloroform/methanol is used to extract all lipids, and gas
chromatography is then used to quantify specific families of FA
-goal: quantify dietary lipids (triglycerides, phospholipids, cholesterol and
specific FA)
- ether extract = crude fat content
% crude fat = weight of crude fat/ wet or dry weight x 100%
3. Ash – minerals or inorganic portion
-dried sample is burned in high temperature oven, and the remaining ash is
weighed and it represents the minerals or inorganic portion
-errors: some volatile minerals are lost during burning and this method doesn’t
quantify individual minerals
-Ash contains most minerals in the sample
-modern improvements: ashing is still first step in most mineral analyses but it is
now followed by acid solubilization and further analysis of specific minerals by
atomic absorption spectroscopy
-importance: nutritional labelling, quality and taste of food, microbiological
stability, nutritional requirements, processing
- % ash = weight of ash/ wet or dry weight x 100%
4. Crude protein – nitrogen content is quantified by Kjeldahl methods (protein is only
component that contains N)
- Steps of the Kjeldahl Method:
1.Digestion with sulfuric acid, converting all N into ammonia
2.Distillation of the ammonia, which is then quantified
3.Multiply grams of N by 6.25 to get grams of protein
-Errors: Nitrogen is also liberated from other components like DNA and RNA;
specific amino acids are not determined through this method **assumes that all
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proteins have 16% N (actual range is 13-19% so accurate conversion factors are
from 5.3-7.7)
-Modern improvements: Kjeldahl is still in use; if %N is known, correction factor
can be specific to samples; specific amino acids are quantified by liquid
chromatography
-Importance: protein is the most expensive macronutrient; accurate analysis is
important for food labelling and agricultural diet calculations ***
-Assumptions:
-all nitrogen is in a protein; all protein contains 16% nitrogen
-%crude protein = (N in sample*6.25) / wet or dry weight *100% ***
-Where do we get the 6.25?
-assumption: all protein has 16% N
- 100% protein / 16% nitrogen = 6.25 ***
-Therefore, nitrogen*6.25 =crude protein
-Other sources of nitrogen: nitrates, nitrites, urea, nucleic acids…etc (can be
interpreted as protein)
5. Crude Fibre – extracted with hot acid and alkali mimics stomach and intestinal
digestion
-Side Notes: *see table below
6. Nitrogen Free Extract (NFE) – all above values are subtracted from initial sample
weight to estimate available (digestible) carbohydrates
-mainly starches and sugars
-measure by subtraction of all other components
-not an extract; nothing to do with nitrogen!!
-NFE accumulates ALL of the errors that exist for the other components of the
analysis
-%NFE= 100–(%moisture + %crude fat + %ash + %crude protein + %crude fibre)
Soluble Fibre
Insoluble Fibre
• Can bind with cholesterol and bile
acids, decreasing absorption
• Can slow stomach emptying
(digestion of simple starches and
sugars) – decreased Glycemic Index
• Provides significant energy and
maintains colon and microbiota health
through fermentation and release of
volatile FAs (really a carb)
• Speeds up rate of movement of gut
contents
• Helps control intestinal pH, can
benefit microbiota
• Increased laxation
• Reduces risk of diverticulitis, colon
cancer
• Provides some energy through volatile
FA production
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Document Summary
Nutr3210 food composition analysis & digestive systems. Gas chromatography for specific fatty acids: southgate/van soest methods: methods to replace nitrogen free extract (nfe) and crude fibre for modern cho labelling. Why look at food composition: food analysis development, application and study of analytical methods for characterizing foods and their constituents. Info about foods allow the consumer to make informed decisions: government regulation stipulate nutrition facts, ingredient lists and health claims, quality control in industries ensures consistent food composition. Nutrition facts: serving size; servings per container, amount per serving (calories per serving and number of calories from fat, calories measure of fuel you get from food, & vitamins and. Errors: loss of other volatile fa and alcohols can cause overestimation of moisture. Importance: further analyses require dried sample, agricultural systems work with food data on a dry matter and human is on wet weight, water content has. % moisture = 100- %dry matter effects on palatability and shelf life **