NUTR 3210 Lecture Notes - Lecture 2: Short-Chain Fatty Acid, Kjeldahl Method, Freeze-Drying

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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 **

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