Chapter 2 Basic knowledge of food analysis Preface

Chapter 2. Basic knowledge of food analysis

Preface • General procedure of food analysis: • Collection, preparation and preservation of sample → pretreatment of sample → analysis → deal with the data → write report

Section 1. Collection, preparation and preservation of sample 1. Sampling 1. 1 Definition: choose a part of representative sample from the large number analytical object as testing sample.

1. 2 Importance of proper sampling 1) For the same kind of food, its composition and content maybe different for the different of breed, producing area or storage condition. 2) For the same analytical object, the composition and content in different position of object maybe different. So if sampling method is improper, the analytical result would be wrong.

1. 3 Principle of right sampling • 1) Representative • 2) Keep the original condition of sample

1. 5 General method of sampling • 1) Random sampling • 2) Representative sampling

Sampling method 1) Uniform solid materials 2) Stiff semi-solid materials 3) Liquid materials 4) Solid foods of uniform composition

1. 6 Quantity of sample • Sample should be divided into three parts , and each quantity should be no less than 0. 5 kg.

1. 7 Attention on sampling • 1) Any sampling tool should be clean. • 2) Keep the original condition of microorganism and physical chemistry of sample. Avoid population or change of sample.

• 3) Samples of different sensory property should not be mixed together. • 4) After sampling, the sample should be analyzed at once to avoid the change. • 5) The vessel of sample should be labeled the following information: sample name, sampling position, sampling date, sampling method, sampling quantity, sampler and analysis item.

2. Preparation of sample • The sample colleted according to sampling rule sometimes has excessive quantity, large granule and asymmetric composition. • So, to ensure the accurate analytic result, the sample must be treated with crush, mix and concentration, etc. • Aim: Make sure the sample uniform, and any part of sample for analysis can represent the composition of whole sample.

Ø Liquid sample: shake and mix round. Ø Solid sample: chop up, crush, mash, mill. Ø Liquid undissolved (water/oil): separate the undissolved phase, and then sampling respectively. • During preparation of sample, escape of volatile ingredient and change of composition should be avoided.

3. Storage of sample • If sample can’t be analyzed at once, it should be preserved properly. • Storage vessel: sealed and clean. • Storage environment: dark, low temperature (0 -5℃), clean and dryness. • Special instance: adding preservative or freeze-dry.

Section 2. Pretreatment of sample • Why pretreatment? • Principle: Ø 1) Eliminate the disturbed factors. Ø 2) Keep the integrity of determined component. Ø 3) Concentrate the determined component.

1. Destroy the organic substance • Mainly used for determination of inorganic elements in food. • Method: organic substances are decomposed into gas state to volatilize in high temp. or high temp. and strong oxidation, and then the determined component is saved.

2. Distillation • Principle: utilize the difference of volatility of components in liquid mixture. • Ex: determination of volatile acid, pennyroyal. • natural pressure, low pressure, vapor distillation

3. Extraction by solvent • Principle: utilize the difference of solubility of components in sample. • Solid-liquid • Liquid-liquid

Extraction method • • Solex extraction Microwave extraction Ultrasonic extraction Super critical extraction

4. Chromatography • Components are separated in some carrier. • • According to separate principle: Absorption chromatography Distribution chromatography Ion-exchange chromatography

• • According to separated carrier: Paper chromatography Thin layer chromatography Column chromatography

Section 3. Selection of analytical method

1. Importance of proper selection on analytical method • Correct sampling → • Right preparation and preparation of sample → • Proper selection of analytical method : is another key for accurate analytical result.

2. Considered factors when select analytical method. 1) Accuracy and precision of analysis requiements 2）Complexity and speed of analysis methods 3）Speciality of samples 4）Exiting analysis conditions

Section 4. Criterion for food analysis

1. Why should construct the analysis standard? • 1) For production departments • 2)For supervision and management departments

3. International food analysis standard • ISO：国际标准化组织—下设 27个组织 • “食品法典”联合委员会（CAC）：FAO/WHO • AOAC：美国分析化学家协会 — 方法先进可靠 《Official Methods of Analysis of the Association of Official Analytical Chemists》， 每 5年修订一次。