食品课件食分终极版9determination of moisture

1、chemical analysis method Classical methods : (also known as wet chemistry methods) Use separations such as precipitation, extraction, and distillation and qualitative analysis by color, odor, or melting point. Quantitative analysis is achieved by measurement of weight or volume. Analytical methods :

2、 Classical and Instrumental Instrumental methods: Use an apparatus to measure physical quantities of the analyte such as light absorption, fluorescence, or conductivity. The separation of materials is plished using chromatography or electrophoresis methods. Although modern analytical chemistry is do

3、minated by sophisticated instrumentation, the roots of analytical chemistry and some of the principles used in modern instruments are from traditional techniques many of which are still used today. These techniques also tend to form the backbone of most undergraduate analytical chemistry educational

4、 labs.Classification Gravimetric analysis Involves determining the amount of material present by weighing the sample before and/or after some transformation. Application：moisture、ash、fat、DF (dietary fiber)、pectin Volumetric analysis Titration involves the addition of a reactant to a solution being a

5、nalyzed until some equivalence point is reached. Different titrations may use different types of indicators to reach some equivalence point. Application： Acid base titration Total acid , protein, benzoic acid Oxidation-reduction titration Reducing sugarTitration modeDirect titrationBack titrationRep

6、lacement titrationIndirect titrationMOISTURE DETERMINATIONTeaching AimsThe significance of determination of moistureThe principle and the methods of moisture determinationHow to improve the accuracy and precision in moisture determination Whats the significance of moisture determination?9Moisture De

7、terminationMoisture or water is by far the most common component in foods ranging in content from 60 95%. The two most common moisture considerations in foods is that of total moisture content and water activity.Importance of Moisture AssayMoisture is used as a quality factor(a) Jams and jellies to

8、prevent sugar crystallization; (b) Sugar syrupsMoisture (or solids) content is often specified in compositional standards (i.e., Standards of Identity), it greatly influences the physical properties and product or raw material qualityThe shelf life of food product is closely related with moisture co

9、ntent (a) Dehydrated vegetables and fruits; (b) Dried milks(c) Powdered eggs; (d) Dehydrated potatoes; (e) Spices and herbsComputations of the nutritional value of foods require that you know the moisture content (Nutrition label).Moisture data are used to express results of other analytical determi

10、nations on a uniform basisMoisture DeterminationImportance of Moisture AssayE.g. Food that should control the moisture content Soybean productCerealSalted Vegetables , SaltMilk powderPork flossE.g. Moisture Content of Selected FoodsMethods of Moisture Loss MeasurementConvection or forced draft ovens

11、 (AOAC) -Very simple; Most commonVacuum Oven -Sample is placed in oven under reduced pressure thereby reducing the boiling point of water.Microwave Oven -Uses microwave as a heat source; Very fast methodInfrared Drying -Uses infrared lamp as a heat source; Very fastHalogen Drying -Twice as fast as t

12、raditional infra red technology GBUniversally applicable, highly efficient干燥法Drying methods重量法是一种经典的测量水分的方法, 包括常压干燥、真空干燥、红外线加热干燥、微波干燥Principle: The total moisture content of foods is generally determined by some form of drying method whereby all the moisture is removed by heat and moisture is determ

13、ined as the weight lost. % water = wet weight of sample-dry weight of sample wet weight of sampleAir-oven Methodput the sample (10g) in flat, tarred dish -specified time and temperature (95105for 34 h) -measure the loss of water. Vacuum oven Methoduse it if you do not want to expose to high temperat

14、ure. e.g. Food rich in fructose must be dried at 70 or below. Longer drying timeMethods of Moisture Loss MeasurementThermo-gravimetric method is universally recognized as an efficient, reliable and cost-effective method for determining moisture content, and can be utilized in virtually any such envi

15、ronment. Not suitable for the sample (16%, two step dryingAbout the method and determination process Volatilization and position of other food components-Different methodTemperature and time ( position of other food components) - Constant weight 2 mg Significant variations in temperatures or power l

16、evels within evaporation instrumentSample pans and sample size-Residue weight 1.5-3gClumping and surface crust formation-Increase evaporation surface areaPractical ConsiderationsAbout Drying MethodsAdvantages: Notably this type of testing is simple and generally does not require high capital investm

17、ent. Disadvantages: Time consuming and non-portability. Spicespeppercornsstar anisecinnamonclovespepperInstrument of Distillation MethodDirectreflux distillationsDistillation methodsDistillation methods are based on direct measurement of the amount of water removed from a food sample by evaporation%

18、Moisture = 100 (MWater/Minitial)Principle of distillation method: Heating a weighed food sample (Minitial) , immerse sample into an organic solvent (placed in a flask) that is immiscible with water (xylene二甲苯, toluene甲苯). The water in the sample evaporates and is collected in a graduated glass tube

19、where its mass is determined (MWater )The organic solvent must be insoluble with water; have a higher boiling point than water; be less dense than water; and be safe to use. E.g. water100,benzol80.2 ,water +benzol 69.25 Distillation methodsDetermination process The flask containing the sample and th

20、e 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 gradua

21、ted tube, distillation is stopped and the volume of water is read from the tube.Practical ConsiderationsThere are a number of practical factors that can lead to erroneous results:Emulsions can sometimes form between the water and the solvent which are difficult to separate;Water droplets can adhere

22、to the inside of the glassware, position of thermally labile samples can occur at the elevated temperatures used.Advantages: Suitable for application to foods with low moisture contents; Suitable for application to foods containing volatile oils, such as herbs or spices, since the oils remain dissol

23、ved in the organic solvent, and therefore do not interfere with the measurement of the water; Equipment is relatively cheap, easy to setup and operate; Distillation methods have been officially sanctioned for a number of food applications. (The only recognized methods for Spice and perfume)Disadvant

24、ages: Destructive; Relatively time consuming;Involves the use of flammable solvents; Not applicable to some types of foods.Practical ConsiderationsNot suitable for the sample ( 1.010-3 g/100 g The precision of the technique can be improved by using electrical methods to follow the end-point of the r

25、eaction, rather than observing a colour change.Titration end point judgment 2Coulomb titration mode：amperometric titationSuitable for the sample 1.010-5 g/100 g CHEMICAL METHODS K-FKarl Fisher Method（ K-F METHODS）Standard technique for low moisture foods (Including bound water ). Especially good for

26、 reducing sugars and protein-rich foods and good for foods with high volatile oils (e.g. dried fruits and vegetables, confectionary, coffee, oils and fats).Karl Fisher MethodCriteria for Selecting an Analytical TechniquePrecisionAccuracyReproducibilitySimplicityCostSpeed(time)On-line/off-lineSensitivitySpecificitySafetyDestructive/ Non-d