Food Analysis Fourth Edition edited by S. Suzanne Nielsen Purdue University West Lafayette, IN, USA ABC II part Compositional Analysis of Foods 6 chapter Moisture and Total Solids Analysis Robert L. Bradley, Jr. Department of Food Science, University of Wisconsin, Madison, WI 53706, USA [email protected] 6.1 Introduction 87 6.2.1.4 Types of Pans for Oven Drying 6.1.1 Importance of Moisture Assay 87 Methods 90 6.1.2 Moisture Content of Foods 87 6.2.1.5 Handling and Preparation of 6.1.3 Forms of Water in Foods 87 Pans 90 6.1.4 Sample Collection and Handling 87 6.2.1.6 Control of Surface Crust Formation 6.2 Oven Drying Methods 88 (Sand Pan Technique) 90 6.2.1 General Information 88 6.2.1.7 Calculations 91 6.2.1.1 Removal of Moisture 88 6.2.2 Forced Draft Oven 91 6.2.1.2 Decomposition of Other Food 6.2.3 Vacuum Oven 91 Constituents 89 6.2.4 Microwave Analyzer 92 6.2.1.3 Temperature Control 89 6.2.5 Infrared Drying 93 S.S. Nielsen, Food Analysis, Food Science Texts Series, DOI 10.1007/978-1-4419-1478-1_6, 85 c Springer Science+Business Media, LLC 2010 86 Part II • Compositional Analysis of Foods 6.2.6 Rapid Moisture Analyzer Technology 93 6.5.4 Infrared Analysis 99 6.3 Distillation Procedures 93 6.5.5 Freezing Point 100 6.3.1 Overview 93 6.6 Water Activity 101 6.3.2 Reflux Distillation with Immiscible 6.7 Comparison of Methods 101 Solvent 93 6.7.1 Principles 101 6.4 Chemical Method: Karl Fischer Titration 94 6.7.2 Nature of Sample 101 6.5 Physical Methods 96 6.7.3 Intended Purposes 102 6.5.1 Dielectric Method 96 6.8 Summary 102 6.5.2 Hydrometry 96 6.9 Study Questions 102 6.5.2.1 Hydrometer 97 6.10 Practice Problems 103 6.5.2.2 Pycnometer 97 6.11 References 104 6.5.3 Refractometry 98 Chapter 6 • Moisture and Total Solids Analysis 87 6.1 INTRODUCTION (d) Glucose syrup must have ≥70% total solids. (e) The percentage of added water in processed Moisture assays can be one of the most important meatsiscommonlyspecified. analyses performed on a food product and yet one of 5. Computations of the nutritional value of foods the most difficult from which to obtain accurate and require that you know the moisture content. precise data. This chapter describes various methods 6. Moisture data are used to express results of for moisture analysis – their principles, procedures, other analytical determinations on a uniform applications, cautions, advantages, and disadvan- basis [i.e., dry weight basis (dwb), rather than tages. Water activity measurement also is described, wet weight basis (wwb)]. since it parallels the measurement of total moisture as an important stability and quality factor. With an understanding of techniques described, one can apply 6.1.2 Moisture Content of Foods appropriate moisture analyses to a wide variety of The moisture content of foods varies greatly as shown food products. in Table 6-1 (4). Water is a major constituent of most food products. The approximate, expected moisture content of a food can affect the choice of the method of measurement. It can also guide the analyst in deter- 6.1.1 Importance of Moisture Assay mining the practical level of accuracy required when One of the most fundamental and important analytical measuring moisture content, relative to other food procedures that can be performed on a food product constituents. is an assay for the amount of moisture (1–3). The dry matter that remains after moisture removal is com- 6.1.3 Forms of Water in Foods monly referred to as total solids. This analytical value is of great economic importance to a food manufac- The ease of water removal from foods depends on how turer because water is an inexpensive filler. The fol- it exists in the food product. The three states of water lowing listing gives some examples in which moisture in food products are: content is important to the food processor. 1. Free water: This water retains its physical prop- 1. Moisture is a quality factor in the preservation erties and thus acts as the dispersing agent for of some products and affects stability in colloids and the solvent for salts. (a) Dehydrated vegetables and fruits 2. Adsorbed water: This water is held tightly or is (b) Dried milks occluded in cell walls or protoplasm and is held (c) Powdered eggs tightly to proteins. (d) Dehydrated potatoes 3. Water of hydration: This water is bound chem- ically, for example, lactose monohydrate; also (e) Spices and herbs · 2. Moisture is used as a quality factor for some salts such as Na2SO4 10H2O. (a) Jams and jellies to prevent sugar crystalliza- Depending on the form of the water present in a tion food, the method used for determining moisture may (b) Sugar syrups measure more or less of the moisture present. This (c) Prepared cereals – conventional, 4–8%; is the reason for official methods with stated proce- puffed, 7–8% dures (5–7). However, several official methods may 3. Reduced moisture is used for convenience in exist for a particular product. For example, the AOAC packaging or shipping of International methods for cheese include: Method (a) Concentrated milks 926.08, vacuum oven; 948.12, forced draft oven; 977.11, (b) Liquid cane sugar (67% solids) and liquid microwave oven; 969.19, distillation (5). Usually, the corn sweetener (80% solids) first method listed by AOAC International is preferred (c) Dehydrated products (these are difficult to over others in any section. package if too high in moisture) (d) Concentrated fruit juices 6.1.4 Sample Collection and Handling 4. Moisture (or solids) content is often specified in compositional standards (i.e., Standards of General procedures for sampling, sample handling Identity) and storage, and sample preparation are given in (a) Cheddar cheese must be ≤39% moisture. Chap. 5. These procedures are perhaps the greatest (b) Enriched flour must be ≤15% moisture. potential source of error in any analysis. Precau- (c) Pineapple juice must have soluble solids of tions must be taken to minimize inadvertent mois- ≥10.5˚Brix (conditions specified). ture losses or gains that occur during these steps. 88 Part II • Compositional Analysis of Foods against the sample. It is critical to control temperature 6-1 fluctuations since moisture will migrate in a sample to table Moisture Content of Selected Foods the colder part. To control this potential error, remove Approximate the entire sample from the container, reblend quickly, Percent Moisture and then remove a test portion (8,9). (Wet Weight To illustrate the need for optimum efficiency and Food Item Basis) speed in weighing samples for analysis, Bradley and Cereals, bread, and pasta Vanderwarn (10) showed, using shredded Cheddar Wheat flour, whole-grain 10.3 cheese (2–3 g in a 5.5-cm aluminum foil pan), that White bread, enriched (wheat flour) 13.4 moisture loss within an analytical balance was a Corn flakes cereal 3.5 straight line function. The rate of loss was related Crackers saltines 4.0 to the relative humidity. At 50% relative humidity, it Macaroni, dry, enriched 9.9 required only 5 s to lose 0.01% moisture. This time Dairy products Milk, reduced fat, fluid, 2% 89.3 doubled at 70% humidity or 0.01% moisture loss Yogurt, plain, low fat 85.1 in 10 s. While one might expect a curvilinear loss, Cottage cheese, low fat or 2% milk fat 80.7 the moisture loss was actually linear over a 5-min Cheddar cheese 36.8 study interval. These data demonstrate the necessity of Ice cream, vanilla 61.0 absolute control during collection of samples through Fats and oils weighing, before drying. Margarine, regular, hard, corn, 15.7 hydrogenated Butter, with salt 15.9 Oil-soybean, salad, or cooking 0 6.2 OVEN DRYING METHODS Fruits and vegetables Watermelon, raw 91.5 Oranges, raw, California navels 86.3 In oven drying methods, the sample is heated under Apples, raw, with skin 85.6 specified conditions, and the loss of weight is used Grapes, American type, raw 81.3 to calculate the moisture content of the sample. The Raisins 15.3 amount of moisture determined is highly dependent Cucumbers, with peel, raw 95.2 on the type of oven used, conditions within the oven, Potatoes, microwaved, cooked in skin, 72.4 flesh and skin and the time and temperature of drying. Various oven Snap beans, green, raw 90.3 methods are approved by AOAC International for Meat, poultry, and fish determining the amount of moisture in many food Beef, ground, raw, 95% lean 73.3 products. The methods are simple, and many ovens Chicken, broilers and fryers, light 68.6 allow for simultaneous analysis of large numbers of meat, meat and skin, raw samples. The time required may be from a few minutes Finfish, flatfish (flounder and sole 79.1 species), raw to over 24 h. Egg, whole, raw, fresh 75.8 Nuts Walnuts, black, dried 4.6 6.2.1 General Information Peanuts, all types, dry roasted 1.6 with salt 6.2.1.1 Removal of Moisture Peanut butter, smooth style, with salt 1.8 Sweeteners Any oven method used to evaporate moisture has as its foundation the fact that the boiling point of water Sugar, granulated 0 ◦ Sugar, brown 1.3 is 100 C; however, this considers only pure water at Honey, strained or extracted 17.1 sea level. Free water is the easiest of the three forms of water to remove.