Ada Marie Campbell

EVALUATING FOOD BY OBJECTIVE METHODS

Methods used by food scientists to evaluate food quality include objective and sensory methods. This chapter describes the objective methods that do not depend on the observations of an individual; these methods can be repeated using an instrument or a standard procedure. Instrumental or objective methods include a wide variety of tests. The advantages of these tests are many. They may offer a permanent record of results and invite confidence because they are reproducible and less subject to error than the sensory methods of evaluation. However, if results of objective and sensory methods do not correlate, then they may not be measuring the same component of quality and hence the chemical or physical method may not be useful for the study. The appearance of foods can be recorded by means of photography or, in some cases, photocopying. A preliminary acceptance or rejection of a food is based on the visual appearance, including the color. In addition, the measurement of the size of food material particles or structural components may be of interest in research and quality control. The objective measurement of texture is complex because it must reflect the action of the mouth in removing food from an eating utensil; the action of the tongue and jaws in moving the food; and the action of the teeth in cutting, tearing, shearing, grinding, and squeezing food.

FATS AND THEIR LIPID CONSTITUENTS

Fats are mixtures of lipids. Lipids are chemical compounds present naturally in many foods. Lipid mixtures in the form of shortenings, frying fats, and salad oils are used in food preparation and are referred to as fats. The major functions of a fat in food preparation include (1) to tenderize, (2) to contribute to batter or dough aeration, (3) to serve as a heating medium, (4) to serve as a phase in an emulsion, (5) to contribute to flavor, and (6) to enhance smoothness, body, or other textural properties. The chemical structure of lipids determines both the physical properties and the chemical reactions that may occur. The relationship between the chemical properties of lipids and the physical properties of lipids and fats underlies the processing methods used for fats. The relationship between chemical and physical properties of lipids also underlies the functional properties of the fats used in food preparation. This chapter discusses the chemical structure of lipids. The chemical structure of lipids is basic to the study of all other aspects of lipids and the fats that contain them.

CHAPTER 16 – STARCH

Publisher Summary Starch is obtained from cereal seeds and certain roots and tubers. Corn is the major commercial source of starch worldwide, but other sources such as wheat, rice, potatoes, and cassava are important in some countries. This chapter discusses the chemical nature of starch, involving the molecular structures, properties, and changes they can undergo; it further discusses the physical nature of starch, involving granular structures, properties, and changes they can undergo. The chapter discusses the changes that occur in starch during food production, the commercial process of starch modification, and the use of modified starches. Huge array of treatments can be applied to raw starches to change the performance in use. Through these treatments (1) improved resistance to heat, acid, and shear stresses can be achieved, (2) temperatures at which thickening of raw starch occurs can be altered, (3) the specific properties of finished products can be controlled, and (4) the shelf stability of foods destined for cold storage and freezer storage can be enhanced. Starches can be treated for many kinds of modification of their functionality in starch-containing foods.

EVALUATING FOOD BY SENSORY METHODS

Sensory evaluation is an essential component of a food research project or product development. The Sensory Division of the Institute of Food Technologists (IFT, 1981b) defines sensory evaluation as “A scientific discipline used to evoke, measure, analyze, and interpret reactions to those characteristics of foods and materials as they are perceived by the senses of sight, smell, taste, touch, and hearing.” Sensory evaluation tests may be used in product development, research, quality control, and shelf-life studies. In each of these applications, sensory evaluation data may be used as the basis for decision-making. Several factors must be controlled in conducting a sensory evaluation test to minimize experimental error in the data. Sensory tests may be divided into three groups based on the type of information that they provide. The three types are discrimination, descriptive, and affective. The selection of the appropriate test should be based on clearly defined objectives for the project.

CHAPTER 21 – SHORTENED CAKES

Publisher Summary This chapter discusses the respective roles of ingredients in cakes. The rules for cake formulation are based on balance between tenderizing ingredients—sugar and fat—and structural ingredients—flour and egg. Earlier, the weight of sugar in cakes could not exceed the weight of flour. With improvements in cake flour and cake shortenings, it became possible to use larger amounts of sugar and liquid. Balance between liquids and the batter constituents that have an affinity for water is the basis for the rule that the weight of the liquids, including fluid milk, water, and eggs, should equal or slightly exceed the weight of the sugar; otherwise, the high sugar concentration interferes with hydration of proteins and gelatinization of starch. Fat melts and batter viscosity decreases in the early stages of baking. Leavening gases are formed and they diffuse into the gas cell nuclei; new gas cells are not formed. Cake quality is assessed by the measurement of volume, compressibility, and breaking strength and by sensory evaluation.

FRUITS AND VEGETABLES

Many parts of different plants are used as fruits or vegetables. They may be roots; tubers; bulbs; stems and shoots; leaves; flowers and fruits; or pods and seeds. The characteristic texture of a fruit or vegetable depends on the presence and the relative proportions and arrangement of the various types of cells. An important textural characteristic of plant foods is juiciness, which pertains to the quality of having extractable liquid in cells and tissues. Much of the appeal of fresh fruits and vegetables is in their varied colors. The attractive color of the raw fruit or vegetable is, however, subject to change under various conditions associated with use, often resulting in unattractiveness. The characteristic flavor of a fruit or vegetable is attributable to a mixture of many compounds, some of which are present in small amounts. The nutritive value of vegetables also varies with cooking method. This chapter discusses the cooking of dried legumes and the production and use of tofu.

QUICK BREADS, EXTRUDED FOODS, AND PASTA

This chapter provides an account of quick bread, extruded foods, and pasta. Quick bread derive their generic name from the speed of their leavening action. The less development of gluten is desired in quick bread than in yeast bread, and the function of flour in some cases is overshadowed by those of other ingredients. Soft wheat all-purpose flour is used for most quick bread. Extruded foods are produced by the commercial process of extrusion. The chapter discusses the principles and some applications of extruded foods. Some extruders are simply forming extruders in which little or no cooking occurs. They are operated at low screw speeds and without external application of heat. Pasta constitutes one group of extruded foods. Pasta is cold extruded rather than extrusion cooked. Although their ingredients vary somewhat, pasta products differ from one another in physical form having a multitude of shapes and sizes, in addition to hollow tubes (macaroni) and solid rods (spaghetti).

INTRODUCTION TO FOOD SCIENCE

This chapter discusses the relationships between chemical and physical properties of food. Water is a dispersion medium in foods. Fat is also a dispersion medium in water-in-oil emulsions. The dispersed particles in a colloidal dispersion are large. The particles are not large enough to be observed with an optical microscope as they are in suspensions. Destabilization is the removal of the stabilizing factor and consequent aggregation into particles of larger than colloidal dimensions. Colloidal dispersions stabilized by hydration are destabilized by the competitive removal of the water layer. Proteins represent a major group of colloids in food. The acidity of foods or of the medium in which they are cooked also has an important effect on certain qualities of the product. Enzyme activity in foods is important from the standpoint of the enzymes present in plant and animal tissues used as food and the enzymes added during food processing. Browning, which is a common occurrence in food, is undesirable in some cases and is desirable in others. Food preparation and food processing involve energy in many ways. Energy in the form of heat is transferred into and out of food in heating and cooling, respectively.

MILK AND MILK PRODUCTS

This chapter discusses the physical properties, components, flavor, and functional properties of milk and of products made from cows milk. Milk is a complex physicochemical system whose various constituents differ in molecular size and solubility. The smallest molecules of salts, lactose, and water-soluble vitamins are in true solution. The proteins, including enzymes, are in colloidal state because of the large size of their molecules. The fat in nonhomogenized milk is present as the globules of larger than colloidal size. Milk is slightly acidic, having a pH of 6.6 at 25°C. It is well buffered by proteins and salts, especially the phosphates. The pH of milk is temperature dependent. Skim milk and whole milk are Newtonian fluids. Therefore, the viscosity of a given sample depends only on temperature, whereas the viscosities of the non-Newtonian creams, concentrated milks, and butter depend also on shear rate. Prior to the consumption of milk as fluid milk or as a product prepared from fluid milk, milk is subjected to one or more treatments that may influence the characteristics of the product. Dairy-based ingredients are used to improve flavor and nutritive value of products.

PREPARING THE REPORT

Reporting experimental work after its completion is usually required. The main sections of the report are introduction; review of the literature; materials and methods (procedures); results and discussion; summary and conclusions; and references. The headings are subject to some variation and the sections are subject to some combining. The “introduction” section includes a statement of purpose and provides justification for the work. The “literature review” section is combined with the introduction, providing the background for the stated purpose of the study and the justification. The “review of literature” may be a separate section if it is long and if the introduction does not depend on it. A long review should not consist only of separate summaries but should be organized into a cohesive exposition of the subject. The purpose of the “materials and methods” section is to state what was done in the experiment. The results and discussion may be either combined or presented as separate sections. In a short report, they are likely to be combined. Tables and figures are helpful in the presentation of results. The “results and discussion” section might end with conclusions or the section might be followed by a separate summary and statement of conclusions. The “list of references” section at the end of the report should follow a definite form consistently.