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Study Guide - Module 2
Food Technology (FDS308)
Charles Sturt University
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FDS
Food Technology
FACULTY OF SCIENCE
Module 2 201830
FDS308
Food Technology
FDS308 Module 2 Technologies of liquid foods
Faculty of Science
Originally written and compiled for FDS201 by Associate Professor Samson Agboola
Adapted for FDS308 by Dr Ester Khosa
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Contents
- Module profile Page
- Topic 1 Milk and dairy technology
- Introduction
- Overview of milk chemistry
- Components of milk
- Physical structures in raw milk
- Chemistry of the coagulation of milk by rennet
- Factors affecting milk yield and composition
- Fluid milk processing
- Cheese making process-an overview
- Technology of ice cream manufacture
- Topic 2 Technology of fats and oils
- Introduction
- Structure and functional properties of fats and oils
- Sources of fats and oils and their functional properties
- Production of butter
- Production of margarine
- Tests for fats and oils
- non-alcoholic beverages Topic 3 Technology of fruit juices and carbonated
- Introduction
- Composition of fruit juices
- Manufacture of orange juice
- Processing of apple juice
- Manufacture of carbonated non-alcoholic beverages
- Ingredients used in the manufacture of carbonated soft drinks
- Manufacture of carbonated soft drinks
- Topic 4 Technology of alcoholic beverages
- Introduction
- Ingredients in the manufacture of beer
- Technology of beer making
- Wine making
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Topic 1 Milk and dairy technology
Objectives
Upon successful completion of this topic, students should be able to:
provide an overview of the chemistry of the raw milk system;
discuss the variations in milk composition and their causes;
provide an overview of unit operations in fluid milk processing, such as separation, homogenisation and pasteurisation;
provide an overview of cheese making processes;
be familiar with the technology of ice cream manufacture; and
give an overview of other milk and dairy products.
Introduction
Milk is a food product that can be consumed with minimal processing and it contributes immensely to human nutrition from infancy. Milk can be separated into its principal components, cream and skim milk, which may be further separated into butterfat, caseins and other milk proteins, and lactose. It can also be processed into many products such as cheese and milk powder, or used as a major ingredient in many manufactured foods such as ice cream and confectioneries. Milk and dairy products are one of the most common food products in the whole world. As a result, most of our discussions on this topic will be overviews considering the amount of material available on each milk and dairy product. You are encouraged to learn more about these products from readily available texts on the individual subheadings.
It must also be stated at this time that dairy processing is a high risk area, very susceptible to microbial and other biochemical hazards. High standards of hygiene is therefore very important for a successful dairy operation. Most operations involve the institution of the food safety program HACCP, and fluid milk processing usually involve the Clean-in-Place (CIP) cleaning regime.
As you will find out from this and other topics in the second part of this subject, you will frequently need to recall aspects of the earlier topics, and even some of the unit operations you have learnt in food processing and engineering. Readings and other relevant chapters in your textbook should be very useful in this regard.
Overview of milk chemistry
There are 2 major definitions for milk. One is a chemical definition and the other, a legal definition. Until recently, milk was legally defined as the lacteal secretion (excluding colostrum) from the udder of a cow. The definition has since been
expanded to include secretions from other mammalian species, e. goat, sheep, buffalo and camel. However, the discussion here relates mainly to bovine (cow) milk.
Chemically, milk is a complex emulsion of fat globules dispersed in a colloidal solution of proteins, carbohydrates, vitamins and minerals. The average composition and range of the major constituents in milks of western breeds of cattle are shown in Table 1 below. Milk composition is dependent on several factors such as breed of cattle, stage of lactation, age, nutrition and weather. The most variable of the components is fat, stemming from the selection and breeding for fat which is the most expensive component. Lactose and ash (minerals) are the least variable components due to the need for the osmotic pressure of milk to match that of blood. The pH of fresh milk is usually between 6.6-6. The components of bovine milk are discussed in more details below.
Table 1: Composition of Bovine Milk from Western Cattle
Component Average percentage Range (Avg. percentages)
Water 86 85-87. Fat 4 3-5. Protein 3 3-3. Lactose 5 4-5. Ash 0 0-0.
Source: Swaisgood, H. R. (1985). Food chemistry, OR Fennema (Ed.) (2nd ed.). New York: Marcel Dekker.
Components of milk
Proteins
Milk proteins can be divided into 2 major parts: caseins and whey proteins (Table 1). They are defined usually by their respective solubility at pH 4; the insoluble fraction being composed of caseins. Other means for separating the proteins include calcium sensitivity, salt solubility (e. ammonium sulphate) and renneting (see later on). Caseins are the most abundant of the milk proteins and exist in form of micelles within the milk serum (plasma).
The micelles are made up of the 4 major caseins, known as αs 1 -, αs 2 -, β- and κ- caseins, present roughly in ratios 4:1:4:1, and held together by colloidal calcium phosphate. Degradation of the β-casein by the indigenous enzyme plasmin leads to the formation of γ-caseins and the proteose-peptones, the later which partition into the whey protein fraction (soluble at pH 4). Caseins are calcium-sensitive due to the presence of phosphoseryl residues in their amino acid sequence, heat stable due to their high proportion of proline, leading to ‘random’ structures (proportion of regular secondary structures like α-helix, β-sheets are minimal) and low in sulphur-containing amino acids (less disulphide bonds).
Table 1: Lipid composition of Bovine Milk
Lipid Weight percent g/litre
Triglycerides 97 -98 31. Diglycerides 0.3-0 0. Monoglycerides 0.02-0 0. Free fatty acids 0.1-0 0. Free sterols 0.2-0 0. Phospholipids 0.2-1 0. Hydrocarbons Trace Trace Sterol esters Trace Trace
Source: Swaisgood, H. R. (1985). Food chemistry, OR Fennema (Ed.) (2nd ed.). New York: Marcel Dekker.
Table 1: Major Fatty Acids in Milk Fat
Fatty Acid # of carbon atoms: # of double bonds
Approx. Content (% weight)
Melting Point (°C)
SATURATED 65. Butyric 4:0 2 -8. Caproic 6:0 2 -4. Caprylic 8:0 1 16. Capric 10:0 3 31. Lauric 12:0 2 44. Myristic 14:0 8 54. Palmitic 16:0 31 63. Stearic 18:0 13 69.
UNSATURATED 34. Myristoleic 14:1 0 -4. Palmitoleic 16:1 1 0. Oleic 18:1 29 16. Linoleic 18:2 2 -5. Linolenic 18:3 0 -14.
Source: Rosenthal, I. (1991). Milk and dairy products: Properties and processing. Weinheim: VCH Pub.
Milk salts and sugars
Milk salts are mainly chlorides, phosphates, citrates and bicarbonates of sodium, potassium, calcium and magnesium. They are distributed between a soluble phase and a colloidal phase (Table 1), and this may be very important in the stability of milk. The distribution of the salts can vary with pH. There is also an inverse relationship between milk salts and lactose, such that the total solutes remain constant for a constant osmolality. As a result of this, milk has a fairly constant freezing point (about -0°C), and can be used as an index of quality (to check adulteration with water).
Lactose is the predominant carbohydrate in milk and it contributes to the characteristic milk flavour. Its synthesis is associated with α-lactalbumin (α-la). Lactose has only one fifth the sweetness of sucrose and can crystallise in 2 forms-alpha (α) and beta (β); β- being the more soluble form. At 20°C, the ratio of the β form to the α-form is 1.
Table 1: Concentration and distribution of principal milk salts and lactose
Component Mean (mg/100g) Range(mg/100g) % in soluble phase
% in colloidal phase
Total calcium 117 110.9-120 33 67 Ionised calcium 11 10.5-12 100 0 Magnesium 12 11.4-13 67 33 Sodium 58 47 -77 94 6 Potassium 140 113 -171 93 7 Citrate 176 166 -192 94 6 Total phosphorus 95 79.8-101 45 55 Inorganic phosphorus 62 51.9-70 54 46 Chloride 104 89.8-127 100 0 Lactose 4900 4800 -5000 100 0
Source: Swaisgood, H. R. (1985). Food chemistry, OR Fennema (Ed.) (2nd ed.). New York: Marcel Dekker.
Enzymes
Enzymes in milk can be indigenous or from microbial growth. Assuming that the milk is well preserved, the most important enzymes are therefore the indigenous ones. Proteases and lipases are probably the most important to the food scientist since these affect the quality of dairy products. They can be present in the serum or associated with either the casein micelles or the fat globule membrane. Lipases can cause rancidity in milk especially after agitation (e. pumping, homogenisation) due to increased access of membrane-bound lipases to the fat and increased fat surface area. Other enzymes located in the fat globule membrane are alkaline phosphatase, catalase, xanthine oxidase, and sulphydryl oxidase.
Vitamins and minerals
The nutritive quality of milk emphasises its role as a complete food. It is composed of the major food groups-proteins, carbohydrates and fats. It also contains vitamins A, B 6 , B 12 , C, D, thiamine, riboflavin, niacin, folacin and minerals like calcium, phosphorus, magnesium, zinc and iron. Milk products may sometimes be fortified with iron and vitamin D, since they are present in somewhat deficient amounts in milk. Milk is also low in vitamin C and folacin.
Chemistry of the coagulation of milk by rennet
Rennets are commercial names for enzyme preparations used in milk coagulation during cheese manufacture. Traditionally, rennet, mainly in form of an enzyme known as chymosin, is obtained from the fourth stomach of a young calf (the commercial calf rennet is about 85% chymosin and 15% porcine pepsin). Due to the high cost of chymosin (rennin), other sources including vegetable, fungus and recombinant DNA have been explored. Most of these enzymes can be classified among a group of enzymes known as acidic (aspartic) proteases because their maximum activity is in the acidic pH range (starter culture produces lactic acid which help reduce milk pH to optimum level for renneting.
(a) (b) (c)
Figure 1: Schematic diagram of the attack by chymosin (shown as C) on casein micelles. Three different points in the reaction are illustrated: (a) the κ- casein coat of the micelles is intact, and chymosin has just been added; (b) some time later, much of the κ-casein has been hydrolysed but sufficient remains to prevent aggregation; (c) at a later time still, nearly all of the κ-casein has been hydrolysed and the micelles have started to aggregate
Source: Dalgleish, D. G. ( 1992 ). The enzymatic coagulation of milk. In P. F. Fox (Ed.), Advanced Dairy Chemistry, Vol. 1, Proteins. London: Elsevier Applied Science.
Commercial rennet attacks casein micelles, cleaving κ-casein (remember this is located towards the periphery) at the Phe-Met (105-1 06) peptide bond, and yielding 2 fractions: hydrophobic (insoluble) para-κ-casein and the hydrophilic (soluble) caseinomacropeptide (Figure 1). Due to its properties, the para-κ- casein can aggregate, forming a network of caseins with fat suspended within its matrix, leaving the whey proteins in a solution known as either rennet or cheese whey. It has been established that aggregation of the micelles does not start until most of the κ-casein have been hydrolysed. Thus clotting of milk proceeds in 2 stages-enzymatic hydrolysis and aggregation of the micelles. The aggregation state is the step that is definitely influenced by the addition of calcium salts (production of firmer curds, shorter coagulation time etc.). It is not clear whether calcium improves the activity of rennets (enzymatic hydrolysis stage).
The action of enzymes does not stop after the coagulation. In fact, they continue to act on all the caseins especially during ripening of the cheese, generating some of the flavour compounds responsible for cheese aroma. The overall aroma is however developed by a range of enzymes from rennets, starter cultures, moulds and natural milk flora. These include proteases, peptidases, and lipases.
Factors affecting milk yield and composition
The udder of a cow produces milk from components withdrawn from the animal’s bloodstream. There are many factors that may affect the yield and composition of raw milk as delivered by the cow. These include factors such as milk flow within the udder, through to breed and genetic manipulations, nutrition, age, management practices and the environment. Some of these factors are further expanded below.
a. Milk flow within the udder: The tightness of the teat muscles can affect the yield of the milk from individual cows. ‘Hard milkers’ are cows having teats with tight smooth muscle bundles because milk is expressed as a fine spray and milk flow is very slow. Teats with weak, relaxed, or incompetent teat muscles are termed ‘leaky’, as such teats milk out in 2-3 minutes, compared to 10 or more minutes with hard milkers.
b. Breed of animal: In western cattle, the heavier breeds like the Holstein and Friesians have the highest mean lactation yield of milk. However, the Jersey and Guernsey tend to have higher fat and casein levels. However, there are still within breed variations especially due to environmental and management factors which we will look at later. It is important to note however, that the animal’s breed has the most influence on the yield and composition of the milk.
c. Genetics: Breeding for certain traits especially percentage of fat and milk yield is very common due to their economic importance. Selecting for one trait however, has effect on other traits. For example, if one selects for milk yield, generally, the fat yield will increase but the percentage of fat will decrease. Similar trend is observed in selecting for protein. So generally, if one selects for milk yield, the yields of other milk solids increase, but their percentages decrease.
d. Environment: Cows of European origin are generally more susceptible to stress than cows of Tropical origin. However, tropical cows have lower basal metabolic rates, lower feed consumption, and as a result, lower milk yield. Environmental effects are usually confounded with nutrition (feed and water). For example, turning cows out onto the lush green pastures of the spring raises milk production, a phenomenon known as the spring flush. Also, the stresses associated with thermal changes (hot and cold environmental temperatures) affect dairy cattle metabolism by increasing maintenance requirements. In winter months, water intake decreases and feed consumption increases, and the milk yield does not decrease much except in adverse conditions e., temperatures below -5°C. In the summer months however, water intake increases while feed intake decreases, which lead to less milk yield.
proteins and minerals but less lactose than the normal milk which appears after several days. Colostrum also contains an elevated content of antibodies (up to 10%) which are transferred to the newborn.
Fluid milk processing
In modern dairies, the milk from the cow is drawn under vacuum from the milking machine cups through pipes leading to a bulk holding tank in another room. The tank is provided with refrigeration to cool the milk to 4°C or lower to control bacterial growth.
The cooled milk is usually transported to processing plants in bulk steel tanks, where it is maintained at 4°C or lower by refrigeration. Simple, on the farm quality control tests like colour and odour may be carried out by the tanker drivers. Alternatively, it may be brought to a central receiving station where it is pooled, before being sent to the processing plant. At the plant, more rigorous tests are performed including microbiological (bacteria, yeast and mould) counts, number of somatic cells, estimation of sediments, fat percentage, total solids content, and freezing point determination. Some of these tests like fat and total solids are necessary for payment, while others like bacterial counts and presence of faecal indicator organisms may be necessary for acceptance/rejection of the milk sample.
Once the quality of the milk as received by the plant is confirmed adequate, the first step in the processing is a further blending to the required fat content. This may be important depending on the breed of the cows dominating the various batches of milk from different suppliers. After that, the milk is clarified. This is achieved by passing the milk through a centrifugal clarifier (Figure 1) to remove sediment, somatic cells, and some bacteria. The clarifier works by distributing the milk in thin layers over conical disks which revolve at high speed. Separation is thus effected by density difference and the impurities flow outwards to the edges of the disks. The same principle (centrifugation) is used in fat separation from milk to produce cream (see later). Non clarification would lead to deposit formation at the bottom of the homogenised milk bottles.
Figure 1: Centrifugal clarifier
Source: Potter, N. N., & Hotchkiss, J. H. ( 1995 ). Food science (5th ed.). New York: Chapman & Hall.
The clarified milk is then pasteurised in order to rid the milk of any pathogens. There is also an attendant reduction in the bacterial load for improved keeping quality. Pasteurisation has been covered in details in the second topic of this module.
We have seen that fat in milk is present in discrete globules which, being insoluble and lighter than water, rise to the top in form of a cream line. While this characterisation is used for the commercial separation of cream from milk, it is a defect in full-fat milk. Therefore, in fluid milk processing, it is necessary to distribute the fat uniformly throughput the milk in such a way that it will not separate out. This is the purpose of homogenisation.
The simplest and most common mechanism is by passing the milk through a narrow opening at very high velocity. This is known as the pressure homogeniser. A pressure homogeniser consists essentially of a homogenising valve and a high pressure pump. The valve provides an adjustable gap 15-300 mm wide through which the crude emulsion is pumped at pressures up to 69 MNm-2. On entering the gap, the milk is accelerated to velocities in the range 50-8000 ms-1. This extreme condition of turbulence makes the fat globules in raw milk (up to
Cheese making process-an overview
You must be aware of the roles of the raw materials including milk, microorganisms (starter cultures and moulds) and enzymes (especially rennet), some of which have been alluded to in the section on milk chemistry. You will realise that different varieties of cheeses are available using different processing methods. All the same, you must be familiar with the major processing steps including fermentation, coagulation, hooping, whey drainage, curd cooking, salting, etc. The role of maturation (ripening), especially in the development of flavour and texture in the finished product should be well understood.
Recommended reading (available online at CSU library as an Ebook)
Johnson, M., Law, B. A (2010). The origins, development and basic operations of cheesemaking technology. In B. A. Law, & A. Y. Tamime (Eds.), Technology of cheesemaking (2nd ed., Chapter 2, pp. 68-97). Chichester, West Sussex: Blackwell.
Technology of ice cream manufacture
Introduction and definition
Ice cream was believed to have originated from Chinese culture and brought into Europe in the 13th century by explorer Marco Polo. It was introduced to North America by the European settlers and it is in America that most of the technological developments in ice cream manufacture occurred. Ice cream, to date, is probably the most popular dessert in the whole world. It is considered a very palatable, nutritious, healthful and relatively inexpensive food. A typical composition of regular (non-diet) ice cream is given in Table 1 below:
Ice cream can be defined as a partly frozen foam (dispersion of a gas in liquid) with an air content of 40-50% by volume. The continuous phase of the foam (liquid) contains dissolved solids i., sugars, proteins and stabilisers and the fatty phase is in an emulsified form. From studies involving microscopy, rheology and thermal analyses, we have concrete ideas about the ice cream structure. It is still recognised as a very complex system of different structural elements including solids (ice, fat and sugar crystals), liquids (oil globules and water) and gasses (air cells). Here, we shall concern ourselves with the ingredients used in ice cream manufacture and how these ingredients act to give us the product known as ice cream.
Table 1: Typical composition of regular ice cream
Component Percent composition
Fat 9. MSNF* (including lactose) 10. Added Sugar (mainly sucrose) 16. Emulsifier 0. Stabiliser 0. Inorganic Ash 1.
- Milk Solids Not Fat
Ingredients and their functions
Table 1 below lists the common ingredients used in the manufacture of ice cream. It should be noted that this list is not exclusive as many other ingredients e., fruits and flavours are used. The legal standard, especially with respect to fat and labelling must be followed for commercial products and this may differ from one country to another.
Table 1: Ice cream ingredients
Butter Buttermilk powder Milk Cream Whey powder Sugar Condensed milk Emulsifiers Skim milk powder Stabilisers
Each of the major ingredients in ice cream serves specific functions and contributes particular attributes to the final product. Many of these ingredients of course have to be processed in order for them to be functional as we shall see shortly. The major ingredients are milkfat, milk solids-not fat (MSNF), sugars, stabilisers, emulsifiers and flavours.
a. Milkfat: Gives the product a rich flavour, smooth texture and body. It is also a source of calories and contributes heavily to the energy value of ice cream
b. MSNF: Contributes to flavour, body and desirable texture. Higher levels of MSNF permits higher overruns. Overrun is the increase in volume of the ice cream mix by whipping in air. The ice cream mix is obtained by mixing and homogenising the ingredients (see also processing steps).
c. Added sugars: Adds flavour (sweetness) and body, and lowers freezing point of mix so that it does not freeze solid in the freezer. Sugars may be added in form of sucrose or as corn syrup, or dextrose-fructose mixtures.
Study Guide - Module 2
Course: Food Technology (FDS308)
University: Charles Sturt University
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