© Food – a fact of life 2009 Foundation Preservation

© Food – a fact of life 2009 Principles of food preservation The aim of preservation is to prevent food spoilage as a result of growth of micro-organisms and breakdown of food by enzymes. There are both short - and long-term methods of preservation. As soon as food is harvested, slaughtered or manufactured into a product it starts to change. This is caused by two main processes: autolysis – self destruction, caused by enzymes present in the food; microbial spoilage – caused by the growth of bacteria, yeasts and moulds. Micro-organisms and enzymes need certain conditions to survive and reproduce. These include: temperature oxygen food time moisture of water will slow or prevent decay pH level

© Food – a fact of life 2009 Some of the factors affecting the growth of micro - organisms can be manipulated in different ways to prolong the life of the food product. Temperature (high and low) High temperatures prevent decay and destroy enzymes and proteins, killing the organisms responsible for decay. Low temperatures slow decay by slowing the rate of reaction of enzymes and will prevent growth and reproduction. Low temperatures slow decay. Chilling or freezing the food to retard growth of micro-organisms and inhibit enzyme activity. Oxygen Lack of oxygen will slow or prevent most forms of decay. Oxygen is needed for respiration by organisms. Some bacteria can survive without oxygen, such as those used in biogas generators. Food kept in an airtight container will deprive micro- organisms of oxygen and prevent contamination. Moisture Lack of water will slow or prevent decay. Water is needed for transport and to support reactions inside organisms. Decay organisms need water to digest their food.. Reducing the moisture content of the food to make water, (which is essential for growth), unavailable to micro-organisms. Alternatively, placing food in a sugary solution will make water unavailable for the growth of micro-organisms. pH level Placing food in an acidic or alkaline solution will inhibit the growth of micro- organisms.

© Food – a fact of life 2009 Methods of Preservation Traditional methods of food preservation began from the essential need to store supplies when they were plentiful and to keep the food fresh for as long as possible to last through the winter months. Although food preservation has been in use for thousands of years, it is only in the last two centuries that many of the ‘new’ food processing techniques have been developed. High temperature Low temperature Removing moisture Use of chemicals Controlling the atmosphere Irradiation.

© Food – a fact of life 2009 Temperature MethodExamplesHow it works CanningVegetables The sealed can is heated to kill bacteria. When it cools, no more bacteria can enter. Cooling or chillingReady made meals The food is kept at a low temperature (4°C) so bacteria cannot reproduce (they're not killed). FreezingVegetables and meat products Bacteria cannot reproduce at low temperatures and the water in the food is frozen. DryingFruit, flour, soups, pasta Removing water prevents the microorganisms from growing, or digesting the food. Adding salt or adding sugarJam, crisps, meatRemoves water by osmosis Adding vinegarPickles Changes the pH to prevent the enzymes in the microorganisms from working. Food can be preserved by removing one or more of the factors that bacteria and fungi need to survive. Methods of food preservation

© Food – a fact of life 2009 High temperatures pasteurization sterilization UHT Canning Milk preservation 1. Milk which is pasteurized is heated up to 72°C for 15 seconds then rapidly cooled to 10°C. Pasteurization only kills pathogenic bacteria so is not a long- term method of preservation. 2. Sterilized milk tastes and looks different to ordinary milk. The milk is bottled and sealed with metal caps and then heated up to 104°C for 40 minutes or 113°C for 15 minutes. Sterilization kills nearly all micro-organisms and enzymes and is therefore a long-term method of preserving milk. Sterilization preserves food without the use of preservatives or refrigeration so is attractive to consumers concerned with ‘artificial’ foods. Tetra Pak cartons are often used as the packaging materials for sterilized products. High temperatures stop bacterial growth by killing the bacteria. Heat processing is used for preserving all sorts of foods and food products. Different methods include:

© Food – a fact of life To produce UHT milk, the milk is heated to between 130°C and 150°C for 1-3 seconds in plate heat exchangers to ensure that spores are destroyed. In the case of milk the product is heated to not less than 132.2°C for one second and is packaged under aseptic conditions. This method kills nearly all micro-organisms and enzymes but there is little change in the flavour and colour of the food. It is a long-term method of preservation and is usually used in combination with aseptic packaging. Packages are hermetically sealed. The packaging materials used are robust and prevent entry of air unless opened or pierced. The products can be stored in ambient temperatures for up to 6 months usually and do not require refrigeration. Until the package is opened the product remains in sterile conditions and is therefore preserved.

© Food – a fact of life 2009 Canning and Bottling Canning Food is packed into a can and then heat processed until the contents are ‘commercially sterile’ at 121°C by passing the can into a special steamer where a vacuum is applied to draw the air from the can as the lid is sealed into place. A double seam is made by rolling the metal edges together in the shape of a hook. Air is driven out by the steam, preventing bacteria from surviving. The severity of the heat treatment can be lessened if the food contains acid, salt or sugar. The most important pH is 4.5, as below this the very dangerous organism Clostridium botulinum is inhibited. Cans are cooled in chlorinated water (bacteria free) to prevent any possible contamination in cans which may have a slight defect. This could infect the food and cause an outbreak of food poisoning some time later. Canned food has a very long shelf life at ambient temperatures. Bottling Toughened glass is used because it can be taken to high temperatures for heat treatment methods such as bottling fruit and vegetables. It is easy to sterilise glass. The bottle (or jar – often referred to as a Kilner Jar) can be filled either with raw foods and then heated slowly to boil the foods, destroying bacteria and spores. Air is expelled through a specially designed lid, and as the jar cools down, a vacuum is formed. The sealed jar prevents the re-entry of bacteria. As there is no oxygen present, bacteria are unable to multiply. Bottling can also be used for high temperature methods such as jamming, pickling and chutney production. Glass is non-reactive and therefore suitable for use with acidic mixtures. Bottled food has a very long shelf life at ambient temperatures.

© Food – a fact of life 2009 Low Temperatures Chilling -A short-term method of preservation. Chilling preserves food by slowing down the growth of bacteria. -Raw ingredients and food products need to be chilled. Over the past 50 years chilling and freezing has become the most popular domestic method of preserving food. This is mainly due to wider ownership of domestic refrigerators and freezers and developments in technology, rather than the discovery of new preservation principles. -Chilling reduces the temperature to between 1ºC -4ºC. -Chilling food cannot preserve a food indefinitely, but can reduce spoilage caused by micro-organisms and enzymes. What are cook-chill foods? Processed food products that are chilled are usually called cook-chill foods. They are prepared and cooked in the normal way before being blast chilled (rapidly chilled to below 5°C (usually 2 °C) within 1½ hours using a special chiller). The food is kept at a low temperature during transport, storage and display up to when it is cooked. Cook-chill foods can only be kept for a few days unless special packaging is used. The food must be cooked to above 70°C and eaten within two hours.

© Food – a fact of life 2009 Freezing Freezing was first used as a method of preserving food in China in 1800BC. It is a long-term method of preservation. It preserves food by stopping the growth of bacteria. The bacteria remain dormant until the food is defrosted. Raw ingredients and food products can be frozen. Reducing the temperature of the food to below – 18ºC reduces the activity of the micro-organisms and enzymes. Freezing also reduces the availability of water because ice crystals are formed. Quick and controlled freezing is needed so that large ice crystals do not form and break the cell walls of the food. In modern methods of freezing, small crystals are formed meaning that the food is much more likely to retain its structure once it has been defrosted. Disadvantages of freezing: -Most food contains large amounts of water. When water is frozen, ice is formed. Large ice crystals are formed when food is slowly frozen, this can damage the cell structure of the food. -When the food defrosts, the water enclosed within the cells is released, e.g. cell damage in soft fruits (strawberries) and the collapse of some colloidal systems in food products, e.g. cream. Freezing food quickly can reduce the size of ice crystals. - When frozen, micro-organisms do not die, they simply become dormant, retarding their growth. -Moulds can still grow in cold temperatures.

© Food – a fact of life 2009 Methods of freezing Plate In this method the food is prepared in the ordinary way and is packed into a flat container, usually a cardboard-based container, often with polythene or wax lining (e.g. beef burgers). The container is placed between flat, hollow refrigerated metal plates, which are adjusted to press tightly to the pack. This method is slowed down if there are any air gaps between the food and the metal plates. It too has been superseded by other more efficient methods. Foods: flat products such as burgers and fish fingers Blast A blast-freezer is a large cabinet in which a fan has been introduced to move the air over the product. Moving air readily takes up heat and loses it again. The air reaches a temperature of -25ºC or lower and should move with a velocity of 400m/minute. The blast freezer is generally a batch system. Continuous systems which have been developed have more throughputs. Foods: irregular shaped foods such as vegetables and prawns, chicken Cryogenic Food is immersed in or sprayed with very cold liquefied gases such as nitrogen and carbon dioxide. It is a rapid method of freezing which can cause thermal shock to some foods because of the sudden contraction. Liquid nitrogen is sprayed onto food on a conveyor belt in a tunnel. Foods: delicate foods such as raspberries and small items like prawns

© Food – a fact of life 2009 Drying Drying is a long-term method of preservation and the removal of moisture by warmth or high temperature. Since bacteria need moisture to grow and reproduce, removing the water stops their growth, thereby preserving the food. It is probably the oldest method of preservation Modern commercial methods such as: spray drying freeze drying roller drying accelerated freeze drying (AFD), tunnel drying fluidised bed drying oven drying, e.g. herbs, coffee, tea, vegetables Traditional methods of removing moisture are by sun drying : tomatoes, dates, figs, chillis, peppers and sultanas are dried in this way. This method needs heat and some wind to dry out the food before spoilage happens. It is a slow process and not a effective long term method. Dried foods can be stored at ambient temperatures. Modern methods of removing moisture include

© Food – a fact of life 2009 Accelerated freeze-drying (AFD) is the most modern method of drying. The food is dried under vacuum at reduced pressure. It is then frozen before being rapidly heated. Since the ice changes to water vapour (steam) without passing through the liquid stage, most of the flavour, colour and texture of the food is preserved. The food also does not shrink as much as in other methods of drying. Foods: fruit, vegetables, chicken, omelettes, herbs and even dog food! Spray drying. Liquids are spray-dried in a funnel-shaped chamber; solid foods are dried in a continuous tunnel; less robust foods are dried in a fixed cabinet.

© Food – a fact of life 2009 Irradiation This is a method of preservation where ionising radiations are applied to foods to kill bacteria (gamma rays / beams of electrons). There is said to be no risk of residual radioactivity and this process should not be confused with radioactive contamination or radioactivity, though some consumers are still concerned about possible, yet undiscovered effects. Opponents of food irradiation sometimes state that large-scale irradiation would increase processing, transportation, and handling times for fruits and vegetables thus contributing to a negative ecological balance compared to locally grown foods. Food labelling regulations are not consistent across the world, so consumers may not be 100% sure that they are consuming non- irradiated foods. Irradiation has the potential to reduce microbial spoilage, insect damage and the need for chemical additives to improve keeping qualities, flavour etc. Often used in prawns, strawberries and potatoes (to prevent them sprouting). It is expensive to carry out and as yet is only permitted in the UK for use with some vegetables, spices and poultry products.

© Food – a fact of life 2009 Modified Atmosphere Packaging (MAP) Modified Atmosphere Packaging is also called Controlled Atmosphere. It is a method of storage where the oxygen level can be lowered or the carbon dioxide level or nitrogen levels increased. In this form of packaging, the packs are gas flushed, reducing food spoilage and prolonging shelf life. Nitrogen prevents rancidity in fatty foods such as nuts. Oxygen is required in packaging raw meat to preserve its red 'bloom'. If oxygen levels are too low, bacteria will respire anaerobically, producing toxins. MAP has to be used in conjunction with temperature control in storing fresh foods (below 5°C). It is a very good medium for storing fresh fruits, vegetables, meats and fish. Spoilage is reduced and shelf-life is increased. Vacuum Packing The food is placed in plastic packaging and the air around the food is then sucked out and the plastic bag sealed. The food is now in anaerobic conditions. Once the package is opened the food needs to be stored in appropriate storage conditions. Bacon, coffee and fish are most commonly packaged in this way.

© Food – a fact of life 2009 Chemical methods Sugar Sugar reduces microbial activity due to its dehydrating effect. The water available to the microorganisms is reduced by osmosis as the sugar solution is more concentrated than the cytoplasm inside the cells of the microorganism. Therefore water passes out of the cell and the cell becomes dehydrated. The initial boiling of the fruit will destroy the enzymes and micro- organisms (but not spores), preventing spoilage later on. The high concentration of sugar added during the jam making process makes the water unavailable thus reducing the microbial activity through dehydration effect. Jam jars are normally heated before the jam is added destroys the micro-organisms found in the jars.

© Food – a fact of life 2009 Salt Salt is used often in solution as brine. This method is used in preserving meats, such as ham or bacon, and is known as curing. Many foods are stored in brine solutions, in cans or jars or sealed sachets, such as olives, tuna fish. The solution is usually 25% sodium chloride, 1% potassium nitrate and 0.1% sodium nitrate. The salt content acts in the same way as the sugar, through the action of osmosis. Coating food in salt or placing it in a salt solution (brine) reduces the moisture content of the food, i.e. reduces the availability of water. With little moisture, micro-organism growth is retarded. However, the taste of the food may change considerably.

© Food – a fact of life 2009 Vinegar (Pickling) Vinegar (acetic acid) can be used to pickle foods such as vegetables or eggs. The lowered pH provides unfavourable conditions for microorganisms to reproduce. Most microorganisms grow bets if the pH of the food is between 6.6 and 7.5 (neutral). Bacteria are less acid-tolerant than moulds and yeasts. The initial boiling of the ingredients will destroy enzymes and micro- organisms (but not spores), preventing spoilage later on. Vegetables and fruits are covered in vinegar and other ingredients, often including spices. The high concentration of acid inhibits bacterial growth and multiplication. Pickle/or chutney jars are normally heated before the product is added to destroy micro-organisms found in the jars.

© Food – a fact of life 2009 Smoke Smoke contains antimicrobial substances which reduce food spoilage. The preservative effect comes from phenols and acids and aldehydes in the smoke (which produce a thin tar like film over the surface). It also causes surface dehydration of the product, and the high temperature and lack of oxygen if ‘hot smoked’ can kill some microbes. Smoking is often used on foods which have already been cured (partially dried through the use of salt). Foods that are preserved through smoking are normally meat, fish, and cheese.

© Food – a fact of life 2009 Advantages and disadvantages to environment of preserving fresh foods by the methods like UHT, vacuum packing, drying, canning NEGATIVES Litter from packaging causing an eyesore, pollution and physical hazards. Tins rusting and posing risks from sharp edges Pollution caused by factories that make the packaging Use of natural resources to make packaging The possibility that the packaging cannot be recycled, e.g. tetra-packs The creation of a society that expects packaged food rather than being willing to buy food at source, e.g. milk from the milk man, using recyclable glass bottles. The costs added to these products as a result of the methods used and the materials consumed Vacuum packs may require refrigeration and will thus have a shorter shelf life. POSITIVES None of these methods require refrigeration, so all of them are environmentally friendly in terms of fuel consumption They can all be kept at ambient temperatures They are easy to stack and to store They can be kept for a long time, meaning fewer shopping trips required They are easy to prepare, some being ready to eat on opening Canning, Drying and UHT are safe methods of preservation, with low risks attached