Preservation systems developed due to centralized production methods involving the separation of the production and service components of the food flow system either by place or time or both. Food that is centrally produced is either then distributed to the point of service in batched or is pre-portioned; it may be transported in a ready-to-serve state, for example hot or it may need some form of regeneration in a satellite or end-kitchen, for example chilled or frozen food. Catering systems depend on the warm-holding of foods during delivery, for example, various types of insulated or heated containers, electrically heated trolleys and insulated trays.
Preservation systems that extend the delivery and storage time are cook-chill, cook-freeze and sous vide.
2. Preservation systems
2.1 Cook-chill
The term cook-chill refers to a catering system that involves the full cooking of food, followed by blast chilling and chilled storage at between 0°C and 3°C. The food can be stored for up to 5 days. Food is distributed in refrigerated vehicles and reheated before service. The principle feature of a blast chiller is that it is capable of rapidly reducing the temperature of hot foods to low, safe temperatures. Therefore, they make it easier for caterers to comply with food safety and temperature control legislations. Cook-chill systems can also reduce the risk of food poisoning and the growth of bacteria in foods.
The procedures and key features of cook-chill:
If raw materials are poor quality when you buy them, they are not going to improve with cooking. It is vital, therefore, that you ensure your supplier only supplies you with high quality products and, if necessary, check their storage, handling and distribution methods.
Having purchased good quality raw materials, it makes sense to keep them in safe storage and in prime condition before they are needed.
Pre-preparation of foods, avoiding cross-contamination by staff handling the different types of raw foods. At the preparation stage, basic Food Hygiene Regulations and HACCP Guidelines apply.
Whatever the food product you are cooking and by whatever method, it is essential that the core temperature of the food reaches at least 70°C, and is held at this temperature for at least two minutes. This is to ensure that any pathogenic micro-organisms that may be present are destroyed.
Once the food is cooked, the chilling process must start as soon as possible and at most within 30 minutes. This leaves time for hot portioning prior to chilling. However, handling of food should be kept to a minimum.
All food should be chilled to between 0°C and 3°C within 90 minutes of being placed in a blast chiller, This is not only to ensure safety, but also preserves the appearance, texture, flavour and nutritional value of food.
Chilled food should be stored in a dedicated refrigerated storage cabinet at a temperature of between 0°C and 3°C, in order to control the growth of micro-organisms. You should use a refrigerated cabinet or cold room designed for chilled food storage, and use it solely for your cook-chill products.
If you intend to operate a centralised cook-chill system and supply food to one or more other locations, the dishes must be transported to the other site whilst in their chilled state (kept below 5°C). The use of refrigerated vehicles is recommended, or at the very least, pre-chilled insulated containers for short journeys.
Cooked and chilled foods that are to be eaten cold or at room temperature, should be consumed within 30 minutes of removal from storage. If the food is to be regenerated (reheated to 70°C), this should start no more than 30 minutes after the food is removed from chilled storage. Regeneration must take place close to the point of consumption.
2.2 Cook-freeze
The term cook-freeze refers to a catering system that involves the full cooking of food, followed by blast freezing and storage of food at a controlled low temperature of -18°C/-22°C, before controlled and thorough thawing and regeneration prior to service. Food can be stored for 2 months at this controlled temperature. For a cook-freeze system, you require a blast freezer rather than a blast chiller, suitable storage for frozen foods and, preferably, a controlled thawing cabinet. Blast freezing can also be used for raw materials and semi manufactured products.
The procedures and key features of cook-freeze:
Raw food should be purchased using a standard purchasing specification to ensure quality and consistency.
All foods should be kept under strict temperature control, in hygienic and clean conditions until required for preparation.
Pre-preparation of foods, avoiding cross-contamination by staff handling the different types of raw foods.
Cooking should be ideally done in batches. At times it may be necessary to adjust the recipes to account for large scale batch production and to account for chemical changes in the food and a result of storage for up to eight weeks at very low temperatures.
Within a time limit of 30 minutes, all hot food should be portioned into single or multi-portions prior to freezing.
In order to preserve food quality and prevent any growth of bacteria all cooked food should be placed in a blast freezer within 30 minutes of final cooking and being portioned. The blast freeze cycle transforms the liquid present in the food into microcrystals which do not damage the tissue structure of the product and ensures the quality of the food is maintained so that you still have a high quality product after defrosting.
The shelf-life of pre-cooked frozen food varies according to type but in cold storage, it may be stored up to 8 weeks without any significant loss of nutrients or palatability.
All distribution should take place using chilled insulated containers for any short journeys or refrigerated vehicles for longer journeys.
Frozen food can be thawed to 3°C prior to being regenerated or be regenerated directly from its frozen state. Food should be heated to a minimum of 70°C for at least 2 minutes. Any foods regenerated and not consumed must be destroyed and not reheated or returned to a refrigerator.
2.3 Sous-vide
The term sous-vide refers to a catering system that involves the preparation of quality raw foods, pre-cooking if necessary, packing food in high-barrier plastic bags, vacuum sealing the bags and cooking the food in the vacuum sealed pouches. Food is then stored in the pouches at a controlled temperature of between 0°C and 3°C, for up to 21 days.
The sous-vide method increases the potential shelf-life of normal cook-chill/cook-freeze in three ways:
By removing the air from the plastic bags or pouches the growth of most bacteria is restricted.
The food is cooked at pasteurization temperatures aiding the destruction of most micro-organisms.
The food being sealed within the bags or pouches is protected during storage and regeneration from any cross-contamination.
The procedures and key features of sous-vide:
Fresh ingredients from trusted suppliers must be used for sous-vide cooking. This can significantly lower initial microbial levels, extending shelf life and product freshness.
Vacuum packed bags of raw food must not be stored for more than two days before sous-vide pasteurization above 60°C takes place.
Separate preparation areas for raw food and cooked food must be allocated in the sous-vide cooking area. All working surfaces and utensils must be kept hygienically clean.
Temperature and cooking times in recipes must be followed precisely and when new recipes are developed, precise temperatures and cooking times must be recorded.
All sous-vide equipment must be checked and calibrated. Vacuum pack seals should be checked for leakage.
Attention must be given to accurate temperature control and monitoring as sous-vide especially since foods lack chemical preservatives, which normally arrest biological activity in processed foods.
Core temperatures should be measured with a temperature probe, which should be hygienically cleaned.
Cooking temperatures must ideally not exceed 70°C to ensure that the juiciness of the food is not lost.
Vacuum packed bags must be labelled accurately with date of pasteurisation and expiration, identification of contents and name of the chef who prepared the food.
Sous-vide food must be cooled to 3.3° C or below in less than 2 hours and consumed within the specified shelf-life storage time.
Sous-vide food must be stored at 3° C or below.
Proper control over refrigerator temperatures must be kept, especially in the case of walk-in fridges where temperatures can rise above the required storage temperature.
The temperature reached in regenerating sous-vide food for serving, must be the same as that required in conventional cooking.
3. Advantages & disadvantages of the preservation systems
Advantages of cook-chill system
Disadvantages of cook-chill system
Batch processing that involves minimal processing. Having a greater control over portioning and reduced wastage.
Production separated from consumption. Therefore, the business can fully utilised staff time, saving costs.
Central purchasing with bulk buying discount, benefitting the business.
Chiller storage is cheaper to install and run than freezer storage.
Blast chillers are cheaper to install and run than blast freezers.
Thawing time is eliminated.
Extended shelf life in the distribution chain. Foods are chilled and then regenerated on site, solving the problem of moving hot foods.
Regeneration systems are simpler - infrared and steam convention ovens are mostly used and only 12 minutes is required to reheat all foods perfectly.
Anaerobic environment that prevents the growth of aerobic spoilage organisms.
Minimises processing impact on sensory and nutritional qualities. Keeping its freshness.
Standards maintained provide quality food to customers.
Services can be maintained at all times regardless of staff absences.
Allows for a foodservice director to have coveted control in all arenas-cost, convenience, labour and quality.
No system is too small to adapt to cook-chill.
Microbiological spoilage due to:
Temperature abuse;
Insufficient pasteurization; or
Aerobic spoilage.
Product shelf lives that are shorter than those demanded by the retailer, consumer and commercial caterer, due to exposure to oxygen during various preparation stages.
Loss of sensory quality due to non-rapid chilling.
Chilled storage is product dependent, for instance, vegetables may develop acidy / pungent flavours within 2 days.
Adequate refrigeration must be maintained at all times.
Advantages of cook-freeze system
Disadvantages of cook-freeze system
Simplified employee scheduling. Skilled employees work eight-hour shifts, Mondays - Fridays.
Individual items can be prepared in bulk, limiting the number of times need to be prepared.
Bulk preparation can reduce labour costs.
There are no more peak production workloads, ensuring that employees can be used consistently during their shifts without the added stress of peak production.
Seasonal purchasing provides considerable savings.
Extended shelf life in the distribution chain. Foods are frozen and then regenerated on site, solving the problem of moving hot foods.
Delivery to units will be far less frequent.
Long term planning of production and menus become possible.
Less dependence on price fluctuations.
More suitable for vending machines incorporating microwave.
Added equipment costs in the beginning including additional freezer space, packaging supplies, reheating equipment. It may also require some facility renovation.
Employees who are required to work weekends may be resentful of employees who don't work weekends.
Large ice crystals formed can damage food, dry it out, and break down the physical structure.
Thawing is required.
Not all frozen foods can be successfully prepared without extensively modifying the ingredients or recipes. Certain foods lose their flavour or texture after freezing.
Advantages of sous-vide system
Disadvantages of sous-vide system
The flavour, palatability and nutrients are improved, relative to normal processing, because all the contents are held within the sealed pouch. Therefore, customers can benefit from more nutritious foods.
Less additives / preservatives needed.
Reduced risk of cross-contamination because of the pouches used that provide a convenient package for safe handling and distribution.
Wider variety of produced goods, giving more choices to customers.
Shrinkage of the cooked product is reduced, increasing the yield by up to 20% compared to normal cooking.
Reduced oxygen packaging retards the oxidative rancidity of fats and oils.
Batch processing that involves minimal processing. Having a greater control over portioning and reduced wastage.
Centralised production, saving staff time and costs.
Has a longer shelf life than cook-chill, of up to 21 days.
Can offer a flexible production method to catering unites of all sizes with particular applications to a la carte and function menus.
Spoilage is often not visible.
Microbiological spoilage due to:
Temperature abuse;
Insufficient pasteurization; or
Aerobic spoilage.
Exceptionally high standards of hygiene are fundamental.
Product safety is dependent on proper handling and processing.
Sous-vide involves higher set-up capital and operating costs than cook-chill.
Adequate refrigeration must be maintained at all times.
Extra cost to consumer.
Complete meals cannot be produced as certain foods need to be processed differently, for instance, meat and vegetables.
4. Effects of each method on appearance, nutritional value & flavour
4.1 Cook-chill
Cook-chill is a healthy way of preparing food. One major advantage is that chilled foods are less perishable and retain nutrients longer than foods cooked and held at serving temperatures for relatively long periods in the cook-and-serve systems. Damaging bacteria is rendered dormant in cook-chill systems, minimising food spoilage.
Colour, texture, flavour, structure and nutritional value are locked in. Chilled foods often look more attractive than frozen foods and therefore can be successfully used in cook-chill vending situations.
Delicate food surfaces such as pasta and fruit are protected, as rapid chilling stops an "ice skin" forming which otherwise dehydrates and damages the products' appearance.
Information on nutrient losses in cook-chill catering systems is reviewed, which is limited and conflicting. In general, it suggests losses of vitamin C, and possibly other labile nutrients appear to be significant in cook-chill systems.
One major drawback is that not all chilled foods can be successfully prepared without extensively modifying the ingredients or recipes. Some foods may lose their flavour or texture after chilling. Food, which should have a crisp texture, cannot be prepared by cook-chilling. Chilling the food causes the food to lose its crispness.
4.2 Cook-freeze
Cook-freeze is very similar to cook-chill, except that the food is frozen not chilled in this system. Blast freezing helps to keep food looking good. The slower the food freezes, the larger the ice crystals formed; and large ice crystals can damage food, dry it out, and break down the physical structure leaving you with an unrecognisable product. Textural changes may occur; soft fruits can become mushy because the cell structure of the fruit collapses. For instance, a food that may not be as good after freezing is strawberries, they often go soft after freezing. Some other foods that are unsuitable for freezing are eggs, cream, potatoes, sauces and cheese.
During blanching of fruit and vegetables, ascorbic acid (vitamin C) and thiamin (B1) are vulnerable. Nutrients in the form of thaw drip may be lost when foods are thawed, for example, thiamine from meat. However, cook-freeze systems result in better vitamin C retention than conventional systems with significant warm-holding periods.
One major advantage just like cook-chill is that frozen foods are less perishable and retain nutrients longer than foods cooked and held at serving temperatures for relatively long periods in the cook-and-serve systems. One major drawback is that not all frozen foods can be successfully prepared without extensively modifying the ingredients or recipes. As mentioned from above, certain foods lose their flavour or texture after freezing.
4.3 Sous-vide
Sous-vide is an extremely healthful method of preparing food. Most of the benefits are directly related to the fact that food is placed in a sealed, reduced oxygen environment and cooked at low temperatures. The net effect is precise control over heat, oxygen, and added water, which are the three elements most responsible for reducing the nutritional content of conventionally prepared foods.
As a flameless, low heat cooking method, little additional fat is required during cooking to prevent adhesion of proteins to cooking surfaces. Any added lipids are strictly for the enhancement of mouth feel and flavour. Because of the plastic barrier, oxidation is significantly reduced, preserving the qualities of essential polyunsaturated fatty acids.
The plastic film also prevents the loss of moisture and flavours. Consequently, flavours are amplified, and fewer spices and less salt is required, lowering the overall sodium content of sous vide foods. Water soluble minerals are typically leached into cooking water, reducing the mineral content of foods processed by traditional means. The pouch eliminates mineral loss, preserving the mineral content of fresh foods.
Regardless of cooking process, many vitamins destabilize during heat treatment. Research indicates that vitamin C retention decreases to 85% after pasteurization and chilled storage for 5 days versus raw product of the same age (). Between different cooking methods, sous vide provides the highest retention of vitamins versus steaming and boiling.
As with all foods, freshness and minimal processing intervals greatly affect the nutritional profile. Restaurant menu items prepared using sous-vide highlight fresh textures and lively flavours, making healthful food more interesting to consume. Because dishes are richer in taste, sous-vide cooking appeals to the American palate, while still maintaining valuable nutritional properties of fresh, wholesome foods.
5. Conclusion
The nutrients that give us energy and help us maintain good health also cause our food to spoil. There are innumerable micro-organisms in the atmosphere that derive their nutrition from these nutrients by breaking them into simpler forms. As these minute life forms start disintegrating the nutrients, they set off the process of food spoilage. With the knowledge of the role that micro-organisms play in spoiling food, numerous methods of food preservation have been developed by man. All these methods work by altering conditions like temperature, availability of water or oxygen in the food, or in the environment in which the food is stored. Changing or altering these factors hinders the growth of these minute organisms, and hence prevents food spoilage.
