Author: Er. Wasiya Farzana
- Snack include ‘’ a slight or casual or hurried meal, a small portion of food or drink, or a very light meal’’.
- Snack foods - potato and corn chips, alkali cooked corn tortilla chips, pretzels, popcorn, extruded puffed and baked/fried products, meat snacks and rice and pulse based snacks.
- Conventionally, snack food embrace a group of savory, crispy items, which are ready-to-eat and are shelf stable for 2 to 16 weeks at normal room temperature.
- Further growth of snack products is expected because of the following reasons: a. Materials for snack products are relatively inexpensive sources of food energy and nutrition. b. Cereals and snack products are convenience meals consumers enjoy easily at many places. c. Changes in formulas, reducing salt and adding nutrients, and functional compounds can be accomplished.
- Deep fat fried : This includes potato crisps and potato strips/sticks/rings and represents by far the major portion of the snack food business
- Quick fried : Preformed partly cooked pellets derived from potato, potato starch and other cereal materials are fried at high temperature (around 200 ) for a short term (10-15 sec) giving expanded, light textured products.
- Extrusion cooked : Cereal potato powder mixes are extruded through dies at high pressure and temperature to give an expanded base which after further drying is enrobed with oil and flavour. A large number of products are available in this category and they display varying sizes shapes textures and flavours
- Roasted : This is generally represented by nuts, particularly in the form of dry roasted peanuts.
- Baked : This generally snacks of much lower oil content as well as offers an opportunity for different flavour development. An example is savoury mini biscuits.
- Savory Snacks : Savory snack foods are multipurpose foods that can be consumed with a meal or on the go and are often associated with picnics and sports events. Many savory snacks are based on the adaptation of a yeast fermentation process and, to some extent, the formulas used for bread. Savory snacks are microbiologically and chemically safe, do not require any refrigeration for product stability and safety, and are usually seasoned with salt and additional flavourings.
- Pretzels : The pretzel had its origins sometime after 610 AD in southern France, where monks baked scraps of dough in the image of arms folded in prayer to reward children for learning their prayers (McCarthy, 2001). The development of extruders marked a major breakthrough in the high-volume production of pretzels. The array of die patterns is very easily interchangeable, which allows extruders to make pretzels of all shapes (Groff, 2001). After the dough is extruded, rolled, and twisted, the pretzels are passed through a spray of hot soda solution, sprinkled with salt, and baked. Pretzels are often dried further after they are baked in the oven
- Rice-Based Snacks : Rice-based snacks are becoming more available and variable. The mixtures in rice-based snacks include vegetables; tofu; tahini (ground sesame seeds); cereals such as oats, wheat = our, or seaweed; nuts; herbs; or seeds (Rice, 1990). Traditional Korean and Japanese cuisines include a range of deep-fried snacks consisting of a carbohydrate-based glutinous batter containing vegetables. Puffing rice by frying is popular in Asia. Rice is first cooked by either boiling or steaming. The cooked rice is compacted, shaped/cut, dried to 12–15% moisture, and fried in oil at 220°C for several seconds in a deep fryer.
All cereals contain starch at high concentrations and so they need to be cooked to be digestible and acceptable for consumption. Frying is not a complicated process but its chemistry is complex and involves extensive changes in both the frying oil and the food being fried.
Fat is used in most fried snack foods as a processing aid to dehydrate the products and develop characteristic flavors (Robertson, 2006). The primary change in frying is moisture reduction or dehydration. When starch-based material is immersed in hot oil, the starch on the surface is rapidly gelatinized and the product is uniformly covered with small steam bubbles as the surface moisture begins to vaporize. The outer layer of the product dehydrates and forms a veneer-like structure (case hardening). As surface moisture diminishes by frying, the internal moisture becomes steam, rupturing channels through the structure of the product.
Continued frying keeps lowering the moisture content of the product. Low moisture content and high temperature support flavor-producing reactions involving amino acids, proteins, and carbohydrates.
Industrial fryers consist of a heating oil, conveying systems, and a hood to exhaust steam and vapors (Banks and Lusas, 2001). Fryers directly heated by gas burners under the fryer pan (direct- fried fryers) are commonly used, but they are not as efficient as fryers with heat exchangers although they are more economical to purchase. Fryers usually operate at 149–218°C at atmospheric pressure (Banks and Lusas, 2001). Vacuum fryers are sometimes used to fry heat-sensitive products. Vacuum frying at lower temperature results in lighter color in the products than those processed at atmospheric conditions (Banks and Lusas, 2001).
During oil frying, oils can undergo oxidation, cyclicization, polymerization, degradation to volatile compounds, and hydrolysis. These chemical changes cause off-flavors, rancid aromas, greasy Food Packaging and Shelf Life mouthfeel, and impaired nutritional value in fried foods. Thus, maintaining the quality of frying oil under high-temperature conditions is critical to the fried foods.
The majority of expanded snacks on the market is manufactured by extrusion and puffing. Extrusion has revolutionized many conventional snack-producing processes and has provided a means of producing new products.
Extrusion offers many advantages over traditional food unit operations such as minimizing time, energy, and cost inputs while adding versatility and flexibility to the manufacturing process. A wide variety of different products can be produced by changing ingredients, operating conditions, and/or minor components of the extruder. The functions of an extruder include feeding, heating, conveying, mixing, kneading, compressing, shearing, cooking, and shaping (forming).
When a pressurized, heated, and sheared mixture of ingredients exits the die of an extruder, the moisture in the mixture vaporizes instantaneously, which puffs and expands the mixture, producing a snack product. The shape and size of the product are determined by the design of the die and the cutting components. The shaped extrudates are dried or baked and coated with oils, flavors, and/or seasonings (Huber and Rokey, 1990; Matz, 1993). The important variables in extrusion are extrusion temperature, screw speed, feed rate, barrel/screw designs (con/ guration), extrusion rate (residence time), and the composition of the feed, including moisture, sugar, salt, and fat content.
Crispiness of expanded snacks is controlled by expansion. More expansion (more crispiness) is expected at a higher extrusion pressure and a higher content of starch in the extruded materials. Fat usually reduces expansion. Salt helps expansion and results in a tender texture. Emulsifiers generally weaken starch structure, resulting in less expansion.
The moisture content of extruded snack products is normally between 8% and 10% (wet basis).
Additional drying or frying is applied to the extruded products to reduce the moisture content to 1–2%, providing desired crispness (Robertson, 2006).Coextrusion has been used to produce snacks that typically have an extrusion-cooked outer shell with a pumpable (but not free flowing) filling (Huber and Rokey, 1990). The extrusion-cooked portion of the snack will flow through the die parallel to the flow direction in the extruder barrel. The filling is pumped into the die and it flows within the shell extrudate.
The base of puffed cereal products contains conditioned whole grain wheat, rice, barley, maize, or dough made from corn meal or oat = our and is mixed with other ingredients, such as sugar and salt, and cooked under pressure, dried to 14–16% moisture content, and pelleted by extrusion (Robertson, 2006). Popcorn is used as the basis for many snack foods due to its pleasing textural effects at low cost, with the basic popcorn snack simply being popcorn coated with butter and salt (Matz, 1993). Blending the puffed products with different flavors provides enormous opportunities for increasing acceptance and the usage of puffed products (Nath et al., 2007). Puffing is also a commercially applied method to produce breakfast cereals (Matz, 1993). The expansion of water by puffing causes a several-fold increase in volume. Puffing is useful in producing cereal products that need to be reconstituted later.
The raw material is heated in an enclosed vessel so that its vapors create a high pressure atmosphere around the product. Under these conditions, the product temperature is above the glass transition temperature, so that it is in a highly deformable, liquid-like state. The pressure is suddenly released, so that the product “explodes” due to evaporation of moisture from within. This evaporation cools the product and its temperature drops below the glass transition, to possess solid-like behavior. The main process parameters affecting puffing are puffing temperature, puffing duration, initial moisture content of the material to be puffed, and starch content of the raw materials.
The Packaging and the Shelf Life of Cereals and Snack Foods
Degree of puffing of the cooked grain is mainly dependent on the suddenness of change in temperature or pressure (Nath et al., 2007). The advantage of puffing is that different cereal materials can be processed together to produce cereal products with different formulations. The puffed product is usually dried to 3% moisture content by toasting and then cooled and packaged (Robertson, 2006).
Two types of popping equipment, wet (oil) and dry, are commercially used to produce popcorn (Cretors, 2001). Popping corn in oil (wet-popping) is simple and enables the end product to be made at the point of purchase. Dry-popping corns without oil relies on heating. A commercial version of dry-popping uses a motorized rotating wire drum over an open flame or electric heating elements (Cretors, 2001).
Various flakes, such as corn flakes, wheat flakes, and rice flakes, can be produced from intact whole grains, grain mills, or individual or mixtures of ground grains (Lawes, 1990). The grain materials are cooked at elevated pressures and then passed between the steel rollers of a drum dryer (15–20% moisture content). The partially dried flakes are scraped off from the rollers and then usually dried on a traveling wire mesh belt or conditioned for 1–3 days (Lawes, 1990; Robertson, 2006). The flakes are toasted or browned by radiant heat before packing in moisture-proof materials. Flaked grains are sometimes coated with sugar or candy to provide a hard and transparent coating that does not become sticky.
Virtually all snack food production needs a baking, cooking, frying, or drying step, generally near the end of the production line (Matz, 1993). Ovens and driers transfer heat by conduction, convection, and radiation. For example, in ovens, heat transfer is by conduction from the pan, convection from hot oven gases, and radiation from flames and hot oven parts.
Baking is carried out to produce biscuits, cookies, and crackers and this is usually done in tunnel ovens. The product from the oven cools and loses traces of moisture. Some of those products are directly packaged, but many other products require additional processing to add non baked ingredients or flavor enrichments.
If the drying is not properly done, various microorganisms such as foodborne pathogens or spoilage microorganisms can grow (Robertson, 2006). Potato chips fried in oil have enriched flavor. Due to the increased market share of low-fat and fat-free chips, drying or partial drying prior to frying is being used, which, however, results in significant reduction in flavor (Gould, 2001).
A high-temperature short-time (HTST) fluidized bed drying technique was developed for the
production of quick cooking products from starch vegetables (e.g., potato, sweet potato, green peas, carrot) (Jayaraman et al., 1982). HTST drying can result in a considerable reduction in the drying time with improvement in the texture of the cooked starch vegetables (Nath et al., 2007).
Freeze-drying where heat generation is not involved during processing is often used when dried fruits need to maintain their nutritional values. Generally, less loss of α- and β-carotenes and ascorbic acid is found in freeze-dried fruit products compared to those dried by conventional convection drying methods
DETERIORATIVE REACTIONS AND INDICES OF FAILURE
Many cereal and snack products are dry and contain lipids. They are stable against microbial growth due to their low water activity (aw < 0.6), but not against chemical and enzymic reactions.
Food Packaging and Shelf Life that result in product deterioration (Labuza, 1980). The most important modes of deterioration in cereal products and snack foods are loss of crispness (i.e., moisture uptake) and lipid oxidation causing rancidity/off-flavors.
LOSS OF CRISPNESS
Dry food systems can lose their desired crispness during storage or upon opening of the package. Loss of crispness due to moisture uptake is a major cause of snack food rejection by consumers (Robertson, 2006). Puffed r ice cakes were reported to lose their crispness and become tough a s theaw increases through moisture absorption; rice cakes with aw between 0.2 and 0.4 have the best crispness and low hardness (Hsieh et al., 1990).
A critical aw for potato chips and corn chips was reported to be 0.4; potato chips became organoleptically unacceptable at aw of 0.5 (Katz and Labuza, 1981; Quast and Karel, 1972; Robertson, 2006). The values for the critical aw for puffed corn curl and extruded rice snacks have been reported as 0.36 and 0.43, respectively (Chauhan and Bains, 1990; Katz and Labuza, 1981).
Fried Snack foods
- Fried snack foods contain different ingredients.
- Common to all these snacks is fat which is used as a processing agent to dehydrate the product (as in case of potato chips) or puff it (as in case of some extruded products) and to develop characteristic flavors.
- These include deep fried potato chips, strips, sticks, rings etc. Banana wafers, jack fruit chips, tapioca wafers, which are popular in South India, also fall into this group.
- Roasted / fried / salted / flavoured nuts, such as peanuts, cashewnuts, almonds etc.
- Two major modes of deterioration: development of fat rancidity and loss of crispness.
- Fats - oxidative and hydrolytic rancidity - formation of objectionable odors and flavors.
- To minimize the development of rancidity, the product must be protected from O2, light and trace quantities of metal ions.
- The addition of phenolic-type antioxidants such as (BHA), (BHT) and (TBHQ) is very helpful but is not always permitted by legislation.
- Crispiness is a salient textural characteristic for fried snack foods and its loss owing to absorption of moisture is a major cause of snack food rejection by consumers. These reactions are accelerated by heat, moisture and light, as well as the added salt in the product. The packaging requirements of snack food are:
- Grease proofness
- Prevention of Rancidity
- Avoid Loss of Crispness
- Physical Strength
- Seal Integrity
PACKAGING MATERIALS FOR SNACK FOODS
- One of the most important functions of food packaging is the protection of the product from environmental factors such as light, water vapour, gases and odour during its shelf life.
- The actual protection required however depends on the nature of the product.
- In respect of the shelf life of snacks , the following are the main properties of flexible packaging materials (ex. coextruded oriented polypropylene (OPP), which can affect the storage of quality of a product:
- i) Water Vapour Transmission Rate (WVTR)
- ii)Oxygen Transmission Rate (OTR)
- iii)Optical Density (OD)
- iv) Flavour / odour barrier property Water Vapour Transmission Rate i) For a given material such as coextruded OPP, this is primarily a function of film thickness. ii) The lower the WVTR of a film, the better is the moisture barrier of the packaging film, of course, may be further enhanced by various processes such as coating with an additional barrier material, lamination and metallisation. Oxygen Transmission Rate (OTR)
- Similar to WVTR, OTR of clear OPP decreases as the thickness of the film increases. Permeability of oxygen is substantially reduced by coating with a suitable barrier material (ex. Acrylic) and by metallisation. Optical Density (OD)
- This is defined as OD= (incident light/ transmitted light). Determined using a suitable densitometer (at 23 2 RH ) ,it is a measure of the light barrier of a film.
- The greater the OD of a film, the better is the light barrier and hence protection against light induced changes (ex oxidation and bleaching)
Flavour/ Odour barrier property
- The significance of flavour /odour barrier performance of a packaging film lies in its practical importance in relation to the flavour retention of packaged foods such as snacks, and also with respect to protection against tainting against extraneous sources.
- Equations can be used to estimate the likely permeation coefficients of wide range of chemical substances through coextruded OPP as well as polyethylene, at low odorant concentrations.
- In the SI system , permeation coefficients have the unit g which represents the weight of the flavourant /odorant permeating a polymer specimen one metre thick, one require metre square in area per second with a vapour pressure difference across the specimen of one pascal (=10 ppm vol/vol) .
- The equation is:
- R=(PC x VP x A)/L
- R=Rate of transmission
- VP=Vapour pressure of flavourant (Pa)
- A=Area of the film
- L= Thickness of the film (m)
Packaging Materials for Snack Foods
- Flexible Plastics Traditional snack food and wafers may be branded or non-branded. Non –branded snacks are packaged for shorter shelf-life in unprinted low density polyethylene (LDPE) and polypropylene (PP) pouches. For branded snacks and nuts laminated structures are used. • BOPP / LDPE • BOPP / Polyester / LDPE • Metallized Polyester / LDPE • BOPP / Metallized Polyester / LDPE • Polyester / LDPE • Polyester / Al foil / LDPE (The sealant layer could also be LLDPE or cast PP) Composite Containers
- Composite containers are also used for packaging of molded chips and nuts.
- The containers are round and the body (side walls) is made of PE coated foil laminated spirally wound paper.
- The top and bottom ends of the containers may be made from metal or plastic.
- The inner face is coated with a plastic film or a combination of film and aluminum foil, the function of these materials is to ensure optimum barrier properties against moisture, oxygen etc.
- Tinplate Containers: Rigid, round tinplate containers, which are internally lacquered are used for roasted and salted nuts. These are packaged with an inert gas like nitrogen for extended shelf-life.
Other Plastic Packages
Commonly used for roasted salts nuts are PET containers and injection molded PET or PP trays with peel able lids. Potato snack products packaged in clear OPP film has a shelf – life (assessed by flavor and moisture changes) of only 8 to 10 weeks compared to over 26 weeks when packaged in metallized OPP film.
- Flushing of potato crisps with nitrogen is said to have increased the shelf-life from about 60 days (without N2) to about 120 days (Anon., 1988).
- An alternative to gas flushing is the use of oxygen scavenger, which is said to be more effective than gas flushing for reducing the residual oxygen level within the packages, thereby further increasing the shelf-life of packaged nuts.
Packaging for extruded and puffed snacks
Two types of extruded foods
- The traditional items - sev, boondi, papdi, gathia, chakli etc.
- The non-traditional pre-formed partly cooked pellets derived from potato, starch from cereals and fried at high temperatures for a short time to give expanded light textured products.
- Many of these products are highly flavoured with spices, herbs or cheese. Typical examples are cheese balls, “cheetos”, “kurkure” etc.
- Puffed snack foods – popcorn, puffed rice
- The major mode of deterioration for extruded and puffed snacks is loss of crispness.
- The critical aw for puffed corn curl has been reported as 0.36, which corresponds to a moisture content of 4.2 g H 2O per 100 g solids.
- For extruded rice snacks the critical aw has been reported to be 0.43, which corresponds to 6.5% moisture content. The development of stale or oxidized and rancid flavors and odors can also be a problem limiting the shelf life of certain extruded and puffed snacks.
- Many extruded and puffed snack foods are packaged in identical material to that discussed above for fried snack foods.
- The major mode of deterioration is loss of crispness, a package that provides a good barrier to water vapor is the primary requirement.
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About Author / Additional Info:
I am currently pursuing PhD in Agricultural Process and Food Engineering from Indira Gandhi Agricultural University