Snacks
One of the most popular salty snacks, the potato chip, was 150 years old in 2003.
George Crum, a cook at Moon’s Lake, invented the potato chip at Moon’s Lake
House in Saratoga Springs, NY, in 1853. It quickly became popular in restaurants
along the East Coast. Later innovations were packaging in small bags by Laura
Scudder in Monterey Park, CA, and larger-scale production by names still familiar
such as Wise, Utz, Mike-sells, and Lay’s. Today, retail sales in the United States are
about $6 billion/year.
Corn-based snacks are increasingly popular in the United States, including tortilla
chips, corn chips, and corn puffs. Rice-based snacks are popular in Asia. These
products, whether based on corn, rice, or mixtures or grains, are formed by extrusion,
sheeting, or pelletizing and then puffing.
There are differences in flavors between snack foods in other countries and
those popular in the United States For example, seaweed and shrimp flavors are
popular in the Orient but are rarely found here. Dairy-based flavors, such as sour
cream, are common in Scandinavia, and almost anything with paprika sells in
Germany.
Other trends are manipulation of fat content in fried snacks by using higher temperatures,
baking instead of frying, and steam stripping to remove excess oil from
fried snacks.
Technical Platforms for Snacks
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Frying is still one of the most significant unit operations or technologies, and it has
many variations. Frying can be continuous or batch; take place under vacuum, atmospheric
pressure, or elevated pressure; and may involve immersion of food pieces in
hot oil or spraying with hot oil.
Normally the oil used is vegetable oil, but the oil can also be olestra, a sucrose
polyester that is less digestible and thus less likely to contribute unwanted calories.
In some fried snacks, oil may be 30% of the final product, contributing to both flavor
and caloric content.
Motivated by health concerns of consumers, manufacturers have developed
snacks with less fat. One technique is steam stripping to remove some oil while
retaining the characteristic flavor and texture of traditional potato and corn snacks.
Other approaches are baking or drying of “fabricated” snacks made from dehydrated
potato and baking instead of frying extruded corn curls. These have no added
oil and usually rely on added flavors. Application of flavor to baked potato and corn
snacks can be challenging because the chips are fragile and lack the oil that in fried
snacks assists adherence of powdered seasonings.
Other baked snacks include pretzels, which are based on wheat dough and formed
into various sizes and shapes. Pretzels acquire their characteristic surface sheen by
a dip in a baking soda or lye solution. This same sheen makes adding flavors a
technical challenge. There are proprietary technologies for applying flavor systems
to coat pretzels, including the application of a water solution of starch to act as a
“glue.” This then requires a drying step achieved either in an additional piece of
equipment or by passing warm air through the coating equipment.
Another major technique to make snacks is extrusion, which may be high shear
or low shear. For example, a high-shear, relatively short extruder is used for puffed
corn snacks and more-dense corn chips. Low-shear extruders are used for pasta and
for unpuffed pellets. Pellets are sometimes shipped from a central plant to locations
close to markets where they are fried to a final form for sale. In other cases, the pellets
are used almost immediately in a complex process to produce three-dimensional
shapes.
Still other snacks are made by sheeting and cutting. Examples are tortilla chips
made from ground whole corn, which has been soaked in lime slurry to soften the
skins. After sheeting, tortilla chips are generally fried but can also be baked.
Washing the soaked corn removes the skins and creates a wastewater stream. It
is possible to conserve water in a corn snack plant by reusing water first to wash
the soaked kernels and then as part of the soaking process. Suspended solids are
removedby filtration
Equipment Innovations
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Twin-screw extrusion is often used for higher volume production of low density,
i.e., highly puffed snacks, such as corn curls. New variations include snacks with
Snacks 19
two colors in the same piece and filled salty snacks, achieved by co-extrusion of
filling and exterior doughs.
Extruders’ screw diameters range from 32 to 187 mm, and are designed to make
scale-up more reliable. Surface-to-volume ratio is not constant as machines become
larger, so some manufacturers have developed proprietary software to model heat
transfer in twin-screw extruders. This enables accurate prediction of performance
based on experiments at a small scale. A useful scale-up parameter is specific
mechanical energy, which is energy per unit mass flow (Valentas et al. 1991, p. 274).
Low-shear extruders and drying systems are used for pasta and to manufacture
pellets, as mentioned above. Pellets enable high-efficiency production at central
locations and completion of the process close to market. The final products usually
have very low bulk density and can be fragile, so shipping them long distances
is expensive.
New Frying Technology
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Most fryers immerse products in a bath of oil heated either by coils in the bath
through which steam or hot gases flow, or heated externally in a heat exchanger.
Products are moved through the oil bath with a chain or belt conveyor, depending
on whether the products sink or float in the hot oil. Some products, such as donuts,
which float, must be turned over part way through the bath to get even cooking. Most
products pick up oil as moisture is driven off. This removal of oil is compensated
by adding fresh oil to keep the bath level constant. Depending on the volume of
the bath and the rate at which oil is removed with product, oil may be in the bath,
constantly being heated and exposed to oxygen, for varying lengths of time. Some
rancidity is almost inevitable.
A new type of fryer uses falling curtains of hot oil to enrobe food products passing
beneath them on a wire mesh belt. The complete fryer system operates with a
very small volume of oil. As a result, oil carried out of the fryer with product is
quickly replaced with fresh oil, resulting in a rapid oil turnover rate, so foods taste
fresh and have a long shelf life. This concept was developed especially to minimize
oil quantity and maximize removal of fines. In standard fryers, product particles
remain suspended in the oil and are carried along with it, causing off-flavors
to develop and promoting degradation of the oil. The new fryer uses the returning
product belt to drag along the oil collection pan and move any fines to the in feed of
the fryer, where they are discharged into a continuous filter for removal.
Most modern fryers have external heaters and filters for the oil to provide good
temperature control and fines removal. Overflow weirs above the moving belt create
the falling curtains. By controlling the oil flow to each weir, the heat transfer rate
can be modified as product moves through the machine. The same fryer can be used
with hot water as a blancher. Deep beds of nuts have been roasted with the falling
curtain
One paradox of snack processing that is illustrated by several of the examples
described here is that new products, especially those catering to health-conscious
Snacks and Baking
consumers, rely on more complex processes and equipment, such as steam stripping,
extrusion, and new types of fryers. On the other hand, these same technologies can
find wide application in other areas of food processing, as exemplified by extrusion
of pet foods, breakfast cereals, and fish foods and by frying of shrimp and roasting
of nuts.
Coating and Seasoning
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Coating and seasoning have different purposes and methods for various products.
The equipment and processes are unique and are interesting to understand. Coating
and seasoning are used to add flavors to the surface of pieces. This may be compared
with adding flavor to the base cake or substrate. Often, flavors added to the dough
used to make a base cake or other substrates are delicate and are damaged by the
frying or baking process. Flavors on the surface are encountered first and so have a
greater impact than flavors within the substrate.
Dry seasoning. Dry flavors and seasoning such as salt, powdered cheese, and
dry onion or garlic may not adhere well to some snacks, while other snacks, with a
residual layer of frying fat may pick up and retain plenty of applied powder. Shiny
snack pieces such as pretzels are especially challenging because the sodium carbonate
or bicarbonate bath in which they are dipped before baking seals the surface
and creates the characteristic surface finish. For these and other baked pieces, it is
customary to spray a light coat of oil first and then apply the powder.
There are many ways to apply oil and powders, but one common method is to use
a rotating metal drum with internal lifters. Drums come in diameters of 28–60 in.
and lengths of 3–18 ft. Typically, the drums rotate at about 10 rpm and are filled so
that there is a bed from about 6 o’clock to 9 o’clock as one looks at the end of the
drum. Pieces are introduced at one end and exit from the other. Flow is controlled
by the rate of feed, the rotation speed, and the elevation of the feed end of the drum.
It can be a challenge to measure the residence time of pieces, but one way is
to spray paint some pieces, drop them into the feed, and retrieve them from the
exit, measuring the time when they appear. Depending on the piece shape, weight,
and flow through the drum, there is some back mixing, so there is a residence time
distribution. Typically, the target is about 30 s average when just applying oil or
other liquid and 60 s when applying both oil and solids.
Oil is applied through a spray bar with up to six piston nozzles on which the
stroke and frequency of pulsing can be adjusted. Oil or other liquid flow is tested by
capturing in a cup for a short length of time. A calibration curve can be prepared for
a given system and given liquid. Sometimes the oil is heated while at other times it
is not. It is usually desired that the oil be relatively viscous so that it adheres to the
piece. Typical oil applications are 2–20% by weight. The higher values are used for
crackers.
Dry seasoning is applied with a screw feeder inserted at the exit end of the drum.
It is adjusted by varying the screw speed and by blocking some of the holes in the
feeder tube. To calibrate, seasoning is caught on a tray for a short time period and
weighed. Target dry application is 5–12% by weight. Seasonings vary widely in
bulk density, so feeder tube diameter and even drive motor power must be adapted
to the material. Straight salt is much more dense than dry cheese. Cheese and other
seasonings may be cohesive, meaning they stick together well, which can inhibit
flow from the feed hopper. It is not unusual for seasoning feed to be interrupted and
some product to escape unseasoned.
It is one thing to set the flows in the anticipated ratios; it may be quite another to
confirm that the desired final composition is achieved. Some pieces are so uniform
that the difference in weight between uncoated and coated pieces can be measured
by weighing the same number of pieces from the feed and exit streams. For other,
more variable, pieces, it may be necessary to measure a tracer like salt or oil by
chemical analysis.
An alternative to oil to achieve adherence is an aqueous starch solution, which
acts like a glue for the dry seasoning. However, this approach then requires removing
the water using heated air. This could be done in a separate dryer or in a
special drum with perforated walls and a plenum or second shell so that hot
air can be passed through the bed. The equipment to dry the pieces adds an
extra cost and the additional handling if using a separate dryer could increase
breakage.
There are other ways to apply oil and dry seasoning, including simply spraying
the products on a flat belt as they exit an oven and sprinkling salt or seasoning from
a feeder over a belt, but these only treat one side of the product. However, that is
adequate in some cases.
Coating with liquids that create a solid shell. Some products are coated by
dipping in chocolate, such as ice cream bars, while others are run through a
falling stream on a wire mesh belt in a process called enrobing. Coating with a
melt that solidifies as it cools is a complex situation to model. There is gravityinduced
flow of a fluid with a changing viscosity, heat transfer that changes the
viscosity, and surface adhesion. In practice, experiments are performed to establish
the correct temperature of application, the optimum time of transit beneath
the waterfall, and the time for cooling. Excess coating is recovered and recycled,
so it is desirable to minimize inventory by keeping the reservoir small and
it is critical to control temperature carefully, especially with chocolate, because
chocolate is sensitive to temperature and can solidify in unstable crystal forms
if it is not properly handled. These forms can create cosmetic defects in coated
products.
Panning. A very wide range of products is made by coating centers with sugar
or chocolate in a process called panning. Centers may include nuts, fruits such as
raisins or cranberries, soft gels as in jelly beans, hard candy, chocolate lentil-shaped
pellets and, in the case of pharmaceuticals, tablets of drugs. The common feature is
that the centers acquire a coating from successive applications of syrups or melted
chocolate, which are transformed into a solid shell by various means. The pieces
polish and shape each other by tumbling together in a rotating vessel. There is a
great deal of art in the specific operations, which are often done in batches and still
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