Fried foods are increasingly popular with consumers. Reasons for their popularity include desirable sensory characteristics such as a soft, juicy interior and a crispy outer crust. However during the frying process, fat content can increase considerably, sometimes even up to 10 times the original weight. Since consumer trends are moving towards healthier and lower-fat products, the food industries are challenged to create new foods with lower fat content while retaining yield and consumer acceptance.
Four phases of frying
There are four phases in the frying process. These phases help explain why methylcellulose can function as a barrier. During frying, food is immersed into hot oil for a specified amount of time depending upon the desired sensory and physicochemical characteristics of the end-product.
Initial heating. During this frying process, the temperature increases and there is little loss of moisture. This phase is called initial heating.
Surface boiling. The second stage is referred to as surface boiling, which occurs when the heat from the oil changes the surface of the product and the free water on the surface is lost. This starts the process of crust formation.
Falling rate. The third and the longest phase of frying is the falling rate, which involves greatest change in moisture content and oil uptake.
Bubble end-point. The final phase is called the bubble end point in which there is no longer any heat transfer and the water loss has decreased.
The role of pressure and moisture migration
In addition to the phases of frying, it is very important to note that the pressure between the oil and moisture in a chicken nugget is what really defines movement of these two substances. For example, positive pressure within the food product during frying causes an increase in moisture loss whereas negative pressure allows for higher oil uptake due to suction.
Yield is very important in fried foods. Even though the poultry industry is able to add up to 30% breading to make a fried product (greater than 30% are referred to as fritters), there is still considerable yield loss associated with cooking of the product.
Additionally, management of the par-fried and fully-cooked processes can impact yield. During par-frying and deep-fry cooking, moisture migrates from the meat to the outer surface of the product and can be lost in the oil. This migration causes yield loss from the product and can lead to some decrease in consumer acceptance.
Most people believe that oil uptake during the par-frying process counter balances the loss from moisture. However, in the studies we have conducted, there is always a net loss in yield during par-frying, mainly during the moisture flash off from the crust. Therefore, the loss is more than the gain.
Minimizing yield loss
In order to minimize yield losses in par-frying and fully-cooked operations, proper management of the operation is critical. Some things to consider are batter viscosity, par-fry time and temperature, oil management, and reducing yield loss from steam after par-fry or cooking.
Lower batter viscosity can decrease yield due to lower pickup, and higher batter viscosity can increase pickup as much as 30%. Therefore, checking the viscosity by using a Stein or Zahn cup and reading the manufacturer’s instructions can optimize yield.
For yield concerns with par-frying time and temperature, the general rule is that with increased temperatures, the less oil pick-up and increased moisture loss occurs. The lower the frying temperature, the higher the oil uptake, which can produce an oily product and lead to decreased consumer acceptance. However, it is best to experiment with the set up at each facility to optimize yield and oil uptake.
Oil management can be critical for quality. The quality of the oil in the par-fry system will determine the quality of the product. Considerations for oil management include using filters for organic material, minimizing free fatty acids, minimizing moisture contamination, and knowing the load that each batch of oil can withstand before quality begins to deteriorate.
A final major concern in maintaining yield during par-frying is reducing steam loss after the frying stage. Once fried, the product can continue through a cook stage via oven or deep frying. Another option, in non-fully-cooked operations, is to immediately freeze the product in a spiral or blast freezer to reduce moisture loss from the steam. Even though it may seem that steam is a negligible factor for yield loss it can have a significant effect on overall plant yield when discussing large volumes of product.
With all of those good management systems in place – what else can be done to improve yield and quality of par-fried and fully fried foods? A possible answer is the use of edible coatings. Edible coatings are a thin layer of edible material that can be used on a food surface to control mass transfer of moisture, gas, aroma, and/or fats.
Methylcellulose as a coating
Ingredients such as methylcellulose are being investigated to determine uses in fried products as an edible coating. Methylcellulose is an odorless and tasteless food grade gum, which has very good film-forming characteristics and is resistant to fat and oil transmission as well as oxygen transmission. These properties help to reduce the final oil uptake of fried foods as they form thermally induced gelatinous coatings.
Current research at Texas Tech University and Texas A&M University are exploring methylcellulose uses in other fried food items such as chicken nuggets. The objective of this research was to evaluate the effect of methylcellulose added in the pre-dust on the yield and texture of the chicken nuggets.
Increased pickup of pre-dust, batter and breading
The results from this study indicate that methylcellulose added in the pre-dust was effective at increasing the percentage of pre-dust, batter and breading pick up by 4% to 5%.
Methylcellulose coated nuggets also had a lower moisture loss and lower fat uptake in both the core (meat only) and crust (batter and breading only) samples.
During the cooking or frying process, methylcellulose coagulates and forms a protective layer or barrier between the batter and breading and the meat. This coagulation effect binds the meat and the batter during frying and can explain the higher percentage of moisture retained in the interior meat sample.
Additionally, methylcellulose has the ability to bind up to 40 times its weight in water. Therefore, this methylcellulose layer helps prevent moisture loss from the meat and helps to prevent excess uptake of the oil.
These are very positive results indicating that methylcellulose, when added to the pre-dust, can improve yield, decrease moisture loss and decrease fat uptake.
In addition to the yield studies, texture analyses were also conducted to determine if differences existed with consumer acceptability. Creep compliance is a very sensitive test that determines textural properties of food in their natural cooking process. It measures texture under a slow progressive deformation of the product under stress (cooking).
The creep compliance (µm²/N) for this study was measured using a Dynamic Mechanical Analyzer.
No difference was observed between the texture of the control and the methylcellulose treatments. These results indicate that the chicken nuggets prepared using methylcellulose require a force similar to control to be deformed and, thus, may be associated with similar mechanical properties.
In addition, both control and methylcellulose samples were highly acceptable to consumers in a taste panel. Consumers did not detect any significant difference between the taste of control and methylcellulose coated nuggets. Therefore, chicken nuggets may be prepared using methylcellulose without disturbing the texture of the product.