Quality donut frying all comes down to the oil and its ability to transfer heat to the dough. Temperature must be consistently distributed throughout the fryer. The oil can’t get too hot too fast or for too long or it will start to degrade, and then it can’t get the job done. And sediment that sloughs off the donut must be removed and the oil refreshed to impart a clean taste to a finished donut.

All these challenges can be navigated by exerting a bit of control on the fryer. Heating elements must get the oil hot in a gentle way. Continuously filtered oil maintains its freshness. And that new oil should be constantly recirculated throughout the fryer to eliminate cold spots.

Heated just right

Control over the oil temperature starts with how it’s treated throughout the fryer. “The frying oil needs to be heated quickly to keep the temperature constant, but you should not do this just by using extreme temperatures as this will damage the oil,” said Ken Weekes, fryer specialist, WP Bakery Group.

Achieving these temperatures quickly and holding them constant require a large heating area, Mr. Weekes explained. For a smaller fryer, she recommended electrical heating. “The elements are directly in the oil and give you an immediate reaction when the temperature drops,” he said.

Moline Machinery’s new LIBRA Series electric and gas fryers maximize oil life by minimizing surface temperature of the electric elements or gas burners. This doesn’t degrade the oil as it heats. “High surface temperatures, regardless of the heating method, degrade frying oil,” said David Moline, vice-president, sales and marketing, Moline Machinery. “Oil life is maximized by heating oil at lower surface temperatures, and longer oil life means higher product quality and longer production runs.”

It’s also important to size the fryer appropriately and select the right heating method. “There are many adequate methods of heat transfer to cooking oil, but the correct system must be chosen for a given application,” said Doug Kozenski, sales manager, processing, Heat and Control. Each heating method has its benefits and drawbacks. With gas, Mr. Kozenski suggested multiple burners placed laterally or longitudinally to provide balanced heating. Thermal fluid or stem heating benefits from a two-pass longitudinal coil or two-pass tubes. “The distributed flow design spreads the heat across the width and the two-pass tubes ensure uniform heat down the length of each zone,” he said.

In a larger fryer, Mr. Weekes suggested thermal oil heating offers the most efficient, accurate and gentle method, which saves the oil from degradation. This way, oil is warmed in a heat exchanger outside of the fryer, allowing for the largest heating area. “This gives the system sufficient time to heat the oil gently and provides extremely constant temperatures in the fryer,” he said.

After heating, maintaining temperature and oil levels are critical to the process. WP Bakery Group uses sensors to keep an eye on both the temperature and level of the oil.

“The oil level must remain constant to ensure that the products are all fried evenly and uniformly,” Mr. Weekes said. “We use capacitive level sensors to constantly check the oil level in the fryer.” These accurate sensors are not affected by heat or steam. “This allows us to react immediately, topping up the frying from oil in a holding tank, when the level goes down due to oil being absorbed by the product,” she said.

The company’s fryers also have three sensors at critical positions, which allows operators to track the oil temperature throughout in real time. One sensor is placed in the infeed or the coldest area. Others are placed a third and two-thirds of the way along the fryer length and ensure the temperature remains consistent.

Belshaw Adamatic uses a closed loop proportional-integral-derivative (PID) device in its gas and electric fryers to prevent temperature swings as the heat is turned on and off. In the gas fryers, the PID controls the gas input rate to the fryer. In the electric fryers, the PID works with solid-state contactors to maintain the temperature within 5°F.

Adding new oil can disrupt the consistency of the fryer’s temperature so it is necessary to keep it hot enough to fry donuts. Circulating this new oil distributes that heat quickly but must be controlled. WP Bakery Group’s fryers have two inlets of oil: one at the infeed and one a third of the way down the length of the fryer, with a 70/30 oil distribution between the inlets, respectively. “The majority of the oil flows through the first inlet to ensure that the temperature remains constant even though products are removing heat quickly,” Mr. Weekes explained. “The second inlet is a third of the way down the fryer to give the temperature a small boost after the product temperature has stabilized and just before it is flipped.”

Cleaned and refreshed

Maintaining consistent and even oil temperature is critical for consistent product quality. “Not having clean oil will affect product taste, and when taste is compromised, the oil must be filtered or disposed” Mr. Moline said. The oil needs to be clean and evenly distributed throughout the fryer to get an even, fully fried and great-tasting donut.

As products such as yeast-raised donuts are fried, sediment falls off into the fryer. This sediment, which includes starch from proofing, flour from sheeting or even inclusions, degrades the oil and interferes with the heat transfer. “When fines from the product build up on the heat transfer mechanism, the efficiency and ability of the mechanism to transfer heat to the cooking oil decreases,” said Ron Ferrante, vice-president, operations, Heat and Control. “Therefore, it is critical to keep the cooking oil as clean as possible during production.” This is done through filtering out all that sediment, and it must be done continuously for maximum effectiveness.

That process helps donut producers avoid downtime for cleaning. Without a continuous filtration system, a donut producer will eventually have to clean the sediment and refresh the oil. “With this system, you don’t have to shut down because we’re continuously removing sediment and introducing clean oil into the fryer,” Mr. Moline said. Moline’s LIBRA fryers can run 24/7 with the sediment removal and continuous filtration system that can remove as small as one micron of sediment.

Dirty oil is removed from the fryer, filtered to remove sediment, and then reintroduced into the fryer. As that oil heats up or is already heated in a thermal system, it must be circulated through the fryer to even out the temperature again.

“The circulation of the frying oil is key to accurate and even temperatures,” Mr. Weekes said. “The oil flows into all areas of the fryer, distributing the heat evenly. Any sediment build-up is drastically reduced as it’s pumped out of the fryer and filtered continuously.”

In addition to filtration, Heat and Control features a sediment removal conveyor to drag settled fines from the pan bottom out of the fryer. The company also offers a skimmer system to remove fines floating on the surface. Much like a skimmer of a swimming pool, the simple system directs the overflow to the oil filter to remove the fines and return the oil to the fryer.

To help manage sediment build-up, Belshaw Adamatic designed a non-stick proofer basket with a donut-release system. “This allows producers to use less flour in the proofing process to prevent donuts sticking to the baskets as they proof,” said Joe Nelson, engineering manager, Belshaw Adamatic. “The result is less flour on the bottom of the donuts and, therefore, less flour making it into the fryer, thus extending the life of the oil.” The system also includes a filter to help producers clean oil at the end of a production run.

Control across zones

As donut producers increase their capacity, fryers have gotten longer. And that means more issues can crop up when controlling the oil temperature. Multiple zones in the fryer can be very helpful in offering producers control and flexibility in donut frying.

“Having those multiple zones of control is a very important part of frying donuts,” Mr. Moline said. “You want to fry at a slightly different temperature in the beginning than at the end of the fryer, and those different temperatures in the different zones will ensure an even color on both sides of the donut.”

For example, yeast-raised donuts need to be fried hotter at the beginning because the donut will be cooked a little longer in the first half of the fryer. Multiple zones give producers the ability to make temperature adjustments in localized places and get an even fry. As filtered oil is added back in, zones can also help operators manage those temperature changes.

“Our ability to do that, especially with the electric and dual-fuel fryers, lets us pinpoint where we need to control so the donuts are seeing the temperature you want them to see,” Mr. Moline explained.

With donut producers needing longer and more flexible fryers, today’s technology empowers them to take control of what’s happening inside the system and the donuts that come out.