MANUFACTURING PROCESS OF POTATO CHIPS

Introduction: Potato Chips

Potato chips are widely consumed, especially by young people, due to their tastefulness. They are delicious fried foods characterized by a salty taste, crispy texture, and fatty mouthfeel.

Production of Potato Chips

The flow-sheet of potato chips production is presented below. This primarily includes receipt of potatoes and other raw materials. Upon receipt in the industry, potatoes are stored; washed, so that sand, dirt, stones, or microorganisms are removed; and mechanically peeled. The damage to potato tubes may affect the color and texture of chips. Peeled potatoes are additionally washed to avoid darkening. After peeling, trimming of eyes, bruises, dark and green spots, and decayed parts takes place. Before slicing, large potatoes are chopped because they easily crumble after frying. Slicing is important for quality of chips; slice thickness should range between 0.7 and 1.8 mm (opt. 1.0–1.2 mm). Sliced potatoes are washed to remove starch, sugars, etc., and protect them against browning reactions. This also ensures the production of crispy and light-colored chips. Blanching of chips before frying by immersing them in water or salt solutions (sodium bisulfite) at 65°C–95°C for about 1 min improves their color. After blanching and before frying, partial drying of slices should occur to avoid excessive oil absorption by the product.

In modern processing plants, potato chips are continuously fried. Owing to evaporation of water during frying, the weight of potato slices decreases, and consequently they float into the oil. Oil temperature ranges, initially, between 175°C and 190°C, and terminally between 160°C and 170°C. Frying time usually ranges between 1.5 and 3 min, and depends on the slices flow into the fryer and the potato dry matter content.

After frying, excess oil is removed from chips, and salt or flavorings are added. Then, they are allowed to cool and conveyed for packaging, after burnt or broken chips are removed. Packaging materials (polymeric, laminated films) are shaped into bags, filled with weighed portions of chips, heat sealed, secondary packaged, palletized, and transported to the storage area (Vorria et al., 2004).

Moreover, a failure mode and effect analysis (FMEA) model has been applied for the risk assessment of potato chips manufacturing. A tentative approach of FMEA application to the snacks industry was attempted in order to analyze the critical control points (CCPs) in the processing of potato chips. Preliminary hazard analysis was used to analyze and predict the failure modes occurring in a food chain system (potato processing and potato chips processing plant), based on the functions, characteristics, and/or interactions of the ingredients or the processes on which the system depends. CCPs have been identified and implemented in the cause and effect diagram (also known as Ishikawa, tree diagram, and fishbone diagram). Finally, Pareto diagrams were employed toward the optimization potential of FMEA (Arvanitoyannis and Varzakas, 2007).

Quality: Safety Concerns during Production of Potato Chips

Several safety and quality parameters should be taken into account during the production of potato chips. These are analyzed at every production stage. More specifically, raw materials (potatoes, oils/fats, and others) are susceptible to safety and quality hazards, and suppliers should assure their specifications.

The possible hazards for potatoes may be microbiological (fungi, molds, bacteria), chemical (fungicides and pesticides residuals), and physical (foreign materials derived from the soil or collected during harvesting) (Doan and Davidson, 2000). The most possible hazards for frying oils and fats are of chemical nature (antifoaming agents, heavy metals). The controls for other raw materials (flavoring materials, salt, additives, and preservatives) should mainly concern chemical and physical hazards. All raw materials should satisfy the required safety specifications in accordance with the legislation; thus, the industry should cooperate with trustable suppliers. Special precaution should be given to the water used, which should satisfy the specifications for drinkable water, and especially those for pathogens and chemical contaminants in accordance with the legislation.

The storage of raw materials is a very important stage for the safety of the final products. Potatoes should be stored at 3°C–10°C, at a relatively high humidity and shadowy environment in order to keep the solanine level under 0.55% per dry weight. The oil/fat should be stored in closed stainless steel silos at about 0°C, and its deterioration can be monitored by peroxide value and p-anisidine measurement.

During potato washing, the specifications for the processing water as well as the reused water should be checked. The knives and drums used for potato slicing must be made of stainless steel, while they must be maintained and frequently inspected for their integrity. During subsequent washing and blanching, the microbiological quality of the water as well as the blanching conditions should be controlled (i.e., 73°C for 5 min). Before deep frying, the potato slices are partially dried. The maximum moisture removed must not exceed 4%, so that the appearance and taste of potato slices are not affected.

Deep frying of chips is considered the most important step in processing for the safety of final products. Fat changes occurring during frying as well as the possible harmful substances produced must be monitored (Gertz, 2000). Oxidation occurs by the reaction of air oxygen with the fat and some of the reaction products that remain in the fat can accelerate further fat oxidation. Since oxidation proceeds more rapidly at higher temperatures, the frying fat temperature must be kept at the levels required for cooking. However, during industrial frying it can reach 204°C, as many of the foods are conveyed through the frying oil for as little as 1–2 min. In order to keep the oxidation level of the frying fat to a minimum, it is important to use a good quality frying fat, keep the temperature of the fat as low as possible, adjust the maximum rate of fat turnover, and remove food particles from the fat. It is recommended that the temperature should not exceed 185°C (optimum temperature 177°C), while the oil/fat turnover should take place every 5–10 h. The limits/specifications for the used oil/fat renewal (mainly polar compounds content) are determined by legislation and differ from country to country (Lin et al., 2001; Miyagi et al., 2001).

Excessive oxidation of oil and fats is often accompanied by polymerization and formation of various decomposition products. Products such as peroxides, monoglycerides, diglycerides, aldehydes, ketones, and carboxylic acids are volatile at frying temperature and have relatively little responsibility in developing polymers. The nonvolatile decomposition products (cyclic monomers, dimmers, trimers, other high molecular weight compounds) include polar compounds, which may result in gumming and foaming. All these products are undesirable in the frying process (Billek, 2000). Hydrolysis that occurs through the reaction of food water with frying fat leads to the formation of free fatty acids. The rate of hydrolysis depends on the amount of water released into the frying fat, the frying temperature, and the fat turnover rate. Foods contain several components/substances (sugars, starch, proteins, phosphates, sulfur compounds, trace metals), which are extracted into the fat during the frying process, and may turn brown and/or react with the fat and cause its darkening.

The most harmful products formed during frying are the trans fatty acids and acrylamide (Hayakawa et al., 2000; Zyzak et al., 2003). Serious consequences for human health caused by such components are coronary and other cardiological diseases (Billek, 2000). Trans fatty acids may be formed during hydrogenation; however, their content in the fats increases during their use. It has also been shown that trans fatty acids increase cholesterol levels in the human blood. Acrylamide is a β-unsaturated active molecule found in starch-containing products, which have been fried, heated, or cooked at temperatures higher than 171°C. Acrylamide is considered to promote carcinogenesis and has only been recognized to affect the human organism by inducing toxic effects in the nervous system (Saguy and Dana, 2003).

Regeneration of frying fats or oils, by means of filtration systems, is recommended in order to eliminate highly oxidized/polymerized fat components and other impurities from the used fat. Most of the filter systems in use today are passive filters or systems. However, active filtering, using porous materials can adsorb undesirable constituents from the used fat. Bleaching earths may be used for the regeneration of spent frying fats, while synthetic silicas or silicates have been proposed by various researchers for such treatments. The filtration system can be adapted to the fryer and operate continuously, circulating the frying oil through the kettle (Lin et al., 2001; Miyagi et al., 2001).

In order to produce safe fried foods, frying should, therefore, be monitored for its deteriorative state. Various factors related to frying medium, food, or process affect the frying of foods, some of them are designed (size of food) while others may enhance fat/oil oxidation and should be controlled during frying. The extent of fat deterioration may be evaluated by various quality parameters of fat/oil. The most critical parameters are considered to be anisidine value, total polar materials as well as UV absorbance values indicating the present oxidative state and the shelflife of the fat, respectively. The recommended limits for the most important parameters are presented in Table 1.

Moreover, the continuous monitoring of fat quality can also be achieved by rapid analyses using test kits or appropriate sensors, which have been recently developed mainly based on dielectric constant measurements. Four well-known rapid tests include food oil sensor (FOS) (Northern Instruments, Lino Lakes, MN), which measures the dielectric constant in frying fat relative to fresh oil; the RAUTest, which is a colorimetric test kit that contains redox indicators reacting with the total amount of oxidized compounds; Fritest (E Merck, Darmstadt, Germany), which is a calorimetric test kit sensitive to carbonyl compounds; and the Spot Test, which assays FFA to indicate hydrolytic degradation and FFA. The FOS correlates better with polar compounds than with RAU-Test, Fritest, or Spot Test.

After frying, chips are inspected for foreign materials or defects (burnt, too small, too fatty), salted or flavored, cooled, and packaged. The packaging materials should be approved for foods, while strict hygienic conditions should be assured in the packaging area. The final products should be inspected for their correct package sealing and the possible existence of metal pieces by the use of a metal detector and stored. Ultimately, to ensure the delivery of high quality products to the consumers, the sensory characteristics of fried snacks should be inspected.

Hygienic conditions should be assured (good manufacturing practices [GMPs] and good hygiene practices [GHPs]) by the industry in each production line stage, particularly after deep frying. The agents for cleaning or disinfection and the lubricants used should be suitable for use in food products (Soriano et al., 2002).

Dietetic Concerns about Fried Snacks

Nowadays, there is an increasing demand for snack products that are organic or all natural, low-calorie type, low-fat, low-carbohydrate, low-sodium, or having health-promoting benefits. Despite the popularity of deep-fried snacks, there are many concerns considering their nutritional and healthy aspects due to their high fat content associated with many diseases, such as coronary heart disease and obesity. During frying, fat is mainly absorbed on the crust of the snack, whereas the fat absorption is affected by the occurred mass transfer phenomena (water migration from the core to the surface, oil absorption into the snack, and leaching of the liquefied components from the food). Many factors control oil absorption, such as:

1. Oil quality
2. Frying duration
3. Frying temperature
4. Food composition
5. Geometry of the food
6. Initial moisture content
7. Porosity
8. Initial surface tension

The latter can be achieved either by the proper design of the frying conditions or by implementing suitable pre-frying treatments (analyzed in the following paragraphs), which aim to decrease oil absorption and, consequently, to reduce fat intake. The proper selection of frying conditions may result in a significant reduction of oil uptake and, moreover, favor the development of the required sensory characteristics, such as crispiness, crunchiness, flavor, and palatability.

Pre-Frying Treatments to Reduce Oil Intake in Fried Snacks

In the aforementioned text, the extent of oil absorption in fried snacks can additionally be controlled by pretreating foods prior to frying. Drying and edible-coating application have been recently investigated toward this trend. The most frequently applied pretreatment techniques in snacks manufacturing include the following:

a) Pre-drying of the Snack

Independently to the method implemented, drying of snacks prior to frying results in the increase of total solids, the reduction of initial moisture content, and the formation of a crust that will inhibit oil and moisture transfer from and to the crust, respectively. Thus, pre-drying of snacks may be helpful for the reduction of oil diffusion from the frying medium to the crust so that fat uptake is considerably reduced. Moreover, pre-dried snack products are characterized by a crisp and firm perception.

1. Air drying: The dehydration of snacks using air driers leads to the formation of a crust, which is characterized by high compactness, low porosity, and increased solid content, leading to the hindering of oil uptake. In general, air drying is considered a good pre-drying method as it may achieve a uniform dehydration effect on the food, which in turn is correlated with desirable sensory attributes.

2. Osmotic dehydration of snacks: Osmotic dehydration of fried snacks has been also successfully applied. In this case, the reduction of moisture content is carried out through the substantially different concentration potentials between the food and the applied solution. Commonly, salt and sugar solutions or a combination of these are used for the osmotic dehydration of food. The most frequently applied materials for osmotic solutions include salt (brine), sugars (glucose,fructose), and maltodextrins. According to this technique, the food is immersed into the solution, or alternatively the solution can be directly sprayed on the surface of the food. There are published data indicating significant reduction of the fat uptake and texture loss and oxidative stability in osmotically dehydrated fried snacks (Ikoko and Kuri, 2007).

3. Microwave drying: The application of microwaves as a pre-drying method in fried snacks has been also investigated with almost similar effect to that of air drying. However, the data suggest that it is more heterogeneous than air drying; within the product, there are areas of high and low moisture that results in uneven oil diffusion during frying and variable fat uptake (Debnatha et al., 2003).

(b) Use of Edible Coatings

The use of edible coatings in fried snacks is based on their functionality; they possess efficient barrier properties to lipids, oxygen, and carbon dioxide. Thus, an edible coating to be applied in fried snacks must be characterized by low oil permeability, which depends on its oil solubility and diffusivity as well as film thickness. Moreover, edible coatings are able to limit moisture transfer from the inside to the crust, and simultaneously from the environment to the crust. Thus, the product does not absorb excessive oil and maintains its sensory characteristics, that is, crispness and crunchiness. The most applicable edible coatings in snacks include proteins (i.e., caseinates, WPCs), cellulose derivatives, alginates, pectins, starches, and other hydrocolloids. The reported decrease in fat uptake may reach up to 99%.

(c) Extrusion

Extrusion can be applied in snacks manufacturing as an alternative process to frying. This can be achieved using the fry type extruders. The snack blend (corn meal) is subjected to heat and shear rate (approximately 120°C), and the material is changed to a plasticized mass. The snacks produced by the fry type extrusion, due to the inherently lower pressure applied, are characterized by considerably lower expansion, higher density, and also they are more irregular and rough shaped with length variations. Thus, the products having such characteristics are more suitable for frying than high shear expanded products, respectively, which absorb excessive oil (like sponges).

(d) Modification of the Snack Composition

In order to produce low-fat snacks, pregelatinized waxy corn starches are used and low-fat snacks are prepared by baking or by indirect expansion processes (low-shear extrusion followed by microwaving, baking, or frying). In addition, high-amylose starches are used to prevent oil absorption in fried snacks as they form a strong film, which firms up texture and reduces oil uptake. Despite the fact that fat and oil have been used for attaching seasonings to the surface of cereal-based snacks, the demand for low-fat and fat-free snacks has led to the finding of new adhesives that can attach seasonings, such as gum-based solutions, and starch-based coatings.

By Constantina Tzia, Virginia Giannou, and Theodoros Varzakas in "Handbook of Food Processing - Food Safety, Quality, and Manufacturing Processes", edited by Theodoros Varzakas & Constantina Tzia, CRC Press  (Taylor & Francis Group),USA, 2016, excerpts pp.574-580. Adapted and illustrated to be posted by Leopoldo Costa.