Plain set and stirred yogurt with different additives: implementation of food safety system as quality checkpoints

Raw milk

Milk is the main component in yogurt production, with the quality of received milk imperative to control the quality of yogurt. The main risks associated with milk include biological hazards. Treatment of raw milk at the farm is a crucial stage, mostly using chilling to ensure eliminating microbial growth. Microbiological contamination influences the efficiency of pasteurization at a later stage, especially that some toxins produced by microorganisms are heat stable even under the low temperature which is commonly used to preserve the yogurt (>0–<10 °C). For instance, Bacillus cereus produces toxins at 6 °C; Staphylococcus aureus produces toxins at 10 °C, whereas Salmonella typhi at 15 °C (Table 2). Therefore, chilling milk in the farm is a mandatory step to ensure preventing any microbial growth, to be stored at 0–4 °C until proceeding with the processing step inside the dairy plant. Total bacterial count is an indicator of the farm conditions, where the raw milk should have a total bacterial count of <100,000 CFU mL⁻¹, (Chountalas, Tsarouchas & Lagodimos, 2009). High somatic cell count is used to indicate and control the hygiene of livestock, which should be less than 4.0 × 10⁵ mL⁻¹ (Arvanitoyannis & Mavropoulos, 2000).

Chemically, although antibiotics do not immediately impact consumer’s health, they inhibit the process of fermentation during yogurt production. Consequently, antibiotics testing is typically conducted in dairy factories to ensure their absence and the safety of milk. Organochlorine and organophosphorus are the two major pesticides affecting flavor alteration and/or starter growth inhibition (Tamime & Robinson, 2007). Organochlorine and organophosphorus residues in milk originate from contaminated feed, grass or corn silage, and direct application of pesticides on dairy cattle (Shaker & Elsharkawy, 2015). Lead, mycotoxin, and dioxins are the primary contaminants that may pass from the surrounding environment into the milk. Lead is associated with cardiovascular diseases and mental disorders, whereas mycotoxin and dioxins are carcinogens (Chountalas, Tsarouchas & Lagodimos, 2009). Milk contamination by heavy metals is a serious concern confronting the community’s health (El Sayed et al., 2011); the main source of contamination lies in grazing animals on contaminated fields (UNB, 2002).

Physically, any foreign fragments present in raw milk as straw, hair, or soil, are considered a high risk of contamination (Tamime & Robinson, 2007). In conclusion, received milk is expected to meet the following specifications including temperature of <10 °C, total count of <100,000 CFU mL⁻¹, inhibitory substances of <0.007 IU mL⁻¹, fat of ≥3.0 g, protein of ≥3.0 g, somatic cell count of <4.0 × 10⁵ mL⁻¹, freezing point depression of <0.520 °C, and titratable acidity of <0.2% lactic acid (Tamime & Robinson, 2007).

Starter culture

Special emphasis is given to the quality of starter culture to assure safe conditions for yogurt fermentation, with yeast and molds for instance reported to influence the coagulation of yogurt and to further pose a health risk. Accordingly, it is essential to assess culture vitality by measuring the amount of lactic acid and coagulation time to be 0.6–0.7% and 3–4 h, respectively (Tamime & Robinson, 2007). The growth of yogurt bacteria is an essential process to control the quality of final products and manage the cost-effectiveness during manufacturing (Jung, Lamsal & Clark, 2014). To control fermentation and coagulation processes, starter culture must be free from foreign bacteria with excellent vitality (Chountalas, Tsarouchas & Lagodimos, 2009). Gas bubbles or nasty smells are considered indicators of infection or contamination by non-starter bacteria; the test of catalase reaction is conducted to confirm the presence of gross contamination in the starter culture (Tamime & Robinson, 2007).

Stabilizers

Stabilizers are used to maintain a reasonable degree of uniformity and to reduce wheying-off from the final product. A proper yogurt stabilizer forms gel structures in water, leaving less free water for syneresis action, and to increase yogurt shelf life. In addition, stabilizer should be effective at low pH without imparting any off flavor (Wang, Yuan & Yue, 2015). In the production of yogurt, modified starch, pectin, gelatin, and whey protein concentrates are the most used stabilizers; however, a combination of starch and gelatin is typical commercial mixture used in yogurt production. Whey protein is used due to their water-binding attributes (Jung, Lamsal & Clark, 2014).

Non-milk ingredients (fruit preparations, juice, sweeteners, flavor enhancer, and coloring agents)

Fruit-for-yogurt preparations are used to improve flavor and color; however, it must exhibit the identical taste and color of targeted fruit after mixing with yogurt (Table 2). Fruit preparation should also not lead to any phase separation or any defects in texture after blending with yogurt. It must have zero yeast and mold to assure the final product safety level (Jung, Lamsal & Clark, 2014). The microbiological hazards are associated with additives, specifically coliforms, yeast, and mold, which indicate a low level of hygiene (Temelli et al., 2006). Besides, fruits addition may represent a contaminated source with heavy metals and/or pesticide residues. Hence, all additives must comply with national and international legislation to alleviate hazards present above certain limits (Chountalas, Tsarouchas & Lagodimos, 2009).

Facility

It is imperative to control air quality inside the facility used as a process plant for yogurt production (Tamime & Robinson, 2007). Total count for bacteria, coliforms, yeast, and mold are valuable tests performed to evaluate the standard of plant hygiene, and to monitor the process of sanitation for all surfaces and dairy equipment (Mostert & Jooste, 2005).

Receiving

The chemical composition of raw milk is typically at 3.5% fat, 3.5% protein, 5% lactose, and 88% water (Chandan et al., 2006). Raw milk should only be received from good hygienic farms where animals are free from diseases and have not been treated with antibiotics (Sandrou & Arvanitoyannis, 2000). High coliform bacterial count is one of the approved tests that can reflect poor or unhygienic conditions of milking practices (Chandan et al., 2006). Some tests must be applied upon receiving the raw milk namely, temperature (10 °C), pH-value, antibiotic residues, somatic cell count (500,000–700,000 cells mL⁻¹), bacterial limits (300,000 CFU mL⁻¹ in the raw milk or 20,000 CFU mL⁻¹ in the pasteurized milk), coliforms (not to exceed 10 CFU mL⁻¹), and drugs (must be negative on drug residue) (Chandan et al., 2006). Mixing newly received milk with stored milk in the same tank should be strictly avoided to eliminate any contaminants; with all tanks to be cleaned and disinfected before storing received milk to eliminate the accumulation of milk residues. Inside the premises, milk must be conveyed in closed pipes throughout the whole production unit (Sandrou & Arvanitoyannis, 2000).

Mixture preparation and heat treatment

Standardization of milk fat, adding of stabilizers and sweeteners should precede heat treatment (Achaw & Danso-Boateng, 2021). Milk is typically treated at 85 °C for 30 min or 95–99 °C for 7–10 min to destroy all pathogenic microorganisms and most vegetative cells (Table 3). Heat treatment inactivates milk enzymes and produces suitable conditions to enhance the growth of the desired starter culture (Jung, Lamsal & Clark, 2014). Next, milk must be rapidly cooled to 4–5 °C (Sandrou & Arvanitoyannis, 2000). A variety of heat treatments can be applied to milk used for yogurt production including pasteurization (80–95 °C), sterilization (115–120 °C), and ultra-heat temperatures (135–140 °C) (Ritota et al., 2017). After ensuring that all raw materials are free from any hazards and safe to use, the hazard in this stage is limited to drop of temperature to 75 °C or below for at least 20 s (Chountalas, Tsarouchas & Lagodimos, 2009). Verifying the effectiveness of pasteurization, records of thermograph should be observed, and an alkaline phosphatase test (ALP) could be further employed to demonstrate the correct pasteurization of raw milk (Payne & Wilbey, 2009). Moreover, Laboratory Pasteurized Count (LPC) is a common test to confirm the absence of heat-stable bacteria as a high LPC will reduce shelf life of final products. For instance, Bacillus cereus is one of the microorganisms that can survive after pasteurization and increase LP. It should be noted that there are no approved limits for LPC by Federal Standards, but in some countries the maximum limit of LPC is 750 CFU mL⁻¹ (Chandan et al., 2006).

Homogenization

At this stage, yogurt milk is an oil in water emulsion, and homogenization step is required to create a stable emulsion by reducing the size of the discontinuous phase. The objective of homogenizing milk is to reduce the diameter of fat globules to two microns, or less. The temperature must be around 50–65 °C and the pressure of 15–40 MPa (Dhankhar, 2014). Besides, the coagulations begin at pH 4.6–4.7, which is called the isoelectric point of casein. This step enhances texture development and eliminates wheying-off and surface creaming. Thus, sterilization of the homogenizer is considered an essential step to avoid contamination and must be operated under aseptic conditions (Papademas & Bintsis, 2010).

Fermentation

The fermentation room must be isolated from other areas of the plant to eliminate any cross-contamination and must be equipped with proper air ventilation. Fermentation tanks are insulated and wrapped in stainless steel with a cone bottom to drain any fluid after the incubation period, with temperature to be kept at 43 °C (Table 4). A clean-in-place (CIP) process is used to clean tanks with hot cleaning fluids (Jung, Lamsal & Clark, 2014). The starter could be a source of yeast and mold contamination; thus, it is imperative to enforce a strict examination of the starter to eliminate fungal contamination. A direct microscopic view of the used starter or plating of the starter on acidified potato dextrose agar is used to confirm the presence or absence of yeast and mold (Chandan et al., 2006).

Packaging and storage

Plastic containers are used to pack yogurt in various shapes, that characterize certain brands, and in different sizes, ranging from 113 to 904 g (Jung, Lamsal & Clark, 2014). The packaging material can present a source of microbiological contamination; hence, aseptic conditions must be applied at this stage. In addition, metal detectors are essential to detect and eliminate any physical hazards including foreign bodies or metal fragments (Chountalas, Tsarouchas & Lagodimos, 2009). Packages should be examined to prevent contamination to occur after filling or due to leakage (Sandrou & Arvanitoyannis, 2000). In case of using glass bottles, they must be inspected by an automatic photoelectric cell device to verify that all bottles are effectively cleaned and sanitized (Table 4). In addition, all suppliers of packaging materials should be audited to confirm their standards of bacteriological quality (Graves, Smith & Batchelor, 1998). All packaging materials must be stored in a room free of dust and humidity, with fumigation using iodine or chlorine regularly performed (Chandan et al., 2006).

The high temperature above 6 °C is the fundamental cause of spoilage during storage period, mainly by coliform, yeast, and mold (Viljoen et al., 2003). Hence, an automatic control loop should be used to alarm when the temperature rises above 6 °C in the storage areas (Sandrou & Arvanitoyannis, 2000). The fluctuating temperature during storage can increase fat oxidation and food spoilage. Besides, it can adversely affect the sensory properties and coagulum stability by changing texture and viscosity due to hydration of protein constituents and changing in the color of fruit as in case of fruit blended yogurt (Phosanam et al., 2021).