INTRODUCTION

Salsa became the number one selling condiment in the United States in 1992, surpassing the long-time condiment sales leader, catsup. Today, the popularity of salsa is established and continues to grow as it is adopted by other cuisines (Brandweek, 1992).

Salsa can be made fresh or it can be thermally processed to increase shelf-life. Fresh salsa has a different taste and tactile sensation than processed products because the thermal processing necessary to remove viable microbial cells also affects the flavor and texture (Qin et al., 1995). Fresh salsas have many desirable characteristics that processed salsas do not. It would be beneficial to the food industry to be able to produce a shelf-stable, processed salsa that retains some or all of the characteristics of a fresh product.

One of the key ingredients used in making fresh salsa is cilantro. Cilantro contributes to the aroma as well as the color and taste of the salsa and tastes best fresh (Gessert, 1983). Heat processing affects cilantro flavor and causes a change in the taste of a salsa. Cilantro aroma and taste is also affected and completely destroyed by most drying processes (Gessert, 1983).

The microwave vacuum dehydration process (MIVAC) integrates microwave drying in a vacuum environment which dries heat sensitive food materials at very low temperatures in a short period of time (Decareau, 1985). The MIVAC process allows food materials to be dehydrated to low moisture contents while retaining most of their volatile compounds (Carlsen, 1997). MIVAC technology has been successfully applied to a large variety of fruits and vegetables, fruit juices, seeds and grains (Decareau, 1985). Preliminary work on MIVAC-treated cilantro has shown promising results.

The objectives of the current study were to 1) determine the effect of increasing batch size on microbial loads of salsa manufactured with "MIVACed" cilantro; and 2) determine the effect of increasing batch size on the sensory characteristics of salsa manufactured with MIVACed cilantro. In order to accomplish these objectives, shelf life studies were conducted on fresh and processed salsa containing cilantro.

The short-term impact of this research would be to make a shelf-stable, fresh-tasting tomato salsa available to the local community through the University Farm Market. The intermediate impact would be to make this same process and/or product available to the food processors throughout California for production of salsa and other unique cilantro-based products. The long-term implications are far reaching. The MIVAC process is a unique and exciting one that is being under-used by food processors, especially dried herb and spice manufacturers. The overall purpose of this study is to develop a "new" processed, shelf-stable salsa that retains the fresh characteristics of cilantro.

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METHODOLOGY

Cilantro processing

Fresh cilantro arrived daily during cilantro processing. Cilantro was weighed, a sample for microbial analysis removed, and the remainder was kept under refrigeration until ready to be treated. All equipment and utensils were sanitized prior to use with a chlorine-based sanitizer (260 ppm). Cilantro bundles were removed from the box, trimmed, the bundle tie removed, and then washed in three different 5-gal. buckets containing approximately 4-gal. of tap water at room temperature. Representative samples for microbiological analysis were collected at this point. The washed cilantro was placed in plastic bins where it was allowed to drain for at least five minutes before it was treated in citric acid solution. A second set of samples for microbiological analysis were taken at this point. Cilantro was then rinsed in clear tap water and allowed to drain.

Following the citric acid wash and rinse, the cilantro was was cut by hand to uniform size (approximately 1/4 inch). A third set of samples for microbiological analysis was removed at this point. Chopped cilantro was collected and dehydrated. Cilantro was subjected to the MIVAC dehydration process for 75 min. at 130°F and 3,000 watts (20 Torr). The dehydration process removed 90% to 92% of water by weight. MIVAC-treated cilantro had approximately 3% moisture on a dry weight basis using a vacuum oven. A representative and random sampling of dehydrated product was collected for microbial analysis. All cilantro samples were aseptically removed and stored for analysis.

All samples were stored in a refrigerator at 38-40°F until microbial analysis could be performed the day following collection. The fresh-washed, citric acid-treated and chopped cilantro samples were taken out of the refrigerator and prepared immediately. One hundred grams of sample were weighed into a sterile, dry, blender jar and blended until fine particles were obtained. These macerated particles were used to prepare serial dilutions as described in the Compendium of Methods for the Microbiological Examination of Foods (1992). The dried samples were removed from foil pouches and serial dilutions prepared as above. Samples from each dilution were plated on appropriate Petrifilms® and incubated for appropriate times (48 hr. at 35°C for total microbial count and coliform; 72 hr. and 120 hr. at 25°C for yeasts and molds). Films having more than 250 colonies per 1 ml of sample were determined to be too numerous to count (TNC); those having less than 250 colonies per 1 ml but more than 25 colonies per 1 ml of sample were counted and recorded (Banwart, 1989).

Salsa Production

The recipe for salsa was standardized in a related study by a graduate student (Carlsen, 1997). The total amount of each of the ingredients necessary for the entire project was obtained at the same time and from the same lot to avoid variability. All ingredients were stored at the Food Processing Research Laboratory (FPRL) under appropriate conditions except for the cilantro, which was processed as described above. Good manufacturing practices were performed throughout the process to avoid microbial contamination and under-processing. The amount of each of the ingredients needed for every production run was pre-calculated based on standardized recipe percentages.

A bench-top batch (a 40-ounce recipe – designated control) was produced for every scaled-up run (14, 28, and 42 gal. – designated treatments 1, 2 and 3, respectively). The procedure for producing bench-top salsa is described in a previous work (Carlsen, 1997). The scale-up of the salsa recipe took place at the FPRL at California State University, Fresno. Three scale-up processing runs were carried out during the study to produce 14-, 28-, and 42-gal. batches. Diced processed tomatoes (undrained), diced processed jalapenos (drained) and diced frozen onions (undrained) were mixed in a 50-gal. steam-jacketed kettle. The mixture was heated to a processing temperature between 200-205°F and maintained at that temperature for eight to 10 minutes. The mixture was mixed intermittently with a stainless steel paddle and the temperature monitored continuously. The system (piping, pump and air-actuated, positive displacement filler) was thermally equilibrated (190-195°F) by pumping hot product through all components until an exit temperature of 190-195° was achieved. Cilantro was then added to the salsa in the kettle and mixed thoroughly. The mixture containing the cilantro was pumped through the system until cilantro was visible from the filler. The temperature was checked and then the jars were filled. Each jar was hand-capped and inverted immediately. Jars were placed in bustrays containing cool water (62-65°F) where they were held for held for three minutes. They were then turned right side, placed in a refrigerator (36-40°F), and held overnight. The next morning, jars were placed in the original cardboard containers and stored at the FPRL for sampling and analysis.

Shelf Life Study

A specified number of randomly selected jars were taken from each bench-top and scale-up processing run and stored at the FPRL at room temperature (68-75°F). As part of the ongoing shelf-life studies, stored jars were periodically inspected to detect leakage or any abnormal conditions (due to microbial activity, processing or storage conditions). The jars from every production run were analyzed for microbial growth during several occasions in a five-week period. Each jar was tested for vacuum prior to being opened using a canner’s test vacuum gauge.

Two 9-oz. jars of finished salsa product were randomly selected from each run at the time of the analysis. The jars were tested for vacuum and analyzed separately. Two hundred grams of sample were aseptically weighed into a sterile, dry, blender jar. The sample was blended until fine particles were obtained. Serial dilutions were then prepared and plating was done as above.

 

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RESULTS AND DISCUSSION

One hundred and forty pounds of fresh cilantro was necessary to produce 11.0 lbs. of dried cilantro needed for the current study (a 7.9% yield). The results of the Total Plate Count (TPC) are shown in Figure 1. A general trend for microbial growth on cilantro is seen in this figure. Raw cilantro contains fairly high counts of bacteria. Washing reduces the bacterial load as does soaking in citric acid. However, microbial counts increase for chopped cilantro. This can be attributed to production personnel not wearing gloves during the chopping phase. All cilantro was chopped by hand. Microbial counts are considerably lower for MIVACed cilantro than for any of the "wet" products. This was also expected as the moisture content of MIVACed cilantro is approximately three percent; a level that makes it difficult for microorganisms to grow (Gavin and Weddig, 1995) or spores to germinate.

Figure 2 shows the results of the coliform plating. The same general trend of reduced microbial loads can be seen for coliform through citric acid treatment, although the extent of the reduction is less than for the TPC. The increased microbial load for chopped cilantro is again seen for the coliform plating, but as with the other data, the relative increase is not as high as for the TPC. The counts for the dehydrated product are also greatly reduced.

Figure 3 shows the results of plating for yeast cells on Petrifilms®. Yeast counts remained relatively constant between the 72-hour and 120-hour platings. The amount of yeast cells/gram of product was reduced by washing the cilantro, indicating that washing is a fairly effective means of removing viable yeast cells from cilantro. However, the amount of yeast cells present was not greatly reduced by treatment with citric acid solution. This makes sense, since the optimum growth rates for yeasts are in the acidic range and some are optimum in the pH range used for the citric acid solution (pH=3.0) (Banwart, 1989). Dried MIVACed cilantro, however, was free of yeasts, which indicates that the combination of the temperature used for processing (130°F for 75 minutes) and the lack of moisture due to dehydration were sufficient to destroy these cells.

Figure 4 shows the results of plating for mold using Petrifilms®. Washed cilantro contained no viable mold counts for the 72- and 120-hour readings, indicating that the washing step is an effective means of removing molds. The number of mold colonies present/gram of product increased during the acid-soaking and chopping steps of the processing. Molds can grow and reproduce in very acidic conditions and some would grow optimally at the pH used for this experiment. In fact, the pH conditions used to process the cilantro would actually be selective for molds over bacteria and yeasts (to some degree). In addition, mold spores are found in the air, on surfaces, and even on hands. This can help to explain the growth of mold from a product which showed no microbial growth after washing but did show microbial growth after the citric acid treatment. Additionally, dried MIVACed cilantro showed mold growth. Molds reproduce using sporulation. The spores that are produced can survive more harsh environments and then germinate when conditions are favorable. No mold growth was observed in the finished salsa products, indicating that the heat treatment used for processing was sufficient to eliminate mold spores.

Jars for sampling of the bench-top and scale-up operations were randomly selected from all batches. Tables 1 and 2 summarize the results form these samplings. Table 1 shows that total processing time, cilantro residence time and total filling time were fairly consistent for for all control products. However, total processing time and cilantro residence time all decrease relative to the 14-gal. baseline time for the treated products. For example, given the total processing time of 47 minutes for the 14-gal. batch (treatment 1), the 28- and 42-gal. batches (treatments 2 and 3) should have been 94 and 141 minutes (2x and 3x of the baseline), respectively. However, the residence times for the 28- and 42-gal. batches are 78 (-16 minutes) and 98 (-43 minutes) minutes respectively. The same trend can be seen for cilantro residence times. This is particularly significant because the total filling times increased as batch size increased. These data show economies of scale. As batch size increases, the time required to process the product per unit actually decreases.

Table 2 shows production yields. Yields for control (bench-top) products were fairly consistent (93.5-96.6%) but yields for the treated products varied greatly (65.43-97.0%). This is undoubtedly due to the volume to surface ratio for the treated products. As the volume in the steam kettle increased, there was a more rapid exchange of heat between the kettle jacket and the product because of the increased heating surface area. The salsa would heat primarily by convective transfer, which is fairly rapid. However, the surface area of the salsa, where the water would be evaporated, would be relatively the same for all batches produced. Therefore, there is less evaporation and more rapid heat exchange which results in a greater product yield because less water is lost.

Additionally, some product was unavoidably left in pipes and the pump following processing. This amount would be fairly consistent for all three treatment runs, but would be greater relative to the smaller volume of product. For example, if 0.5 gal. of salsa were left in the pipes for a 14-gal. run, this would result in a 3.6% loss. The same product left in a 28-gal. batch would represent a 1.8% loss and in a 42-gal. batch a 1.2% loss. There was also some slight spillage during processing runs which might account for some of the losses of product.

Table 3 summarizes the vacuum values (given in inches of mercury) of processed salsa. In some sample jars the vacuum was low (7.5-8.6 mm of Hg.) while in most the values were higher (9.0-13.2 mm of Hg.). A drop in product temperature at the moment of filling could have been one of the possible reasons for a low vacuum in those jars. The product sealing temperature (190-195°F) has an important effect on the final vacuum in a glass container. The higher the filling temperature, the higher the vacuum. Therefore, if the sealing temperature is insufficient, the effect of product contraction upon cooling is affected resulting in a low vacuum (Gavin and Weddig, 1995). Another reason for this discrepancy could have been the amount of air present in the product at the time of filling and its interaction with product temperature. Higher filling temperatures usually result in less air in the product and a good vacuum can be expected (Gavin and Weddig, 1995).

Table 4 shows the pH values of the product after mixing in the steam-jacketed kettle prior to processing and of the finished salsa products. The thermal process applied to the salsa (Hot-Fill-Hold) destroyed vegetative cells and some heat labile spores. The product’s low pH prevented any remaining spores from sporulating (Gavin and Weddig, 1995). All initial and final pH values indicate that the Hot-Fill-Hold Process is appropriate for this product and that no acidification is necessary. The pH values for the finished products is lower than the initial due to removal of water and concentration of acid within the finished products.

Table 5 shows the results of the samples plated for total plate count and coliform after 48-hr. incubation and for yeast and mold after 72-and 120-hr. incubation time. No growth was seen for any sample during the time of this study. This indicates that the Hot-Fill-Hold process is effective in destroying viable organisms during processing and that the anaerobic conditions in the jars are sufficient to prevent other microbial growth.

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CONCLUSIONS

Initially, there was a concern that adding MIVACed cilantro to a salsa batch near the end of processing might lead to microbial growth because of the decreased residence time in the kettle for MIVACed cilantro. This concern was alleviated as all batch-size samples analyzed showed no microbial growth. The Hot-Fill-Hold method applied to produce the salsa was an effective method to eliminate viable microbial loads.

Increasing batch size led to decreased total and cilantro residence time even though filling times increased disportionately. This also led to increased yields without a loss in vacuum, pH, or an increase in microbial counts. Larger batch sizes should be always used.

The low vacuum obtained in some of the jars analyzed was probably due to the cooling method applied to cool the jars after processing. Jars were cooled in water baths prior to placing them in a refrigerator. The time required to cool the jars was too long, which led to lower vacuum readings than expected. Therefore, in order to increase the vacuum in the jars produced, it is necessary to find a more effective method to cool them or a method to remove more headspace from the jar prior to capping.

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REFERENCES

Banwart, G.J. 1989. Basic Food Microbiology. 2nd ed. Chapman & Hall Publishers, New York, N.Y.

Brandweek. 1992. The Salsa Revolution Takes on an International Flavor. Food Industry-Marketing 33 (36): 36.

Carlsen, K. 1997. Development of a New Shelf Stable Salsa with Microwave Vacuum (MIVAC) Dehydrated Cilantro. MS Thesis, California State University, Fresno, Fresno, CA.

Compendium of Methods for the Microbiological Examination of Foods. 1992. 3th Ed. Edited by Carl Vanderzant and Don F. Splittstoesser. American Public Health Association, Washington, DC.

Decareau, R. V. 1985. "Microwaves in the Food Processing Industry." Academic Press, Inc., Orlando, FL.

Gavin, A. and Weddig, L. 1995. "Canned Foods: Principles of Thermal Process Control, Acidification and Container Closure Evaluation" 6th Ed. Food Processors Institute, Washington D.C.

Gessert, K. R. 1983. "The Beautiful Food Garden." Van Nostrand Reinhold Company, New York, NY.

Qin, B-L., Pothakamury, U.R., Vega, H., Martin, O., Barbosa-Canovas, G.V., and Swanson, B.G. (1995). Food pasteurization Using High-Intensity Pulse Electric Fields. Food Technology 49 (12): 55-60.

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