As a former toddler teacher, trainer of child care providers, college instructor (in Early Childhood Education), and a parent, I implore you to reconsider using lemon juice as a biting deterrent and allowing your child care provider to so as well. Research and experience have taught me that this kind of punitive reaction will NOT help the child who bites. In fact, knowing something punitive is coming when they bite adds to their anxiety and can actually lead to causing more of the behavior you are trying to prevent.
Biting in toddlers is a way for them to meet a need. As adults, we need to observe the child and figure out what that need is so we can teach the child pro-social ways to get their needs met. Discipline is about teaching children - guiding their behavior. To truly do this we need to know why the behavior occurs to address it appropriately. My first thought with the lemon juice method was that it sounds like what I do when my cat does something she isn't supposed to do - I spray her with water. It makes her stop the behavior and the lemon juice may make your child stop biting. However, I am not trying to teach my cat to grow up to be a well-adjusted human being. To teach kids the right thing to do, we need to know why they bite and address the issues specifically.
You said he bites when frustrated - this is VERY common for kids his age. The important thing is to catch him at the beginning of his frustration and model appropriate reactions (tell her to stop hitting you, taking your toy, etc.). It may help to teach your child to walk away when they feel very angry and come back to the situation when they are calmer. In my last classroom for kids between 16 months and 2 yrs. we had a safe face where kids could go to collect themselves. At first, teachers encouraged them to use the space when they seemed to need it and eventually, they learned when they needed to be away from the group to calm down.
You also said he seems to bite for no reason. Trust me, there is a reason. Sometimes it's REALLY hard to find the reason - but there is a reason. The first thing to do may be to start a Biting Log that keeps track of when the child bit and what was going on in the room before the biting happened. Is the child getting hungry, tired, etc.? The book, "Prime Times for Infants and Toddlers" by Greenman and Stonehouse has a sample biting log and great information for child care programs on working through this stage.
Another thing to consider, I had a couple of kids who found it helpful to have a wet paper towel or wash cloth that they kept with them and when they felt the need to bite for whatever reason, they bit the cloth. Some kids are just very oral and have a need to bite. Having the cloth kept them from biting other kids and helped us in our search in the "why" of their need to bite. Neither child carried the cloth around for more than a few weeks. It was kind of a "biting transitional aide."
I'm going to copy a couple of web sites with more information that are geared toward the child care environment and I encourage you to share them with your son's program. I highly recommend the technique of having the child who bit (or hit or whatever) help the child that they hurt. It is a wonderful way to teach empathy.
It may help to have a professional child psychologist observe your son in his child care setting. I had parents do this once for a child that we really couldn't figure out what was setting him off and it was so helpful. Come to find out, the child was trying to make friends and was biting out of frustration from not being able to do that. I would not have figured that out at that stage in my career without outside help.
I know that it's a horrible feeling when your child is hurting others and that you want to do everything you can to stop the behavior. I've worked with a lot of families in your shoes and have never known anyone to not be upset when their child bites. I know it's hard for the child care program too. The teachers are responsible for keeping children safe and guiding their behavior - when these situations occur it's stressful. By working together, you will help your child and help each other. With loving guidance, you will get to the bottom of the cause of the behavior and it will stop. Good luck to all of you.
Sites:
http://www.childcarelounge.com/Parents/biting.htm
This one has some typos, but the info is sound.
Unfortunately, I cannot access the other sites at the moment. I recommend the Prime Times book mentioned above and the book "No Biting! Policy and Practice for Child Care Programs" as resources for your son's child care setting. Both should be available via amazon.com